Review
a r t i c l e i n f o
Article history:
? 2013 The Authors. Published by Elsevier B.V. All rights reserved.
. . . . . . . . . . 17
Geoderma 221?222 (2014) 11?19
Contents lists available at ScienceDirect
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j ourna l homepage: www.e lse1. IntroductionReferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2. Forms of organic phosphorus in pasture soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3. Factors affecting organic phosphorus species and concentrations in pasture soils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4. Stabilisation and mineralisation of organic phosphorus in soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
5. Reaction of phosphatases and their ef?cacy in soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
6. Implications for the P nutrition of pastures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16? This is an open-access article distributed under the t
Attribution-NonCommercial-No Derivative Works L
commercial use, distribution, and reproduction in any m
author and source are credited.
? Corresponding author. Tel.: +61 3 5624 2253.
E-mail address: david.nash@depi.vic.gov.au (D.M. Nas
0016-7061/$ ? see front matter ? 2013 The Authors. Pub
http://dx.doi.org/10.1016/j.geoderma.2013.12.004P. Integrated approaches analysing the soil matrix, soil water and soil biology are suggested to address this
knowledge gap.Received 8 August 2013
Received in revised form 10 December 2013
Accepted 10 December 2013
Available online xxxx
Keywords:
Phosphorus
Organic
Pasture
Transformations
Inositol
Enzymesa b s t r a c t
Organic phosphorus (P) in grazed pastures/grasslands could sustain production systems that historically relied
on inorganic P fertiliser. Interactions between inorganic P, plants and soils have been studied extensively. How-
ever, less is known about the transformation of organic P to inorganic orthophosphate. This paper investigates
what is known about organic P in pasture/grassland soils used for agriculture, as well as the research needed
to utilise organic P for sustainable plant production.
Organic P comprises N50% of total soil P in agricultural systems depending on location, soil type and land use. Or-
ganic P hydrolysis and release of orthophosphate by phosphatase enzymatic activity is affected by a range of fac-
tors including: (a) the chemical nature of the organic P and its ability to interact with the soil matrix;
(b) microorganisms that facilitate mineralisation; (c) soil mineralogy; (d) soil water electrolytes; and (e) soil
physicochemical properties.
Current biogeochemical knowledgeof organic P processing in soil limits our ability to developmanagement strat-
egies that promote the use of organic P in plant production. Information is particularly needed on the types and
sources of organic P in grassland systems and the factors affecting the activity of enzymes thatmineralise organicAgResearch ? Invermay Agricultural Centre, Puddle Alley, Private Bag 50034, Mosgiel 9053, N
f CSIRO Plant Industry ? PO Box 1600, Canberra, ACT 2601, Australiad Faculty of Agriculture and Life Sciences, Lincoln University, Lincoln, PO Box 85084, 7647 Chri
eUsing organic phosphorus to sustain pasture productivity:
A perspective?
David M. Nash a,?, Philip M. Haygarth b, Benjamin L. Turner c, Leo M. Condron d, Richard W. McDowell e,
Alan E. Richardson f, Mark Watkins a, Michael W. Heaven a
a Victorian Department of Environment and Primary Industries ? Ellinbank, RMB 2460 Hazeldean Road, Ellinbank, Victoria 3821, Australia
b Centre for Sustainable Water Management, Lancaster Environment Centre, Lancaster University, Bailrigg, Lancaster LA14YW, United Kingdom
c Smithsonian Tropical Research Institute, Apartado 0843-03092, Balboa, Ancon, Panama
stchurch, New Zealand
ew Zealanderms of the Creative Commons
icense, which permits non-
edium, provided the original
h).
lished by Elsevier B.V. All rights reserrma
v ie r .com/ locate /geodermaPhosphorus (P) is an essential input for many agricultural produc-
tion systems. For example, the productivity of most Australian soils in
their native state was limited by P and nitrogen (N) availability (Sale,
1992). Through the use of P fertilisers, productive pasture-based grazing
systems have been developed in which legumes, predominantly intro-
duced clovers, ?x atmospheric N, helping to rectify both P and N
ved.
12 D.M. Nash et al. / Geoderma 221?222 (2014) 11?19limitations (Magid et al., 1996). While some recent studies suggest that
the need for P fertilisers has diminished, especially in themore intensive
systems (Condron, 2004; Gourley et al., 2012), ongoing P additions are
often required to sustain optimal production (Frossard et al., 2000). In
most instances the use of P remains inef?cient and there is clear need
to increase P-use ef?ciency (Burkitt et al., 2002; Haygarth et al., 2013;
Richardson et al., 2011; Simpson et al., 2011).
