Twelve testable hypotheses on the geobiology of weathering
S. L. BRANTLEY,1 J . P . MEGONIGAL,2 F. N. SCATENA,3 Z. BALOGH-BRUNSTAD,4 R. T. BARNES,5
M. A. BRUNS,6 P. VAN CAPPELLEN,7 K. DONTSOVA,8 H. E. HARTNETT,9 A. S. HARTSHORN,1 0
A. HEIMSATH,1 1 E. HERNDON,1 L. JIN,1 C. K. KELLER,1 2 J . R. LEAKE,1 3 W. H. MCDOWELL,1 4
F. C. MEINZER,1 5 T. J . MOZDZER,2 S. PETSCH,1 6 J . PETT-RIDGE,1 7 K. S. PREGITZER,1 8
P. A. RAYMOND,1 9 C. S. RIEBE,2 0 K. SHUMAKER,2 1 A. SUTTON-GRIER,2 R. WALTER2 2 AND
K. YOO2 3
1Earth and Environmental Systems Institute, Pennsylvania State University, University Park, PA, USA
2Smithsonian Environmental Research Center, Edgewater, MD, USA
3Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, PA, USA
4Departments of Geology, Environmental Sciences and Chemistry, Hartwick College, Oneonta, NY,USA
5Department of Geological Sciences, University of Colorado, Boulder, CO, USA
6Department of Crop and Soil Sciences, Pennsylvania State University, University Park, PA, USA
7School of Earth andAtmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA
8Biosphere 2 Earthscience, University of Arizona, Tucson, AZ, USA
9School of Earth and Space Exploration, andDepartment of Chemistry and Biochemistry, Arizona State University, Tempe, AZ,
USA
10Department of Geology and Environmental Science, James Madison University, Harrisonburg, VA, USA
11School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA
12School of Earth and Environmental Sciences, Washington State University, Pullman,WA,USA
13Department of Animal and Plant Sciences, University of Sheffield, Sheffield, UK
14Department of Natural Resources and the Environment, University of NewHampshire, Durham, NH, USA
15USDA Forest Service, Pacific Northwest Research Station, Corvallis, OR, USA
16Department of Geosciences, University of Massachusetts Amherst, Amherst, MA, USA
17Department of Crop and Soil Science, Oregon State University, Corvallis, OR,USA
18College of Natural Resources, University of Idaho, Moscow, ID, USA
19Yale School of Forestry and Environmental Studies, Yale University, NewHaven, CT, USA
20Department of Geology and Geophysics, University of Wyoming, Laramie, WY,USA
21College of Natural Sciences andMathematics, andDepartment of Biological and Environmental Sciences, The University of West
Alabama, Livingston, AL, USA
22Department of Earth and Environment, Franklin&Marshall College, Lancaster, PA, USA
23Department of Soil, Water, and Climate, University of Minnesota, St Paul, MN, USA
ABSTRACT
Critical Zone (CZ) research investigates the chemical, physical, and biological processes that modulate the Earth?s
surface. Here, we advance 12 hypotheses that must be tested to improve our understanding of the CZ: (1) Solar-
to-chemical conversion of energy by plants regulates flows of carbon, water, and nutrients through plant-
microbe soil networks, thereby controlling the location and extent of biological weathering. (2) Biological stoichi-
ometry drives changes in mineral stoichiometry and distribution through weathering. (3) On landscapes experi-
encing little erosion, biology drives weathering during initial succession, whereas weathering drives biology over
the long term. (4) In eroding landscapes, weathering-front advance at depth is coupled to surface denudation
via biotic processes. (5) Biology shapes the topography of the Critical Zone. (6) The impact of climate forcing on
denudation rates in natural systems can be predicted from models incorporating biogeochemical reaction rates
and geomorphological transport laws. (7) Rising global temperatures will increase carbon losses from the Critical
Zone. (8) Rising atmospheric PCO2 will increase rates and extents of mineral weathering in soils. (9) Riverine
solute fluxes will respond to changes in climate primarily due to changes in water fluxes and secondarily through
changes in biologically mediated weathering. (10) Land use change will impact Critical Zone processes and
exports more than climate change. (11) In many severely altered settings, restoration of hydrological processes is
possible in decades or less, whereas restoration of biodiversity and biogeochemical processes requires longer
 2010 Blackwell Publishing Ltd 1
Geobiology (2011) DOI: 10.1111/j.1472-4669.2010.00264.x
timescales. (12) Biogeochemical properties impart thresholds or tipping points beyond which rapid and irrevers-
ible losses of ecosystem health, function, and services can occur.
Received 12 May 2010; accepted 29 October 2010
Corresponding author: S. L. Brantley. Tel.: 814-865-1619; fax: 814-865-3191; e-mail: brantley@eesi.psu.edu
INTRODUCTION
The impact of humans on the Earth?s atmosphere, water, sedi-
ments, soil, and biota is thought to be of such magnitude that
a new geological epoch ? the Anthropocene ? has been
proposed (Crutzen, 2002; Zalzsiewicz et al., 2008). The pace
of anthropogenic change threatens sustainable use of soil and
water, and impairs critical ecosystem services (Vitousek et al.,
1997; DeFries & Eshleman, 2004; McNeil & Winiwarter,
2004; Foley et al., 2005; Wilkinson, 2005; Wilkinson &
McElroy, 2007; Holdren, 2008; Mann & Kump, 2008).
Predicting changes in earth resources requires quantitative
models to forecast ? Earthcast ? the behavior of the zone
extending from the vegetation canopy to groundwater (Bach-
let et al., 2003; Yang et al., 2003; Steefel et al., 2005; Qu &
Duffy, 2007; Minasny et al., 2008; Godde?ris et al., 2009;
Rasmussen et al., 2010). This realm, where rocks meet life,
has been named the Critical Zone (CZ; US National Research
Council Committee on Basic Research Opportunities in the
Earth Sciences, 2001). Within the CZ, biota ? including
humans ? interact with earth processes to define the chemis-
try, texture, and topography of our surface habitat through
weathering, element cycling, and erosion.
To describe the behavior of a system as multifaceted as the
CZ, Earthcasting models need to be developed and parame-
terized from observations of the atmosphere, water, surface
Earth materials, and biota, made over a range of spatial and
temporal scales. It is not sufficient, for example, to focus only
on rocks at depth if the goal is to solve problems related to
human-land-air-water interactions. Likewise, if the goal is to
understand the long-term implications of climate change, it is
not sufficient to measure the short-term responses of vegeta-
tion, because nonlinear, highly complex responses can emerge
over the long term, in strong contrast to more predictable,
shorter-term linear responses (Swetnam et al., 1999; Gunder-
son, 2000; Chadwick & Chorover, 2001; Bachlet et al.,
2003). One of the best time-integrated records of the CZ is
the soil itself: soil horizons, thicknesses, texture, structure,
composition, biological activity and their spatial patterns in
landscapes integrate and record the flow of material, changes
in climatic and tectonic forcing, and the influence of humans
and other biota on the CZ (Dokuchaev, 1883; Jenny, 1941,
1980; Yaalon, 1983; Retallack, 1990; Richter & Markewitz,
2001). Indeed, humans ultimately depend on weathering
and erosion processes to maintain the natural resources under-
lying society and ecosystems (McNeil & Winiwarter, 2004;
Montgomery, 2007). Productive and sustainable agriculture,
forestry, and aquatic resources are directly linked to CZ
processes that transform rock into soil and sediment.
In this study, we pose 12 hypotheses developed by more
than 30 scientists from a range of scientific disciplines (see
Acknowledgements). The group sought to state provocative,
important, and testable hypotheses that are related to the com-
plex interactions among biology, weathering, and erosion.
Although some of the hypotheses have been implicit in scien-
tific research conducted since the late 1800s, we argue that
new analytical, modeling, and field opportunities now allow
advances that can test these hypotheses over the next decade.
EARLY DISCUSSIONS OF THE EFFECTS OF
BIOLOGY ON WEATHERING AND EROSION
Understanding how biology interacts with earth materials to
form the CZ has long been a subject of curiosity, scientific
inquiry, and social necessity. Historically, numerous proposals
were advanced to explain weathering, the process that breaks
down and solubilizes rock components, leaving residual soil
minerals and exporting solutes. Some of these early ideas
match our current understanding; for example, in the early
to mid-1800s, the decomposition of igneous rocks was
attributed to the reactivity of water and carbonic acid (Four-
net, 1833; Hartt, 1853) and landscape evolution was thought
to be influenced strongly by the production of physically
mobile soils (Gilbert, 1877, 1909). Furthermore, very early
on, the role of vegetation was noted: for example, Belt (1874)
wrote that ?the percolation through rocks of rain water
charged with a little acid from decomposing vegetation? accel-
erated weathering. In roughly the same time period, Charles
Darwin studied the effects of earthworms on soils, concluding
that ?? all the vegetable mold over the whole country has
passed many times through, and will again pass many times
through, the intestinal canals of worms? (Darwin, 1881). By
the 1880s, the concept of soils as natural systems that are gov-
erned by the interaction of climate, living matter, parent mate-
rial, relief, and time had been developed (Dokuchaev, 1883).
Perhaps the modern era of biota-weathering research began
in the latter half of the 1900s as researchers began to explore
the role of microbial communities in weathering of historic
stone buildings (Ehrlich, 1990; Krumbein et al., 1991). Today,
a wide variety of researchers from many disciplines are focus-
ing on the relationships between biota and weathering and
erosion. Such efforts now focus on quantifying both the
importance of microbial activity and plants in elemental
cycling and rock weathering in natural systems (e.g. Banfield
2 S. L. BRANTLEY et al.
 2010 Blackwell Publishing Ltd
& Nealson, 1997; Berner et al., 2003). Furthermore, research-
ers are also modeling the effects of fauna on weathering and
erosion (e.g. Gabet et al., 2003). Recent work on nutrient
cycling and material flows within ecosystems also acknowledges
the importance of rock weathering in structuring ecosystems
(Banfield & Nealson, 1997; Buss et al., 2005; Osterkamp &
Hupp, 2010).
Despite the long history of inquiry, quantitative insights on
many fundamental issues have been elusive. For example, the
contribution of rock weathering to ecosystem-level biogeo-
chemical cycling has rarely been measured directly and is
typically calculated as the difference between atmospheric
inputs and stream flow exports. Likewise, linking the role of
biologic agents of erosion and climatic change to both short-
and long-term denudation has been elusive. Fortunately, new
isotopic techniques (Bourdon et al., 2004; Johnson et al.,
2004; Von Blanckenburg, 2006) and advances in the use of
molecular biological (Banfield et al., 2005), elemental (Herr-
mann et al., 2007), and organic (Johansson et al., 2009)
analyses now make it possible to quantify the rates of rock
weathering and to define the role of organisms.
DEFINITIONS: TALKING ABOUT THE CZ
Although the CZ has been researched since the onset of
science, the term itself is new. CZ science incorporates a holis-
tic yet quantitative approach to understanding Earth surface
processes, integrating across diverse scientific disciplines and a
range of spatial and temporal scales. To promote communica-
tion across the CZ disciplines and to begin to achieve consen-
sus about terminology, we present working definitions of key
CZ terms that have been variously defined over the years by
different groups.
Defining the CZ system
Beginning at the base of the CZ, we define parent material as
earth material as it was before alteration by surface processes.
In some cases, parent material is consolidated bedrock. In
other cases, it can be unconsolidated sediments, or fragments
of rocks and organic matter transported from elsewhere.
Regardless of the nature of the parent material, its alteration
causes the development of regolith, here defined as the mantle
of unconsolidated and altered material that was generated
from the parent material. In this context, soil is the surface
sublayer within regolith where the parent material has been
extensively altered, generally by chemical, physical, and biolog-
ical transformations. Likewise, saprolite is the zone in regolith,
if present, where parent material has weathered isovolumetri-
cally in place. Saprolite generally retains evidence of parent
material texture and fabric.
Biological, chemical, and physical alteration processes are
important in both saprolite and soil, but soil is more affected
by biology due to its shallow depth and proximity to the plant
canopy, which is the source of chemical energy for much of
the biological activity that drives weathering. Defined this
way, soils are the surface-most zone of the regolith where
biological, chemical, hydrological, and physical processes are
most active, characteristically driving the evolution of layers
known as horizons. Although CZ interactions between
biology and weathering are most intense and dynamic in the
soil, it should be emphasized that biological activity ? both
in situ and ex situ ? is also an important feature of weathering
processes in the saprolite and deeper zones of regolith.
Importantly, these definitions do not match those used in
all the CZ disciplines. For example, some scientists define
?soil? to include saprolite. Also, geomorphologists describe
regolith (Small et al., 1999) or the mobile soil layer (Heims-
ath et al., 1997) on hillslopes as the layer that is physically
mixed and transported downslope. Clearly, the mobile layer
and the chemically altered layer do not have to be coincident.
Lower limit of regolith
The bottom of regolith is defined here as the point where
chemical and physical properties change from unaltered to
altered. This leads to two operational definitions for the
parent?regolith interface: (i) the physical interface where the
underlying parent material has altered and can be easily
sampled without drilling or hammering, or (ii) the chemical
interface where regolith is chemically distinct from parent
material. The first definition is useful for field work. Thus, the
depth of refusal during augering or digging is often used to
define the base of regolith. However, at this depth, the ?par-
ent? may be both physically and chemically changed from the
true parent and therefore the operational definition may be
problematic. Furthermore, when parent is a deposit such as
alluvium or colluvium, the physical base of regolith may not
be easy to define using this criterion.
In addition, it is important to reiterate that geomorpholo-
gists often define the parent?regolith interface with a third
operational definition: (iii) the interface between ?mobile? and
?immobile? material, generally identified in soil pits from grain
size and textural relationships. The mobile layer is also some-
times denoted in the geomorphological literature as the ?soil?
and the underlying material is referred to either as ?weathered
bedrock? or as ?saprolite? depending on the degree of chemical
weathering. Importantly, these geomorphological definitions
may differ in many situations from definitions in this paper.
Whereas interfaces (i) and (iii) can be identified in the field,
the chemical base of regolith (interface ii) requires measure-
ments that can only be completed in the laboratory. Even with
such laboratory analyses, an operational definition is still
needed ? e.g. 10% alteration ? to define the distinction
between regolith and parent. Such laboratory measurements
can be used to define weathering reaction fronts for chemical
reactions ? zones of regolith across which a given chemical
reaction for a given mineral occurs.
Hypotheses on geobiology of weathering 3
 2010 Blackwell Publishing Ltd
Both regolith and parent material are often described with
respect to their lithology. The term lithology refers to the rock
type that includes information about composition and miner-
alogy. Also important is the rock texture ? a term referring to
the size and distribution of mineral grains and porosity, as well
as the presence or absence of banding or other patterns.
CZ processes
The term weathering is used here to denote all processes that
change the parent material to regolith. As discussed further
below, some of these weathering processes result in net loss of
material from a system whereas others represent alteration
with little or no mass flux out. Here, we define denudation as
the net loss of all material due to chemical, biological, and
physical processes.
Although weathering is used here in a very general sense,
we will define chemical weathering specifically to include only
chemical reactions that transform the chemistry, mineralogy,
or texture of a solid phase in the CZ. For example, the original
minerals in the parent, i.e. the primary minerals, are often
chemically weathered to produce solutes (dissolved species in
the aqueous phase) that may precipitate as new secondary
minerals. The driving force for weathering is the impulse to
decrease the free energy of the system as primary minerals
equilibrated to deep conditions re-equilibrate to the condi-
tions at Earth?s surface. Chemical weathering can release
elements into water that are transported out of the system as
solutes (chemical denudation), into the gas phase, or into
ecosystem pools (biological uptake). Importantly, precipita-
tion of secondary minerals is a crucial component of the
chemical evolution of the CZ, but it is not included in the
chemical denudation term.
Physical weathering is defined here to include only pro-
cesses that fracture, change the density, or reduce the grain
size of the parent material without changing mineral chemis-
try. Thus, physical weathering does not change the chemistry
but rather tends to increase the surface area per unit mass, or
specific surface area, of parent material and thus increases its
susceptibility to chemical weathering and denudation. By dis-
rupting the coherence of the parent, physical weathering may
also promote physical denudation, which is conceptually paral-
lel to chemical denudation in that it denotes the net loss of
regolith as solids rather than solutes. In this paper, erosion is
defined as the sediment outflux minus the sediment influx for
a given location and is thus identical to physical denudation.
To some researchers, the term erosion connotes losses due to
chemical, physical, and biological processes ? a usage that we
reserve for denudation. Here, we adopt the convention that
erosion refers to material losses due to physical movement of
earth material, without reference to losses of solutes from the
system.
Critical Zone science aims to quantify rates and spatial dis-
tributions of these processes. The rate of chemical or physical
denudation is defined as the net loss per unit time of solutes or
sediments, respectively, from a given system. These rates are
expressed in units of mass or moles or volume of material per
unit time. To compare denudation rates among different
watersheds or hillslopes typically requires normalization
relative to the area in question. Thus, chemical and physical
denudation fluxes are typically expressed as volume or mass
per area per time (m3 m)2 year)1 or m year)1; kg m)2 year)1).
To transform from units of volume per area per time to mass
per area per time requires knowledge of bulk density (mass per
unit volume) of regolith or parent material.
Biological weathering refers to all the processes by which
biota ? including humans ? change the chemical or physical
properties of parent or regolith. Distinguishing biological
from chemical and physical weathering is ambiguous: for
example, the transformation of a primary mineral to a second-
ary mineral within a biofilm that releases solutes to bulk pore-
fluid might be described as both chemical and biological
weathering. Similarly, plant roots and burrowing animals take
advantage of existing pores and fractures in bedrock to
advance physical weathering. The distinction between chemi-
cal and physical weathering can be ambiguous as well. For
example, chemical reactions are thought to drive fracturing
during weathering; conversely, opening of fractures promotes
chemical weathering of new surfaces.
Water in the CZ
In general, groundwater is water in the saturated zone, where
pore space is filled with water. In contrast, the unsaturated or
vadose zone is the layer where pores are filled by both air and
water. The original definition of the CZ stated that the
bottom of the CZ is roughly the bottom of groundwater (US
National Research Council Committee on Basic Research
Opportunities in the Earth Sciences, 2001). However, the
base of groundwater is not precisely defined. At some point at
depth, water is no longer described as groundwater and is
better described as diagenetic water, i.e. water that has been
heated to a minor degree along the geothermal gradient.
Diagenetic water may have been trapped during rock forma-
tion or it may consist of meteoric water circulated downward
from the Earth?s surface.
As the residence time and temperatures of diagenetic waters
increase, the water chemically equilibrates with higher-tem-
perature assemblages of minerals at depth. In contrast, in some
locations at the land surface, water may be in equilibrium with
low-temperature assemblages of minerals. However, most of
the CZ is characterized by water that is present at ambient
temperature and that is not chemically equilibrated: shallowly
circulating meteoric waters are not hot enough nor have they
experienced long enough residence times to be everywhere
chemically equilibrated. The CZ is the zone defined by the gra-
dient from mineral assemblages equilibrated to above-ambi-
ent temperatures at depth to mineral assemblages equilibrated
4 S. L. BRANTLEY et al.
 2010 Blackwell Publishing Ltd
to ambient temperatures at Earth?s surface. The CZ, the zone
that is largely at near-ambient temperatures and that has
not attained chemical equilibrium, is therefore the zone that
hosts organisms living off energy derived from chemically
non-equilibrated rocks and from the sun.
Evolution of the CZ
It is also important to note that some landscapes are experi-
encing active erosion while others are relatively stable, without
significant erosion. Stable landscapes may be characterized by
soil profiles that deepen with time and whose characteristics
change with time. Sets of soils on the same lithology and in
the same climate that have developed on such stable land-
scapes over different exposure periods are referred to as chro-
nosequences. In contrast, the thickness of soils developed on
actively eroding landscapes may reach steady state such that
the rate of weathering advance downward into parent material
is roughly equal to the rate of erosive loss of material at the
surface. The weathering advance rate can be used to describe
the rate of transformation of bedrock to saprolite (saprolite
advance rate) or saprolite to soil (soil production rate).
In both actively eroding and stable landscapes, the CZ is an
example of a complex, non-equilibrium dynamical system
owing to its existence in a thermodynamic gradient, over
which it dissipates energy and may exhibit emergent proper-
ties (Prigogine, 1980; Corning, 2002). As an open system, the
CZ experiences inputs and outputs of energy and matter.
Although the properties of a dynamical system generally vary
with time, they may under certain conditions remain at steady
state. For an open system, this requires the inputs and outputs
across the system?s boundaries to balance one another. Under
steady-state conditions, it is possible to define the mean resi-
dence time or turnover time of a set of particles or elements as
the mass of particles or elements in the system divided by the
rate of throughput. The term residence time is usually applied
to physical, chemical, and biological constituents of the CZ.
The application of the concepts of residence time and steady
state requires a careful definition of the system?s boundaries
and the timescale of interest.
THE HYPOTHESES
The complex interplay of weathering and biota in the CZ
poses challenges for advancing research on how Earth?s
surface evolves in response to biological processes and other
factors. Essential to overcoming these challenges is the use of
an interdisciplinary approach. Using such an approach, we
developed 12 provocative hypotheses that can be tested today,
without regard for whether one or more may turn out to be
untenable or whether some elements of the different hypothe-
ses are contradictory. The goal in advancing these hypotheses
is to stimulate important research at the nexus of weathering
and biology.
Understanding the CZ
Hypothesis 1. Solar-to-chemical conversion of energy by
plants regulates flows of carbon, water, and nutrients
through plant-microbe soil networks, thereby controlling the
location and extent of biological weathering
The fossil record documents that, for more than 400 million
years, the majority of land plants have formed symbiotic asso-
ciations with mycorrhizal fungi (Taylor et al., 2009). In the
Devonian, the evolution of land plants with mycorrhizal asso-
ciations coincided with an order of magnitude decrease in
atmospheric CO2. This was followed by a more gradual
decline in PCO2 from the middle Cretaceous that has been
linked to increased continental weathering of Ca and Mg from
silicate minerals and their re-precipitation in marine carbon-
ates (Berner, 2006). Increasing evidence implicates plant?
mycorrhizal fungal co-evolution as a key driver of these
processes (Taylor et al., 2009).
Plants and their mycorrhizal fungi form an integrated
network transporting carbon, water, and nutrients through
the CZ, with organic carbon flowing down from the top of
the vegetation canopy to the tips of the deepest roots and
mycorrhizal mycelia in soil, whereas in return water and nutri-
ents are taken up and transported through the organisms back
to the canopy (Fig. 1). Flows of soil water and dissolved nutri-
ents into mycorrhizal fungi and roots (Fig. 1, Box B) are
largely controlled by the photosynthetic activity of plants. As
stomata open in leaves to allow CO2 uptake and fixation, loss
of water is promoted to the atmosphere (Fig. 1, Box A). This,
in turn, serves to draw replacement water from elsewhere in
the soil through the network of interconnected plant and fun-
gal tissues. Biotic networks can affect geochemical processes at
considerable distances beyond the site of biological origin due
to transport of metabolic products (e.g. carbon and acids gen-
erated by biota) to significant depths where they stimulate
reactions (Oh & Richter, 2005).
Roots and their mycorrhizal fungal associates are the major
conduits for transport of the chemical energy fixed by plant
photosynthesis into the soil. Mycorrhizal fungi are supported
by 10?30% of the sugars synthesized by their host plants,
enabling these fungi to develop extensive mycelial networks,
often exceeding 200 m cm)3 of soil (Leake et al., 2004).
These fungal networks selectively absorb nutrients that are
required by the host plants. The networks also contribute to
labile C pools by actively secreting organic acids, sidero-
phores, and enzymes (Landeweert et al., 2001; Finlay et al.,
2009). Such organic molecules provide carbon sources to
communities of associated bacteria and archaea, promoting
the dissolution and uptake of mineral nutrients by the fungi
(Fig. 1, Box C). Mycorrhizal fungal networks proliferate on
the surfaces of some minerals due to strong physical bonding,
secretion of organic compounds and protons, and active,
selective, ion uptake, thereby directing the solar-to-chemical
energy converted by the plant shoots into intense localized
Hypotheses on geobiology of weathering 5
 2010 Blackwell Publishing Ltd
weathering of selected soil minerals (Leake et al., 2008; Bon-
neville et al., 2009). In addition, the associated bacteria and
archaea form biofilms around the mycelial network (Fig. 1,
Box C). This biofilm may also enhance weathering and may
reduce the loss of weathered products to bulk soil water (Bal-
ogh-Brunstad et al., 2008).
As transpiration draws water up through roots and mycor-
rhizal mycelia, the surface soil often becomes relatively dry
although the deeper roots continue to access water (Fig. 1,
Box B). Thus, water is hydraulically redistributed via roots
from wetter to drier regions of the soil (Fig. 1, Boxes B and
C). This process normally happens at night when transpiration
is negligible. Hydraulically redistributed water (Warren et al.,
2008) can allow mycorrhizal fungi to remain active in dry soil.
These processes can therefore lead to increases in localized
concentrations of ligands, protons, and other secretions that
may accelerate and enhance mycorrhizal weathering activity
(Fig. 1, Box C). This may, in turn, enhance the supply of
nutrients to host plants to enable greater growth and photo-
synthesis (Fig. 1, Box A), returning more chemical energy to
weathering processes (Fig. 1, Box C).
The existence of functional connections between plant pho-
tosynthesis and mycorrhizal nutrient uptake are well estab-
lished (Graustein et al., 1979; Landeweert et al., 2001; Finlay
et al., 2009), but only in the past decade have researchers
begun to quantify the potentially major role played by mycor-
rhizal fungi and plants in weathering of minerals (Banfield &
Nealson, 1997; Berner et al., 2003; Taylor et al., 2009) and
in hydraulic redistribution of water (Brooks et al., 2006).
Advances in the use of isotope tracers to study the integrated
Fig. 1 A conceptual model of chemical energy in the form of organic carbon formed by photosynthesis driving carbon, water, and element flows in the CZ through a
networked community of plants, mycorrhizal fungi, bacteria, and archaea. Plant roots and their associated mycorrhizal fungal networks are supported by substantial
fluxes of recent photosynthate (red) fixed in plant shoots from atmospheric carbon dioxide. They use this energy to play a pivotal role in the uptake of water (blue)
and nutrients (green) from soil. There is strong inter-dependency between these dynamic flux pathways that act synergistically to enhance biological weathering. (Box
A) Water flow and nutrient fluxes are driven by transpiration losses associated with open stomata to allow carbon dioxide uptake into the leaves. The main demand
for nutrients is in the leaves that provide the photosynthate carbon flux to roots and into mycorrhizal fungi for acquisition of nutrients. (Box B) Water uptake from the
wetter deeper soil layers via mycorrhizal hyphal networks and roots includes the uptake of elements in soil solution and is supported by recent photosynthate provided
by the plant. The mycorrhiza-root network can also contribute to hydraulic redistribution of soil water (dotted blue lines in main figure and Box C). (Box C) Mycorrhizal
fungi release substantial amounts of carbon through respiration and exudation that promote biofilm development on mineral surfaces (pink areas) facilitated by spe-
cialized mycorrhizosphere bacteria and archaea (purple cells). Localized moisture films (blue patches) provided by hydraulic redistribution (dotted blue lines) may
enable the fungi and associated micro-organisms to actively weather minerals during dry soil conditions and to capture and transport essential nutrients to the plant.
Mycorrhizal networks of multiple species connect between adjacent plants, so they can transport some of the hydraulically redistributed soil water and the released
nutrients into other plants (right side).
6 S. L. BRANTLEY et al.
 2010 Blackwell Publishing Ltd
transport pathways through mycorrhizal mycelia, roots, and
shoots (Warren et al., 2008), together with advances in meth-
ods to study mineral weathering at the scale of individual
grains and fungal hyphae (Bonneville et al., 2009), and
molecular, microscopic, spectroscopic, and modeling tech-
niques provide the necessary tools to test this hypothesis at
appropriate spatial and temporal scales.
For example, chemical energy fixed by plants and allocated
to support mycorrhizal mycelial networks in soil can be mea-
sured using 13C- or 14C-labeled CO2 supplied to shoots
(Leake et al., 2004). These isotopes can be traced through
mycorrhizal mycelia as exudates are taken up into associated
bacteria and archaea. Pathways for hydraulic redistribution
can similarly be studied using deuterium or tritium tracers.
The absolute and relative magnitudes of hydraulic redistribu-
tion can be assessed through continuous monitoring of
changes in the volumetric water content of deep and shallow
soil layers (Warren et al., 2008) and measurements of the
direction and magnitude of sap flow in deep and shallow roots
(Brooks et al., 2006; Scholz et al., 2008). Weathering can be
measured by characterization of the alteration of natural min-
erals or by inserting mineral grains into regolith (Nugent
et al., 1998). Synthetic minerals labeled with rare earth or
radioactive elements can also be used. These studies require
an interdisciplinary approach that combines laboratory and
field experiments and involves integrative studies across differ-
ent bioclimatic regions.
Hypothesis 2. Biological stoichiometry drives changes in
mineral stoichiometry and distribution through weathering
Stoichiometry is here used to refer to the quantitative ratio of
elements in a phase or in a chemical reaction. The composi-
tions of primary minerals such as quartz (SiO2) or biotite
(K(Mg, Fe)3AlSi3O10(OH)2), and secondary minerals such as
goethite (FeOOH) are defined with respect to stoichiometry.
Where reactants and products are known, the stoichiometries
of mineral?water reactions that comprise chemical weathering
are also well defined. Similarly, the stoichiometry of redox
reactions that constitute plant and microbial respiration are
also fairly well known. Even the stoichiometry of biota has
been explored. In marine systems for example, the stoichio-
metry of marine phytoplankton is nominally defined by the
ratios of C:N:P = 106:16:1 (Redfield, 1958; Redfield et al.,
1963). Relatively recently, ratios for other elements in phyto-
plankton ? K, Mg, Fe, Mn, Zn, Cu, Mo ? have been shown
to reflect the intrinsic metabolic requirements of specific spe-
cies (Ho et al., 2003; Quigg et al., 2003). The concept of
biological stoichiometry is also rooted in the terrestrial and
limnological literature (for a comprehensive synthesis see
Sterner & Elser, 2002). Ecological stoichiometry has gener-
ally (but not always, see Townsend et al., 2007) proven to be
a useful framework for examining the transfer of matter at
scales ranging from cells to organisms to ecosystems to the
globe.
The stoichiometries of organisms and minerals in the CZ
are intrinsically coupled because most of the 30 or so bio-
essential elements are derived from soil minerals. However,
plants do not always require elements in the same proportions
at which they occur in the Earth?s crust (Table 1, Fig. 2). As a
result, biological uptake by plants and micro-organisms alters
the composition and distribution of elements in soils.
Although it has been shown that plants and micro-organisms
affect such distributions (Hinsinger et al., 1993; Markewitz
& Richter, 1998; Jobbagy & Jackson, 2001; Street-Perott &
Barker, 2008; Brantley & White, 2009), and conversely, that
element distributions influence plant community composition
(Lopez et al., 2009), few studies explicitly link the stoichio-
metric requirements of biological processes to the changes
Crust element content (log10 atom percent)
?6 ?5 ?4 ?3 ?2 ?1 0 1 2
P
la
nt
 e
le
m
en
t c
on
te
nt
 (l
og
10
 a
to
m
 p
er
ce
nt
)
?6
?5
?4
?3
?2
?1
0
1
2
C
O
H
N
K
Ca SiMgP
S
Cl Al
Fe
Mn
B
Zn
Cu
NiMo
Enriched in the crust
Enriched in plants
1:1 Line
Fig. 2 Abundance of elements in Earth?s crust compared with abundance in
plant tissues. Data compiled by J. R. Leake.
Table 1 Average elemental ratios in plants and in Earth?s crust*
Element ratio Plant tissue Crustal rock
C:N 37 8.0
C:P 650 0.6
N:P 18 0.07
Al:P 0.08 90
Si:P 1.8 293
Fe:P 0.04 27
Cu:P 0.002 0.03
Si:Al 23 3.3
Fe:Al 0.50 0.30
P:Al 13 0.01
Cu:Al 0.02 0.0003
Mo:Al 0.0002 0.000005
*Data from J. R. Leake.
Greater Si:Al, P:Al, and Fe:Al ratios in plants compared with the crust suggest
that plants extract and retain Si, P, and Fe from earth materials.
Hypotheses on geobiology of weathering 7
 2010 Blackwell Publishing Ltd
these processes exert on soil mineralogy. Furthermore, the
range of ?typical? stoichiometries for biological systems reflects
many different organisms, life strategies, and metabolisms.
The stoichiometric effects of these processes have yet to be
fully explored (Sterner & Elser, 2002).
A corollary to this hypothesis is the idea that abiotic chemi-
cal weathering would generate a substantially different set of
mineral compositions for a given set of conditions, one that
reflects equilibrium thermodynamics and abiotic reaction
kinetics that are not affected by biological stoichiometry. For
example, geologic evidence suggests the rise of land plants
was responsible for the shift from predominantly mechanically
derived sediments to the phyllosilicate-rich pedogenic clay
minerals found in the Neoproterozic (Kennedy et al., 2006).
Despite such arguments, we currently have a limited mecha-
nistic understanding of how mass and energy transfer by
plants and micro-organisms alter mineralogy (see Fig. 1), and
even less about the role of stoichiometric constraints on these
processes.
Intriguingly, because many enzymatic reactions require
redox-sensitive metals such as Fe or Mo, organisms may
extract these metals and leave behind metal-depleted minerals
(Kemner et al., 2005; Liermann et al., 2005; Tang & Valix,
2006). In fact, in arid-land systems, trace elements that are
present in minerals are strongly depleted in biological soil
crusts relative to uncrusted soils (Beraldi-Campesi et al.,
2009). Nutrient and metal limitation studies suggest that the
N2-fixing cyanobacteria that form these crusts have specific
requirements for Mo that are satisfied via microbial alteration
of soil minerals (H. Hartnett, unpubl. data).
The role of biological stoichiometry in weathering and its
potential control on soil mineralogy is amenable to investiga-
tion via field studies and laboratory experiments at a variety of
spatial scales. For example, high-resolution chemical mapping
(Carlson et al., 1999; Fenter et al., 2001; Ketcham & Carl-
son, 2001) could reveal how micro-organisms associate with
minerals containing bio-essential elements and what minerals
remain after micro-organisms have extracted those elements.
At larger scales, soils have a characteristic bio-architecture
(Fierer et al., 2003) wherein organisms exist in discrete zones;
e.g. phototrophs at the soil surface, fungi at locations associ-
ated with plant roots, and heterotrophs at specific redox fronts
or depths that are protected from exposure to UV radiation.
At watershed or regional scales, the range of soil types encom-
passed by observatory networks enables the investigation of
patterns along gradients in soil chemistry and biology.
Hypothesis 3. On landscapes experiencing little erosion,
biology drives weathering during initial succession, whereas
weathering drives biology over the long term
The interplay between weathering and biology is clearly
evident in relatively stable landscapes where erosion is insignif-
icant compared with the rates of weathering advance. In such
landscapes, when soils are young and rich in nutrients, the net
primary productivity is not limited by inorganic nutrient sup-
ply (Fig. 3). As summarized previously in Fig. 1, organisms
increase mineral dissolution rates through a variety of
processes such as providing organic acids (Drever & Stillings,
1997). Organisms also remove weathering products from
solution (Berner & Cochran, 1998), a process that is influ-
enced by the hydrological flux (Velbel, 1993; White et al.,
1996; Bhatt & McDowell, 2007; Brantley & White, 2009).
As nutrients are removed from the system or incorporated
into the biota over time, the net primary productivity can
become limited by the inorganic nutrients derived from
mineral weathering; this evolution is clearly observed in some
chronosequences (Wardle et al., 2004). Where such chrono-
sequences are developed on stable landscapes, we hypothesize
that weathering is at first controlled by biological processes,
but later is controlled by physical processes that determine the
contents and accessibility of nutrients in the CZ. The timing
of this transition is dictated by climate, lithology, and denuda-
tion.
An example of this general phenomenon is the idea that
terrestrial ecosystems shift from N- to P-limitation over the
course of ecosystem development as weathering and denuda-
tion deplete biologically available P pools in the regolith
(Walker & Syers, 1976; Wardle et al., 2004). Once P in rego-
lith is depleted, the only P available is from underlying parent
minerals or incoming dust (Derry & Chadwick, 2007). This
idea of a shift from N- to P-limitation has been around for
decades, yet it has generally been confirmed only for extre-
mely old and chemically weathered soils in tropical settings
(Tanner et al., 1990; Herbert & Fownes, 1995; Vitousek &
Farrington, 1997; Cleveland et al., 2006). Furthermore, the
Degree of soil weathering
B
io
av
ai
la
bi
lit
y 
of
 m
in
er
al
-d
er
iv
ed
 n
ut
rie
nt
s
N
utrient lim
itation by m
ineral-derived nutrients
Fig. 3 A conceptual figure illustrating the hypothesized shift in the relationship
between weathering and the biosphere for stable landscapes as discussed in
Hypothesis 3. Soon after biota become established at a given location, the bio-
availability of mineral-derived nutrients increases due to biotic stimulation of
chemical weathering rates (see Fig. 1). Over time, as the degree of soil weather-
ing progresses and nutrients are lost at a slow rate due to leakage from the sys-
tem, biotic demand will eventually exceed the supply of mineral-derived
nutrients, causing the system to shift into a state of nutrient limitation.
8 S. L. BRANTLEY et al.
 2010 Blackwell Publishing Ltd
availability of elements other than N and P (e.g. Ca, Fe, K,
Mn, Cu, Mo) must also be considered as discussed above in
Hypothesis 2 (Clarkson & Hanson, 1980).
Intriguingly, limitation can also be due to multiple elements
(Bern et al., 2005; Elser et al., 2007; Barron et al., 2009).
The interpretation of multinutrient limitation and how it
might affect ecosystems must be carefully considered with
respect to the spatial and temporal scale of observations
(Wiens, 1989; Hunter et al., 1998). The spatial controls on
soil ecosystems (Ettema & Wardle, 2002) and how stable eco-
system states are controlled by environmental drivers (Beisner
et al., 2003) is of great current interest.
Variations in lithology represent additional complexity. The
elemental composition of bedrock sets the size of the nutrient
pool, whereas the weatherability of the minerals affects how
quickly the pool is made biologically available. Therefore,
different bedrock types may sustain different degrees of
ecosystem development or succession depending on the
abundance and susceptibility of minerals to weathering. Addi-
tionally, soils develop a number of other properties such as
Eh, pH, porosity, clay content, and cation exchange capacity
that all depend at least partly upon their parent material (Oh
& Richter, 2005), which in turn strongly influence elemental
cycling between biota and minerals. Lithology also affects the
spatial variation in biotic activity (Fierer et al., 2003, Philippot
et al., 2009).
One corollary to this hypothesis is the growing consensus
that denudation replenishes inorganic nutrients that other-
wise become depleted (e.g. Porder et al., 2007). The means
by which this occurs is likely a part of complex feedbacks
among tectonics, climate, and lithology (Anderson et al.,
2007). For example, climate may be particularly important
in controlling chemical weathering rates in systems with
high denudation rates (West et al., 2005). Thus, the resi-
dence time of minerals within soils may be sensitive to physi-
cal denudation rates in geomorphically dynamic landscapes
(Almond et al., 2007; Yoo & Mudd, 2008a). Unraveling
long-term changes in biological availability of inorganic
nutrients in physically dynamic landscapes will require
greater understanding of how (if at all) the propagation of
weathering fronts are coupled with ground surface denuda-
tion (Hypothesis 4).
Hypothesis 4. In eroding landscapes, weathering-front
advance at depth is coupled to surface denudation via biotic
processes
In stable landscapes (Hypothesis 3), regolith development
depends on rates of weathering front advance and the progres-
sive depletion of parent material by chemical denudation (Yoo
& Mudd, 2008b). In contrast, regolith development in
eroding landscapes reflects the balance between advance of
weathering at depth and lowering of the surface by chemical
and physical denudation. Unlike chemical denudation, which
encompasses losses due to weathering reactions over all
depths in the regolith profile, erosion removes material prefer-
entially from the near surface, without leaving behind any
secondary minerals or chemically depleted rock fragments
(Fig. 4). Hence, physical denudation tends to keep soils
relatively fresh, compared with those developed in stable set-
tings. Note that if erosional renewal of regolith is fast enough,
it could preclude development of the latter stages of ecosys-
tem succession identified in Hypothesis 3. To obtain a
comprehensive understanding of how biological processes
interact with regolith development, it is important to quantify
relative rates of regolith production and removal in eroding
landscapes. Cosmogenic radionuclide (CRN) measurements
in samples of soil, sediment, and rock permit quantitative
study of landscape denudation (Granger & Riebe, 2007) and
soil production (Heimsath et al., 1997). Over the last two
decades, we have learned much about the CZ from CRN
studies of surface processes in diverse settings (Von Blancken-
burg, 2006). In contrast, we know comparatively little about
how climate and biological stoichiometry (see Hypothesis 2)
influence the advance of weathering into parent material at
depth. Recent studies of weathering of clasts of known age
have provided clues about how weathering initiates and
advances into geologic materials (Sak et al., 2004) but it is not
clear how this scales to understanding weathering in eroding,
soil-mantled watersheds (Navarre-Sitchler & Brantley, 2007).
Important progress has been made in studies of root wedging,
faunally induced bioturbation (Gabet et al., 2003) and the
grain-scale processes discussed in Hypothesis 1. Yet, linkages
between biological processes and weathering in the subsur-
face remain qualitative at best. Theoretical considerations
Fig. 4 Schematic showing regolith with thickness Hregolith (here equal to the
combined thickness of the soil, Hsoil, and saprolite, Hsaprolite) overlying unweath-
ered parent material (labeled bedrock). Upward arrows denote fluxes of solid
Earth materials across horizons; diagonal arrows denote fluxes out of the sys-
tem. Chemical and physical weathering of parent material results in a flux of
?fresh? minerals into the regolith (Pregolith). Chemical denudation in saprolite
(Wsaprolite) prepares it for conversion into soil (Psoil). Physical and chemical denu-
dation of soil (Esoil and Wsoil, respectively) removes it from the landscape. The
sum of the denudational fluxes is the total denudation rate (D). Aeolian fluxes
(not shown) may be an important additional source of mass in the CZ. Adapted
from Riebe et al. (2003, 2004) and Dixon et al. (2009).
Hypotheses on geobiology of weathering 9
 2010 Blackwell Publishing Ltd
(Dietrich et al., 1995) and field data (Heimsath et al., 1997)
imply that variations in production rates of the mobile soil
layer may often be driven by biotic processes. This raises the
question of whether a similar coupling exists between biota at
the surface and the processes that control regolith production
at depth. If this is the case, then it would provide a mechanism
for equilibrating rates of surface lowering and weathering
front advance. By extension, this would provide a means to
keep regolith thickness roughly stable over time. Steady-state
regolith thickness is an a priori assumption in many studies of
weathering (Yoo & Mudd, 2008b). Although recent model-
ing efforts suggest that steady-state regolith can prevail in
some landscapes (Fletcher et al., 2006; Lebedeva et al.,
2007), we lack empirical evidence for it in nature and further-
more do not understand how biota might contribute to (or
disrupt) any tendency toward development of steady state.
Hypothesis 4 addresses this knowledge gap head-on, propos-
ing that weathering-front advance is coupled to surface
erosion via biotic processes. In the limit of very tight coupling,
the regolith production rate should be equal to the total
(physical plus chemical) denudation rate (Fig. 4), and, by
extension, the surface lowering rate should be equal to the
weathering advance rate.
To test whether surface denudation is coupled to weather-
ing-front advance via biotic processes, we need reliable, widely
applicable methods for quantifying rates of weathering front
advance in landscapes. To date, no such method has been
available. However, recent advances in multicollector induc-
tively coupled plasma mass spectrometry promise to yield
quantitative understanding of the initiation of weathering
from measurements of weathering-induced fractionations in
non-traditional stable isotopes (Johnson et al., 2004) and
U-series disequilibrium dating (DePaolo et al., 2006; Maher
et al., 2006; Dosseto et al., 2008). These approaches may
allow quantification of the time elapsed since weathering
began (i.e. the regolith residence time; Dosseto et al., 2008).
For example, if the production rate of regolith (Pregolith)
equals the denudation rate of the landscape (Fig. 4), the rego-
lith thickness (Hregolith) would be at steady state and we could
write expressions (1) and (2):
Hregolith ? Pregolithsregolith ?1?
and
Pregolith ? Psoil?Zrsaprolite=?Zrparent: ?2?
Here Psoil is the production rate of soil (measured from
CRN), sregolith is the residence time for material in the regolith
(for example, estimated from U-series disequilibrium dating),
and [Zr]saprolite and [Zr]parent are the concentrations of an
immobile element such as zirconium (i.e. an element which is
relatively insoluble and thus cannot be chemically denuded
away) in saprolite and parent material, respectively.
If the regolith is in steady state, regolith thicknesses predic-
ted from expressions (1) and (2) should be equal to thicknesses
observed from cores or shallow geophysical images, suggesting
that landscape lowering is quantitatively coupled with weather-
ing front advance. Discrepancies between observed and pre-
dicted values of Hregolith would imply departures from steady
state. Hence, the comparison of predicted and observed thick-
nesses should shed light on linkages between or decoupling of
surface and subsurface processes. To the extent that surface and
subsurface processes are coupled, we should be able to explore
how the coupling works, using results from expressions (1) and
(2) from many points on the landscape.
Such observations could then be combined with measures
of biotic parameters to address questions about the geobiolo-
gy of weathering and erosion. For example, do differences in
vegetation and other biota (e.g. the mycorrhizal networks
discussed in Hypothesis 1) help regulate the advance of the
weathering front? If so, we should see correlations between
regolith depth and the penetration depth of the process in
question. If tree roots and associated mycorrhizal networks
regulate the depth of bedrock ? as part of the interchange of
water and nutrients outlined in Hypothesis 1, for example ?
then depth of rooting may correspond systematically to the
depth of fractured rock, where weathering advance is focused.
On the other hand, if vegetation has changed over the time-
scales of regolith formation due to ecosystem succession
factors discussed in Hypothesis 3, we might observe disagree-
ment between regolith thickness and the penetration depths
of biota, even in systems that have nominally steady thickness
over the long term. In any case, we are poised to make signifi-
cant advances in documenting how biological, chemical, and
physical processes interact across depth, from the surface to
the regolith-rock interface in the CZ.
Hypothesis 5. Biology shapes the topography of the Critical
Zone
In some ways, coming to grips with the importance of life on
the Earth?s air, water, and land is simply a matter of recogniz-
ing the obvious: life in all shapes and forms mantles most of
the planet?s surface. Hypotheses 1 through 4 emphasize the
phenomena that control elemental distributions and thickness
of regolith. As such, investigations driven by these hypotheses
could provide answers to the question, what would this planet
? any planet ? look like without life? One important aspect of
this question is not addressed by the previous hypotheses
however: can life also affect macro-scale topography of Earth?s
surface?
Quantitatively connecting CZ processes to landscape evo-
lution is a broadly studied topic (Amundson et al., 2007;
Anderson et al., 2007) that even has implications for the
search for extraterrestrial life. The ways that biota may, or
may not, shape topography is of critical importance for
extraterrestrial exploration for two very simple reasons. First,
if there is a distinct topographic signature of life, then explo-
10 S. L. BRANTLEY et al .
 2010 Blackwell Publishing Ltd
ration for life beyond Earth could focus on searching for
such topography. Second, topographic metrics can be remo-
tely sensed. Therefore, it remains an exciting possibility that
topographic evidence of life might be discerned with remote
measurements.
Of course, studies of life?s role in shaping both micro-
and macro-topography are extensive (Brady et al., 1999;
Buss et al., 2002; Vanacker et al., 2007; Bonneville et al.,
2009). However, no mathematical model has been proposed
to define such shaping factors (Dietrich & Perron, 2006) at
least partly because the effect of biological productivity is
difficult to deconvolve from factors such as temperature
and precipitation (Fig. 5). Life contributes to most surface
processes both directly, by modulating the chemistry and
transport of sediment and solutes as described in previous
hypotheses, and indirectly, by influencing the climate and
atmospheric boundary conditions that regulate erosion,
weathering, and sediment transport (Gabet et al., 2003;
Berner et al., 2003). It seems safe to say, therefore, that this
planet?s topography would be different if life on Earth had
not evolved.
Interestingly, however, a recent review suggests that there
may not be a topographic signature of life (Dietrich & Per-
ron, 2006). Nonetheless, this hypothesis must be explored
further. For example, well-designed physical experiments
and coupled numerical models that integrate biogeochemi-
cal and hydrological processes such as those at Biosphere 2
in Arizona (Dontsova et al., 2009; Huxman et al., 2009)
are needed. The connections between micro- and macro-
organisms and their impact on Earth?s surfaces must be
studied explicitly when testing this hypothesis: thus, obser-
vations at the scale demanded by Hypotheses 1, 2, 3, and 4
must be related to observations at the larger scale demanded
by Hypothesis 5.
Ironically, the most difficult aspect of deciphering the role
of life on topography is identifying a control experiment
where life does not exist. At the field scale, attempts to under-
stand chemical-weathering processes in the absence of vascu-
lar plants and most organisms larger than micro-organisms are
restricted to artificially designed ?sand boxes? (Gabet et al.,
2006), or observing recent volcanic flows and the like. All
such approaches are characterized by unique initial conditions
and short temporal scales (Berner et al., 2003).
The most glaring gap in our current understanding of what
shapes the Earth?s surface (see Dietrich & Perron, 2006) is
the lack of quantitative geomorphic transport laws necessary
for landscape evolution models. In the absence of such mathe-
matical models, it may be possible to set up physical (e.g.
sandbox or shaking table), or numerical experiments to quan-
tify how topography changes as a function of soil residence
time. Experiments should be designed in concert with control
experiments that are maintained abiotic. Furthermore, it
may be possible to design experiments where biota are allowed
to change and the consequent changes in topography are
measured, providing initial tests of Hypothesis 5.
PROJECTED RESPONSES OF THE CZ TO
FUTURE PERTURBATIONS
Hypothesis 6. The impact of climate forcing on denudation
rates in natural systems can be predicted frommodels
incorporating biogeochemical reaction rates and
geomorphological transport laws
Perhaps because of the intimate coupling among chemical,
physical, and biological processes, we cannot currently project
how the shape and composition of the CZ evolves over time.
If we could parameterize quantitative models for the evolu-
tion of natural Earth surfaces in the face of ongoing climate
change, as well as other human impacts on the Earth?s surface,
we could calculate how the Earth?s surface will change into
the future. Some have referred to such predictions as Earth-
casts (C. Paola, pers. comm.). Hypothesis 6 contends that
such models can be built and parameterized based on simple
laboratory and field observations and that they will be success-
ful in projecting future changes, at least in natural systems that
are not highly impacted by human land use. Eventually, we
should be able to augment these models to also project
anthropogenic impacts under different scenarios of human
activity. The term ?biogeochemical? in this hypothesis is meant
to include ecological phenomena that govern ecosystem
responses, including community composition shifts and
species that adapt to changing conditions.
To parameterize numerical models, extensive work in the
laboratory over the last several decades has quantified the
equilibrium states and the rate behavior of mineral?water?gas
4000
3500
3000
2500
2000
1500
1000
500
30
0
4000
3500
3000
2500
2000
1500
1000
500
4000
5000
5000
3000
2000
1000
0
4000 3000 2000 1000 0
0
15
20
25
5
10
30
0
15 20
25
5 10
MA
T (C
)
MAT (C)
MAP (mm)
MAP (mm)
N
P
P
 (g
 m
?2
)
N
P
P
 (g
 m
?2
)
Fig. 5 A diagram illustrating the relationship between net primary production
(NPP, grams of plant matter per square meter of land area), vs. mean annual
precipitation (MAP, mm year)1) and mean annual temperature (MAT, C).
Individual data points are shown as black cubes. A multivariable regression
model used to fit the data is illustrated as a smooth surface. Data from Lieth
(1973), figure made by R. Amundson.
Hypotheses on geobiology of weathering 11
 2010 Blackwell Publishing Ltd
reactions and, to a lesser extent, microbiota?mineral?water
reactions (White & Brantley, 1995; Palandri & Kharaka,
2004; Brantley et al., 2008; Oelkers & Schott, 2009).
Significant effort is also underway to identify appropriate
geomorphic transport laws (Dietrich et al., 2003). These
mathematical laws express the mass flux caused by one or
more processes and generally must be validated by field mea-
surements. In addition to the availability of such data, many
researchers are formulating numerical models to simulate CZ
processes (Kang et al., 2006; Steefel, 2008; Godde?ris et al.,
2009).
Despite these developments, model testing against natural
systems is still in its infancy and many problems remain to be
resolved so that quantitative, accurate predictions can be made
(White, 2008; Dixon et al., 2009). For example, upon com-
parison of laboratory and field rates of mineral reactions, many
researchers have discovered that laboratory rates are much
faster than natural rates (Fig. 6). This observation has driven
efforts to reconcile model simulations with field observations
(White & Brantley, 2003; Maher et al., 2009; Zhu, 2009). In
addition, geomorphological transport laws are not currently
available for processes such as landslide formation, debris
flows, surface wash, and glacial scour (Dietrich et al., 2003).
Testing this hypothesis will require the development of new
modeling approaches to allow projection from laboratory
measurements to field observations for soils, sediments, and
landscapes. Earthcasting approaches must allow delineation of
climate variables as a function of time both in the past and
forward into the future as drivers of denudation. Full coupling
of climate and weathering models is currently not possible,
and instead, climate model outputs must be used to drive the
weathering or erosion models separately (Williams et al.,
2010). A rigorous test of Hypothesis 6 will require develop-
ment of new coupled models of the climate-CZ system.
Another frontier in modeling is the incorporation of biotic
effects such as those discussed in Hypotheses 1 through 4
(Godde?ris et al., 2009).
Hypothesis 7. Rising global temperatures will increase
carbon losses from the Critical Zone
The Earth is warming. Average air temperatures have
increased 0.13 C per decade globally (IPCC, 2007), and
fivefold greater increases have been observed in the Arctic (up
to 0.6 C per decade north of 62N; Polyakov et al., 2002).
Projections based on six emission scenarios suggest warming
could reach 1.8?4.0 C per decade by 2100 (IPCC, 2007).
Positive feedbacks operating on CZ carbon pools ? both
plants and soils ? have the potential to accelerate these tem-
perature increases.
Importantly, because most CZ processes respond to tem-
perature, warming can have complex effects on the CZ carbon
balance (e.g. Norby & Luo, 2004). Warming has the potential
to increase decomposition of soil organic matter and the resul-
tant export of dissolved organic carbon (DOC; e.g. Rustad
et al., 2001; Knorr et al., 2005; Davidson & Janssens, 2006),
but the extent to which this potential is realized is regulated
downward by changes in composition, activity, and density of
soil microbial communities and roots (e.g. Luo et al., 2001;
Carney et al., 2007; Strand et al., 2008). While increases in
DOC fluxes from some temperate watersheds experiencing
warming have recently been identified, these increases have
been attributed to decreases in sulfate deposition (Evans et al.,
2006; Monteith et al., 2007). On the other hand, increased
DOC export has been observed, possibly in response to warm-
ing, in other regions that have historically received little atmo-
spheric S deposition (Freeman et al., 2004). Finally, for the
few studies where changes in soil organic carbon have been
documented over time, the effects of land use change must be
considered as an alternative to climate change (Bellamy et al.,
2005).
The net, temperature-driven change of the CZ carbon pool
will represent the summation of all the net changes in individ-
ual soil and plant carbon pools. Model and observational stud-
Fig. 6 A plot showing how the rate of dissolution of plagioclase feldspar varies
as a function of the duration of observation (plotted in years on the x-axis).
Measurements of this rate in the laboratory are orders of magnitude faster than
the rates estimated in natural systems. Filled symbols represent rates measured
in the field that have been normalized by the surface area of the feldspar mea-
sured by BET analysis. Half-filled data denote field rates corrected for geometric
surface area. Open symbols represent rates measured in the laboratory for
freshly ground unaltered samples; open symbols with crosses delineate labora-
tory rates measured for samples that were previously weathered in the field.
The dashed line is a regression fit to the data. Solid lines indicate the measured
laboratory rate of dissolution of pristine or previously weathered Panola granite
as a function of time. Figure revised from the original publication: references for
original data as indicated by numbers are found in the original reference (White
& Brantley, 2003). Many explanations have been advanced for why rates vary
with duration of observation, including the effects of changing soil structure
with time, changing mineral surface chemistry with time, changing chemical
affinity with time, the effects of biota, inadequacies in measurement of surface
area, hydrologic factors, clay precipitation effects, etc.
12 S. L. BRANTLEY et al .
 2010 Blackwell Publishing Ltd
ies of plant responses to elevated temperature tend to predict
there will be an increase in net primary production (Rustad
et al., 2001), but the range of responses is large and in some
systems plant growth may decline with increasing tempera-
ture. In addition, the effects of temperature on plant growth
or NPP (see Fig. 5) result from interactions controlled by
many other factors such as the partial pressure of CO2 and
annual precipitation (e.g. Norby & Luo, 2004; Bradley &
Pregitzer, 2007).
Global feedbacks will depend on the sign and magnitude of
CZ responses to warming, most of which are difficult or
impossible to project for specific regions (Fig. 7). For exam-
ple, decreased DOC concentrations from the Yukon have
been attributed to destabilization as warming increases soil
CO2 fluxes (Striegl et al., 2005). Conversely, it has been sug-
gested that Siberian DOC exports will increase nonlinearly
with warming (Frey & Smith, 2005). The effects of increased
C export ? solid, liquid, or gas ? will depend not only on the
net effect of fluxes related to the organic carbon pools but also
relative to changes in C sinks such as silicate weathering
(Hypothesis 8) and upon changes in discharge (Fig. 7). Fur-
thermore, changes in soil erosion (Harden et al., 1999; Smith
et al., 2001) will also affect organic carbon pools (Van Oost
et al., 2007). Importantly, small changes in soil carbon con-
tent can exert a disproportionate effect on the global C cycle
because soils contain more than double the C in the atmo-
sphere and vegetation combined (Bajtes, 1996; Houghton,
2007).
From affordable, networked, and miniature datalogging
sensor grids to coupled, remotely sensed data products, large
No increase in Q
C
ar
bo
n 
ex
po
rts
Time
CO2
DOC
Increased Q
C
ar
bo
n 
ex
po
rts
Time
CO2
CO2
CO2DOC
Permafrost
Thawing Thawing
Steppe/Grasslands
C
ar
bo
n 
ex
po
rts
Time
DOC
C
ar
bo
n 
ex
po
rts
Time
DOC
No increase in Q Increased Q
Fig. 7 Predictions of carbon export (CO2 and DOC) under a warming climate, with and without increases in discharge. The two examples chosen are from northern
latitudes which are expected to see the largest temperature increases. The difference in magnitude of the carbon exports is meant to reflect the difference in carbon
storage between the regions. There are a variety of factors affecting carbon exports including those that affect net primary productivity and therefore total carbon
stocks (e.g. climate, fertility, species composition) and those that affect the decomposition and export of those carbon stocks (e.g. temperature, water, substrate
quality, oxygen). Base map is the UNEP ? GRID-Arendal Maps and Graphics Library (2009).
Hypotheses on geobiology of weathering 13
 2010 Blackwell Publishing Ltd
data sets are now being collected that will allow us to test how
ecosystem C fluxes respond to temperature and moisture
changes (e.g. Baldocchi et al., 2004). Various modeling
approaches are increasingly being used to model data-rich
systems at single scales. In the future, powerful models will be
parameterized by real-time data and compared with results
from warming experiments across multiple scales. Experi-
ments that manipulate temperature, moisture, nutrient, and
CO2 concentrations are starting to provide insights into the
sign and magnitude of potential ecosystem responses to
warming. From this information we will eventually predict
how carbon fluxes will respond (e.g. Fig. 7; UNEP ?GRID-
Arendal Maps and Graphics Library, 2009). Necessarily, these
models will be parameterized using observations over short
timescales but will be used to predict longer term effects.
Three interrelated research areas of great importance
should be targeted: (i) analysis and modeling of CZ carbon
fluxes in solid, liquid, and gases across a matrix of experimen-
tally manipulated sites or in locations along natural gradients;
(ii) ongoing observations on Arctic ecosystems with special
focus on the greenhouse gas CH4; and (iii) multifactor experi-
ments designed to identify tipping points (see Hypothesis 12)
in ecosystem responses to warming. These latter experiments
could allow, for example, investigation of the inter-relation-
ships of temperature, moisture, ozone, and elevated CO2, or
the inter-relationship of N deposition fluxes and elevated
CO2.
Hypothesis 8. Rising atmospheric PCO2 will increase the rates
and extents of mineral weathering in soils.
The accelerating rate of increase in atmospheric CO2 concen-
trations (Canadell et al., 2007) is expected to impact the CZ
in many ways, as discussed in the previous hypothesis. For
example, elevated CO2 causes changes in plant physiology
that can cause ?top-down? perturbations of the CZ. In this
regard, plants are transducers that sense changes in the atmo-
sphere and actuate changes in the CZ (Bradley & Pregitzer,
2007). Changes in CO2 are causing alterations in the type,
distribution, respiration rates, and growth rates of plants
(Ainsworth & Long, 2005; Norby et al., 2005, McMahon
et al., 2010), but these effects may be species-specific (Frans-
son et al., 2007), short term (Oren et al., 2001), or subject to
change as elevated CO2 interacts with other global change fac-
tors (Langley and Megonigal, 2010). The sum effect of all
these factors on primary mineral weathering is currently
unknown. However, given the discussion in Hypothesis 1 that
emphasizes that plants as a source of the inorganic and organic
compounds that enhance chemical weathering, we hypothe-
size here that elevated CO2 will cause significant increases in
the rate and extent of primary mineral weathering over much
of Earth?s surface (Fig. 8). Importantly, this increase in weath-
ering could furthermore feedback to alter the primary produc-
tivity (Fig. 5) and could potentially impact aquatic resources
downstream. Altered rates of mineral weathering could even
affect the flux of solutes to drinking water (Natali et al.,
2009).
Soil CO2 concentrations are naturally elevated well above
atmospheric concentrations due to biological activity. Ele-
vated atmospheric CO2 accelerates biological activity directly
by stimulating primary productivity (Ainsworth & Long,
2005; Norby et al., 2005), indirectly increasing microbial
respiration and decomposition (Pendall et al., 2004; Carney
et al., 2007). Increased plant and microbial respiration
dictates an increase in overall soil respiration (King et al.,
2004; Comstedt et al., 2006), resulting in higher soil CO2
and dissolved inorganic carbon concentrations (Andrews &
Schlesinger, 2001; Karberg et al., 2005). These latter changes
can then supply more protons for dissolution reactions
(Godde?ris et al., 2006). Elevated atmospheric CO2 may also
accelerate weathering through an increase in plant production
of root exudates that contain organic acids and organic poly-
phenolic compounds (see Hypothesis 1). In addition,
increases in ecosystem productivity could increase the size of
the sink for products of mineral dissolution. The magnitude
of these responses and the responses of different ecosystems
are largely unknown (Fig. 8).
To investigate these many factors, a few free-air CO2 enrich-
ment (FACE) studies have examined the effects of elevated
CO2 on mineral weathering. Andrews & Schlesinger (2001)
observed higher soil PCO2, a 271% increase in soil solution cat-
ion concentration, a 162% increase in alkalinity, and a 25%
increase in Si over only 2 years of elevated CO2 in soil plots
located in a loblolly pine plantation in North Carolina (USA).
However, a longer-term (5-year) data set from the same
experiment revealed a more complex pattern, including years
that showed significant effects of the treatments on soil CO2
concentrations, but no differences in chemical weathering
rates (Oh et al., 2007). Elevated atmospheric CO2 has also
Elevated
concentration 
in atmosphere
Increased 
primary 
productivity
Increased 
autotrophic 
respiration
Increased root 
exudation
Increased 
heterotrophic 
respiration
Increased soil 
CO2 
CO2 
concentration
Increased 
mineral 
weathering
Increased 
lithogenic 
nutrient 
uptake
Fig. 8 Conceptual diagram for Hypothesis 8 that reflects a hypothesized rela-
tionship between elevated CO2 in the atmosphere and rate and extent of min-
eral weathering. Length of the arrows and size of the pools do not indicate
magnitude of the response or the reservoirs, as these are largely unknown,
especially in different ecosystem types.
14 S. L. BRANTLEY et al .
 2010 Blackwell Publishing Ltd
been observed to increase soil gas CO2 and dissolved inor-
ganic carbon concentrations during the growing season by
14% and 22%, respectively, whereas total alkalinity increased
by 210%, likely due to enhanced chemical weathering (Kar-
berg et al., 2005). In addition, a significant decrease in soil
pH and higher surface soil trace metal concentrations were
observed with CO2 enrichment (Natali et al., 2009). These
limited studies highlight the variability of responses to ele-
vated CO2 and the need to study the effects across all biomes.
Although some of the published studies to date provide
strong indications that elevated CO2 can increase weathering
rates (Karberg et al., 2005), there is no conclusive evidence in
the form of increased fluxes of rock-forming elements or
changes in mineral mass balance to prove this hypothesis. For
example, depending on the initial soil mineralogy, an increase
in PCO2 could cause the base cation content of the soil to
either decline due to accelerated export (chemical denuda-
tion) or remain relatively unchanged due to increased rates of
mineral weathering accompanied by secondary mineral pre-
cipitation.
At this time we cannot project how weathering will respond
to changing CO2. Small-scale mesocosm studies under con-
trolled conditions and field-based elevated CO2 experiments
using standardized CO2 enrichment methodologies and state
of the art characterization techniques are needed. At the field
scale, investigators should fully sample existing elevated CO2
studies (e.g. FACE sites) and initiate new elevated CO2 field
studies that encompass the full range of ecosystem and CZ
types. In addition, to predict decadal and centennial changes
across landscapes will require implementing field experiments
using observatories sited along environmental gradients and
using models to predict chemical denudation fluxes as a func-
tion of changing CO2 (e.g. Banwart et al., 2009; Godde?ris
et al., 2009). Finally, Fig. 8 only emphasizes the short term
impacts: we also need models that are written to incorporate
how vegetation modulates or responds to the important long-
term feedbacks between weathering and climate (e.g. Berner,
2006).
Hypothesis 9. Riverine solute fluxes will respond to changes
in climate primarily due to changes in water fluxes and
secondarily through changes in biologically mediated
chemical weathering
Riverine export of terrestrial elements mobilized by chemical
weathering, i.e. chemical denudation, is an important flux in
global elemental cycles. Models have shown that weathering
processes exert a negative feedback to atmospheric CO2 levels
and global climate over very long timescales (Ridgwell &
Zeebe, 2005; Berner, 2006). It is less clear, however, how
riverine export will respond to climate change over the com-
ing decades. Understanding this is critical to our ability to
predict future changes in river quality, ecosystem sustainabil-
ity, mobility of pollutants, and response of coastal systems to
climate change.
The solute export of a watershed (Fi) is calculated from the
product of the river discharge (Q in m3 s)1) and the concen-
tration of solute i (Ci in mol L
)1). Recent studies have
demonstrated that the export of weathering products scales
with discharge for watersheds (Fig. 9) representing a range in
climate and lithology (Raymond & Cole, 2003). Figure 9
suggests that biota exert little to no control on the decadal
timescale of riverine export of weathering products. There-
fore, we hypothesize that factors that alter discharge rates,
namely precipitation and evapotranspiration, will be the major
source of change to riverine chemical export in the coming
decades.
Of course, this hypothesis may in fact contradict several of
the preceding hypotheses that were largely addressing obser-
vations made at the scale of a pedon or smaller (e.g. Hypothe-
sis 8). Perhaps, observations made at different scales lead to
different conclusions. Clearly such contradictions must be
investigated. Why might biota-facilitated chemical weathering
play only a minor role in changing riverine fluxes? This could
be because, although chemical weathering in topsoil is biolog-
ically mediated, its kinetics is controlled by the interplay of
several factors (White & Blum, 1995; Brantley & White,
2009). For example, concentrations of dissolved weathering
products, Ci, are often diluted with increases in discharge (Q),
as hydrology alters weathering regimes by changing the resi-
dence time and the relative contribution from deep ground-
water vs. shallow porewaters (Kirchner, 2003; Raymond
et al., 2008). Watersheds rarely show an increase in the con-
centration of cations during periods when a large volume of
water moves through the watershed because the weathering
signals are diluted by the water inputs. Nonetheless, the dilu-
tion of Ci is small compared with the increase in Q during wet
years, and the overall flux therefore increases with Q. One
inference from such observations is that Ci in many water-
sheds is always at or near the maximum in concentration as
dictated by mineral solubility. Such solubility-controlled
Fig. 9 Annual export of water and bicarbonate from the Mississippi River.
Data are from Raymond et al. (2008) for 1900?1950. The main source of the
bicarbonate is soil-weathering processes.
Hypotheses on geobiology of weathering 15
 2010 Blackwell Publishing Ltd
weathering has been commonly observed in watersheds domi-
nated by carbonate minerals (Williams et al., 2007) or those
with low precipitation (Raymond et al., 2008).
It is possible that biological processes maximize weather-
ing intensities so that the relative changes in concentrations
of the dissolved loads in response to climate-induced changes
are insignificant, especially on the century to decade time-
scales. Furthermore, despite the arguments in the preceding
hypothesis, although elevated PCO2 levels are commonly
observed in soil atmospheres, a small rise of atmospheric
CO2 may not lead to significant increases in weathering if
such changes do not affect the solubility limitations of weath-
ering. It may also be important to point out that biologically
facilitated weathering is highest during the growing season,
when discharge is typically low due to high rates of evapo-
transpiration.
Of course, variation in biologically mediated chemical
weathering may become important in some instances such
that the impact of climate change on Ci is more significant
than impact on Q. The thawing and decomposition of peats in
northern latitudes will expose soils with a very high weather-
ing potential to solutions with DOC (Striegl et al., 2007) and
may therefore show enhanced chemical denudation (see
Fig. 7). Similarly, periods of low sea level during glacial peri-
ods expose carbonate-rich continental shelf that are then sus-
ceptible to weathering (Foster & Vance, 2006). Also, farming
? another example of a biotic process? can alter bicarbonate
production and export from watersheds through processes
such as liming (Hamilton et al., 2007). Recent studies have
demonstrated that suburban growth and urbanization can
also increase chemical denudation rates due to organic matter
loading (Barnes & Raymond, 2009).
The impacts of climate change on watershed-scale chemical
weathering rates and denudation fluxes will undoubtedly be
governed by a complex interplay between changing water
delivery and alterations in stream chemistry (Ci). In most areas
of the globe, however, alterations in the water budget will pre-
cede large-scale land-cover changes. It is our contention that,
in the short term (decades to a century), the alteration of
water throughput will trump changes in biologically mediated
chemical weathering. This argument is buttressed by the fact
that examination of contemporary watershed data has not
revealed any significant biologically driven variation on river
fluxes.
Data required to test this hypothesis are available due to
advances in remote sensing techniques, data compilation, and
real-time measurements (Raymond & Cole, 2003). For
example, the US Geological Survey has discharge and riverine
water chemistry data for most watersheds in the USA, with
different climate, vegetation, and land use history, and these
records often cover many decades. These databases allow
identification of changes due to anthropogenic forcing. With
such data, process-based models can be parameterized to fore-
cast the response of riverine element export to climate change
in the coming decade (e.g. Godde?ris et al., 2006). Such reac-
tive-transport models should be linked to terrestrial ecological
models to take into consideration the impacts of biological
activity on water budget, C cycles, soil erodibility, and mineral
dissolution.
Hypothesis 10. Land use change will impact Critical Zone
processes and exports more than climate change
The fluxes of matter and energy from the CZ are sensitive to
the effects of human activities (Vitousek et al., 1997; Yang
et al., 2003; Green et al., 2004; Montgomery, 2007; Alexan-
der et al., 2008), perhaps most evidently in the direct effects
of road building, timber harvesting, and agriculture on
erosion of the Earth?s surface (Cronin et al., 2003; Wilkinson,
2005). The evidence, which includes degraded soils, altered
patterns of ecological succession, and accelerated sedimenta-
tion in streams, rivers and floodplains, hails from innumerable
studies of agricultural erosion, fertility loss in soils, logging,
deforestation, and urban development that span a wide range
of geographic settings.
Human activities have modified landscapes and land-cover
types over such a massive scale (Cronin et al., 2003; Barnes
& Raymond, 2009) that these effects have been more
important than climate effects in determining changes in CZ
fluxes to date. For example, Fig. 10 shows that sediment
accumulation rates increased in Chesapeake Bay (USA) coin-
cident with land use changes associated with European set-
tlement and farming in the watershed. Because erosion
originating from agricultural lands and construction sites is
often high compared to forests (Swaney et al., 1996) ? the
dominant vegetation type in the absence of significant dis-
turbance ? it is unlikely that climate change alone could
have caused the significant increase in erosional fluxes into
the Bay.
Although climate change may not have been the biggest
driver to date in determining changing erosion patterns, this
hypothesis probes whether ongoing changes in temperature
and precipitation (Canadell et al., 2007; IPCC, 2007) will
be more or less important to CZ fluxes than changes in land
use and cover. Human-induced land-use change has been
drastic in both developed and developing regions, and is
likely to stay high for decades to come if the standard of
living rises in the latter localities. At the same time, the
effects of climate change on the CZ will be increasing with
time globally and will impact landscapes regardless of human
population density for long durations of time (Yang et al.,
2003). Moreover, climate change will also modulate ecosys-
tem composition and CZ function in ways that will likely
increase fluxes associated with chemical and physical weath-
ering (Banwart et al., 2009). Rising soil temperatures and
changing precipitation regimes will also shift rates and spa-
tial distributions of biogeochemical processes (see Hypothe-
ses 7 through 9). Understanding the relative importance of
forcing due to changes in land use and climate is needed for
16 S. L. BRANTLEY et al .
 2010 Blackwell Publishing Ltd
reliable prediction of changes in ecosystem services. As
discussed in Hypothesis 12, it will be important to deter-
mine whether changes in climate and land use might push
landscapes across tipping points into undesirable states of
ecosystem function.
Addressing this hypothesis will require an array of
approaches from observational field studies conducted at
multiple scales to the analysis of historical databases of
human effects on the CZ (Walter & Merritts, 2008). By
examining how land use has affected the susceptibility of the
CZ to denudation in the past, we should be able to place
constraints on how it will respond as human influence
changes in the future. For example, analyses of sediment
loadings in rivers can be related to climate fluctuations (Rossi
et al., 2009), as well as to areal changes in highly erodible
cultivated lands following establishment of programmatic
change such as the USDA Conservation Reserve Program in
1962 (Amelung et al., 2001). Geographic information sys-
tem and remote sensing technologies are now available to
facilitate regional-scale assessments (Cronin et al., 2003).
Moreover, there is much to learn from paleosols, which doc-
ument effects of significant climate changes in the geologic
past (Retallack, 1990).
MANAGING THE CZ
Hypothesis 11. In many severely altered settings, restoration
of hydrological processes is possible in decades or less,
whereas restoration of biodiversity and biogeochemical
processes requires longer timescales
Although advances have been made in restoring and main-
taining agriculturally impacted topsoils and in restoring CZs
associated with stream channels and riparian zones, less is
known about how systems-level interactions influence the
response of CZs to perturbations and restoration efforts.
Hypothesis 11 proposes that restoring CZ function is possi-
ble for degraded systems, but the timescales of restoration of
the hydrologic system is faster than that of the biogeochemi-
cal system (Fig. 11). A corollary to this hypothesis is the
inference that restoring hydrologic pathways is a prerequisite
to re-establishing biodiversity and biogeochemical processes.
It is common to note that ecosystem services are altered in
soils and streams as hydrological processes are changed (Daily
et al., 2000; Rogers, 2006). However, we need metrics and
techniques for identifying and quantifying such alterations in
CZ systems so that we can learn how to reclaim, rehabilitate
and restore function (Fig. 11c; Wallace, 2007; Heneghan
et al., 2008). Given the complexity of inter-relationships
among chemical, physical, and biological factors endemic to
the CZ (Fig. 11a), process-based models for soil erosion, soil
carbon evolution, water quality evolution, and the evolution
of ecosystem function must be developed and tested against
field data (see, for example Hypothesis 6). Such tests will
extend our ability to Earthcast at the spatial and temporal
scales required by natural resource managers (e.g. hectares
and years).
We suggest that addressing this hypothesis requires a
three-stage approach. First, regionally meaningful metrics of
both healthy and altered CZ systems must be developed
and evaluated to ensure they are statistically robust, even in
a changing environment. The choice of metrics will depend
on needs of natural resource managers (namely, what they
can feasibly measure), and also on the relevant CZ processes
and ecosystem services in a range of disturbed landscapes.
The second phase requires developing, implementing, and
monitoring restoration projects to restore altered CZs. Such
strategies should include: (i) taking account of past, present,
and future land uses; (ii) conducting hydrological studies
with attention to groundwater interactions; (iii) analysis of
physical and biological properties of systems; (iv) study of
how soil and water chemistry (both at the surface and at
depth) influence water and soil quality (Mitsch & Jorgen-
sen, 2004). These techniques will most likely include
Fig. 10 Long-term sediment fluxes estimated between 1000 C.E. and 1880
C.E. and post-1880 based on sediment cores in the Chesapeake Bay. Of the 16
core sites, 10 show marked increases in sediment fluxes post-land clearance
when compared with the long-term averages (Cronin et al., 2003). Figure
reproduced with permission.
Hypotheses on geobiology of weathering 17
 2010 Blackwell Publishing Ltd
biotechnology, geo-engineering and landscaping, plantings,
and species introductions. Both hydrologic fluxes and bio-
geochemical processes must be monitored along with the
metrics used to define healthy and altered CZ?s. Finally,
both the societal and economic implications of these tech-
niques must be evaluated with the help of resource manag-
ers. Such a long-term approach will allow both testing of
this hypothesis and the development of efficient restoration
strategies.
Hypothesis 12. Biogeochemical properties impart thresholds
or tipping points beyond which rapid and irreversible loss of
ecosystem health, function, and services can occur
A dynamical system may experience a threshold or tipping
point when its state is such that even a small perturbation
can cause an irreversible change in the system?s functioning
and properties. Ecosystems, deep ocean circulation, and
earth?s climate have all been previously suggested to exhibit
threshold behavior (Lenton et al., 2008). One common
characteristic of systems with thresholds is that variables can
be difficult or impossible to measure at the precision needed
to predict changes around the threshold.
Here, we hypothesize that the CZ, as a dynamical system,
most likely exhibits tipping points. These thresholds are pre-
sumed to be irreversible on timescales relevant to human life
(decades to centuries), because, unlike in Hypothesis 11, once
a tipping point is passed, restoration may not be possible to
the previous state within those timescales. Although thresh-
olds have been presumed to exist for the CZ, few have been
precisely defined. As an example, Fig. 12 shows a CZ tipping
point in which nutrient buffering collapses as fertilizer applica-
tion increases beyond a threshold (Rabalais, 2002). Other CZ
tipping points include thresholds for erosion by rill formation
and landsliding (e.g. Favis-Mortlock, 1998). Tipping points
for accelerated desertification, acidification, decline in carbon
storage, collapse of nutrient buffering, or growth of harmful
algal blooms could also be important (Millenium Ecosystem
Assessment, 2005).
Given the increasing pressures on natural systems, it is criti-
cal to identify tipping points as quickly as possible. Further-
more, because the tipping point concept is relatively intuitive,
it is a very useful concept for both policy makers and scientists.
The term ?tipping point? is now being used by both groups to
describe global challenges such as climate change (Post et al.,
2009; Russill & Nyssa, 2009), biodiversity losses (Pimm,
2009), and crop yields (Barley, 2009). However, policy can
only be formulated appropriately if we identify where and
when tipping points occur. Currently, thresholds are only rec-
ognizable after passing the threshold. As a result, one current
focus of research is to identify early warning signals of change
and threshold behavior in ecosystems as well as effective
responses to such threats (Jordan et al., 2006).
Many of the coupled biological, geological and hydrolog-
ical processes are well enough understood to open the way
to use models to explore tipping points in regolith or soil
formation. To understand the biogeochemical resilience of
soils with respect to acidification, nutrient depletion,
drought, and erosion, however, we need to quantify the
rates and mechanisms of chemical weathering, hydrology,
erosion, and biotic processes in regulating biogeochemical
resilience. With such knowledge, we can use models to
explore tipping points as a function of variables such as
climate, lithology, and vegetation as we attempt to propose
regulatory frameworks. It is furthermore crucial to examine
B
Progress towards specific target condition
U
til
ity
 o
f S
E
K
P
B C
Degraded state
Desired 
state
P
CB
P
C
A
Fu
nc
tio
na
lit
y
Fu
nc
tio
n
Degraded Objective
Degraded
Objective
Physical
environment
Vegetation 
manipulation
Complex 
management
Structure
Recl
ama
tion
Reh
abili
tatio
n
Rest
orati
on
C
B
Fig. 11 Framework for integrating soil ecological knowledge (SEK) with theo-
retical models of ecosystem restoration. (a) When sites are heavily degraded,
some improvement in soil function may be achieved by manipulations of either
the chemical (C), physical (P), or biological (B) attributes of the soil. For greater
progress toward a target condition, however, an increase in the degree of com-
plex SEK is required, and there are more complex interactions among P, C, and
B attributes. To achieve the desired state of restoration requires very high level
understanding of P, C, and B processes. (b) Physical, biological, and manage-
ment thresholds that must be overcome if restoration is to be successful.
Hypothesis 11 is consistent with the likelihood of faster restoration of function-
ality with respect to physical processes such as hydrologic flow and slower resto-
ration of biogeochemical processes. (c) The integral relationship between
structural and functional attributes in ecosystem restoration (figure reproduced
with permission from Heneghan et al., 2008).
18 S. L. BRANTLEY et al .
 2010 Blackwell Publishing Ltd
how the biogeochemical resilience and tipping points of soil
environments relate to ecosystem services within water-
sheds.
Like the other hypotheses, methods by which research
priorities for Hypothesis 12 can be addressed include experi-
mental watershed studies as well as smaller-scale mesocosm
experiments. However, to understand tipping points also
requires modeling efforts to build conceptual understanding
of biogeochemical resilience, allowing for scaling from the
mesocosm to the watershed to the region to the globe. Exam-
ples of properties that are expected to be important in impart-
ing thresholds to the CZ, and which should therefore be
targeted in these studies, are listed in Table 2.
Finally, the geologic record may be a source of additional
understanding if we can identify tipping points that have been
crossed in the past. For example, ancient soils ? paleosols ?
could allow us to identify tipping points that may have been
involved before, during, or after the Paleocene?Eocene ther-
mal maximum (PETM). Considered an analog for future
global warming, the PETM was characterized by releases of
greenhouse gases that drove rapid warming of 5?9 C that
resulted in alteration of terrestrial ecosystems (Wing et al.,
2005; Zachos, 2005). Changes in chemical weathering related
to such greenhouse episodes may reveal evidence for or
against tipping points in the surface Earth system (Thiry,
2000; White et al., 2001).
CONCLUSION
We have advanced 12 hypotheses that can be tested concern-
ing the geobiology of weathering through the use of tools and
approaches available today. Interestingly, several of the
hypotheses are contradictory: nonetheless, each hypothesis is
thought to be defensible by at least one segment of the CZ
scientist community. This suggests that even at a very basic
level, we do not understand CZ function.
To build understanding, new observatory initiatives are
underway to investigate the CZ in a more holistic fashion.
For example, in the USA, six CZ Observatories were funded
since 2006. Likewise, in Europe, a group of CZ scientists
has been funded as the SoilTrEC International CZO Net-
work with partners in the European Union, China, and
USA. This program has explicit links to the CZO program
in the USA. Other researchers are similarly designing or
Table 2 Variables and properties that contribute to biogeochemical tipping
points in the Critical Zone
Population density, land use, water allocation, and economic development
within a watershed
Minimum and maximum rainfall, air temperature, water table depth, and
baseflow
% Vegetation cover, rooting depth, and % impervious cover
Regolith thickness, soil age, and maturity
Acid buffering capacity, clay content, cation exchange capacity, and organic
matter content
Nutrient filter capacity, internal nutrient recycling, and nutrient ratios
Functional and genetic diversity of microflora and microfauna
Textural threshold(s) and soil aggregates
Fig. 12 Hypothetical response (lower figure) in buffering of nutrients by CZ
processes as anthropogenic inputs of nitrogen (N) and phosphorus (P) via fertil-
izer application are increased (upper figure). At relatively low fertilizer loadings,
processes such as denitrification that eliminate or processes such as sorption or
plant uptake that retain N and P can limit the export fluxes from the soil
environment. Beyond a threshold in fertilizer application, however, the natural
buffering capacity of the soil is exceeded and there is a rapid transfer of bioavail-
able N and P to the stream network. If the export of bioavailable silicon (Si),
primarily produced by chemical weathering of silicate minerals, remains con-
stant, then the increase in N and P export fluxes is also accompanied by a drop
in the Si:N and Si:P ratios. Both the absolute and relative changes in nutrient
fluxes as the tipping point is crossed may cause significant downstream damage
to in-stream ecosystems, freshwater reservoirs, and the coastal zone (Rabalais,
2002). Upper figure reproduced with permission from the Millenium Ecosystem
Assessment (2005).
Hypotheses on geobiology of weathering 19
 2010 Blackwell Publishing Ltd
extending observatory networks in other countries around
the world.
However, observatories alone cannot test the hypotheses in
this paper. Quantitative models are being developed to under-
stand observatory data and these models are needed to test
the hypotheses (e.g. Parton et al., 1987; Tucker & Slinger-
land, 1994; Iverson & Prasad, 1998; Godde?ris et al., 2006;
Yoo et al., 2007; Steefel, 2008; Banwart et al., 2009).
Discrepancies between models and observations will drive
improvements in our understanding of many of the important
phenomena. Only with such efforts will models yield accurate
?Earthcasts? that project how the three forcing factors ?
tectonism, climate, anthropogenic activity ? individually and
together cause the CZ to change.
Of these three forcings, anthropogenic activity could be the
most important driver of CZ change into the immediate
future. In other words, the largest impact of biology on
weathering today may be that of humans. In turn, this impact
impairs the ecosystem services and soil health that human
society relies upon. Unlike the other two forcings, however,
the direction of anthropogenic activity can sometimes be
reversed by learning and behavioral change. In this regard, the
CZ is a powerful integrating concept for both scientists and
the public alike because it fosters comprehension of the inter-
connectedness of water, land, air, and biota. It is perhaps reas-
suring therefore that in November 2009, a Google Scholar
search generated 303 000 unduplicated hits of web-pages
dedicated to educating the public about foundational con-
cepts in CZ science. However, the increasing use of web-deliv-
ered education for the public (Hill & Hannafin, 2001) is
actually a symptom of another trend that is cause for concern.
In industrialized societies, children increasingly spend less
time outside but more time online (Sallis et al., 2000; Nor-
man et al., 2006). Without experiential learning, the dialog
between natural scientists and stakeholders is limited and
people increasingly experience a disconnect from the natural
environment (Fischer, 2004; Rogers, 2006). Thus, just as
humans have attained the status of a geological force (Vito-
usek et al., 1997; Wilkinson, 2005), they ironically are
disconnecting from their natural environs. Given all of these
trends, the observatories and models needed by scientists to
understand the CZ must also be designed to engage the
public in the puzzle of how to live sustainably during the
Anthropocene.
ACKNOWLEDGEMENTS
Funding from NSF EAR-0946877 is acknowledged. We also
acknowledge the following people for contributions to the
workshop, ?Frontiers in Exploration of the Critical Zone II:
The Geobiology of Weathering and Erosion?, held 5?7
October 2009, at the Smithsonian Institution National
Museum of Natural History: R. Amundson, T. Anderson, E.
Barrera, T. Crowl, R. Cuenca, R. Davis, P. Glynn, K. Maher,
D. McKnight, C. Monger, L. Patino, M. Pavich, C. Pilcher,
E. Sztein, and N. Woodward. Special thanks to C. Samper,
T. Anderson, and T. Karl of the Smithsonian Institution
National Museum of Natural History. M. Hopkins, D.
Lambert, and T. Bernier are acknowledged for organization
and editing. L. Sanford and T. Cronin are acknowledged for
help with Fig. 10. T. White is acknowledged for information
concerning paleosols and C. Paola for the term, ?Earthcast-
ing?. C. Anderson is acknowledged for help with figures. The
manuscript benefitted from three helpful reviews and editorial
handling by D. Beerling and K. Konhauser.
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