Inorganic P in the formof orthophosphate is a primary constituent of
most P fertilisers. When added to soil, orthophosphate is either seques-
tered into forms that are not immediately available to plants (Engelstad
and Terman, 1980;McLaughlin et al., 2011) or extracted from soil water
and incorporated into plant and microbial biomass. In grazing systems,
P is further transferred into animal biomass and may be exported from
farms as animal (or plant) product. Phosphorus in biomass is otherwise
returned to the soil when plant and animal biomass, and their wastes,
are recycled and decomposed.
Decomposing plant and animal products, along with the soil micro-
?ora and fauna that undertake such decomposition, provide a signi?-
cant store and a source of organic P, which commonly comprises
N50% of total soil P in agricultural systems (Stutter et al., 2012), and in-
organic P (Tisdale et al., 1985). In grazed pastures, up to 85% of the P
taken up by plants is returned to the soil in dung. Such deposits can rep-
resent P inputs of 35 and 280 kg P ha?1 annually for individual sheep
and cattle, respectively (Haynes and Williams, 1993). Importantly, P is
often returned to soil at a location far removed from where it was con-
sumed, so stock transfer represents a loss in pastoral systems (Kemp
et al., 2000) that can account for approximately 5% of P fertiliser inputs
(Simpson et al., 2011).
Despite predictions of future P shortages (Cordell et al., 2009), there
appears to be no immediate concerns regarding the supply of P
fertilisers (Heffer and Prud'homme, 2011). However, the offsite impacts
of P derived from farmland (Schr?der et al., 2010) and the susceptibility
of the supply chain to short-term disruptions in nations supplying raw
materials (i.e. Morocco) and associated market volatility, has increased
scienti?c interest in accessing ?stored? organic P for plant production,
thereby lowering the short to medium term (i.e. b10 year) need for
continued P additions (Haygarth et al., 2013; Stutter et al., 2012).
An important consideration for more sustainable use of world P re-
serves and agriculture production is whether the productivity of
pasture-based grazing systems can be sustained by greater utilisation
of soil organic P if fertiliser applications are curtailed? In this paper we
consider the forms of organic P in pasture soils and factors that regulate
their bio-availability, with an emphasis on phosphomonoesters, the sin-
gly most abundant form of organic P. In this context we use the term
?pasture? to refer to native or introduced grass species that are grazed
as part of an agricultural production system established through the
use of P amendments. Our aim is to identify gaps in knowledge that
need to be addressed in order to optimise the use of organic P for pas-
ture production, particularly in Australia and New Zealand.
2. Forms of organic phosphorus in pasture soils
Orthophosphate, the predominant form of inorganic P in soil, can be
derived from dissolution of primary P containing minerals such as apa-
tite, through the application of mineral P fertilisers or by mineralisation
of organic forms of P by microorganisms (Frossard et al., 2000; Stewart
and Tiessen, 1987). Other forms of inorganic P commonly found in soils
include pyrophosphate and polyphosphates which may be associated
with high levels of fungal activity (Bunemann et al., 2008; Makarov
et al., 2005). Organic P on the other hand is derivedmainly from biolog-
ical processes involving assimilation of orthophosphate and subsequent
release asmicrobial, animal and plantmaterials mature and decompose
(Condron et al., 2005).
Typical organic P compounds (Figs. 1 and 2) include: (i) phos-
phomonoesters (compounds with a single ester linkage to ortho-
phosphate) such as the inositol phosphates; (ii) phosphodiesters(compounds with two ester linkages to orthophosphate) such as ri-
bonucleic acid (RNA), deoxyribonucleic acid (DNA), lipoteichoic
acid, phospholipid fatty acids (e.g. lecithin); and (iii) organic
polyphosphates such as adenosine triphosphate. Phosphomonoesters
and diesters, including speci?c compounds such as myo-inositol
hexakisphosphate (IP6) and DNA are routinely determined by extrac-
tion with a solution containing 0.25 M NaOH and 50 mM Na2EDTA
and detection by solution 31P nuclear magnetic resonance (NMR) spec-
troscopy (Cade-Menun, 2005; Condron et al., 2005; Murphy et al.,
2009). Lower extractant concentrations may enhance phosphodiester
recovery compared to phosphomonoester recovery (Turner, 2008),
while hydrolysis of phosphodiesters during extraction and analysis
has been shown to occur with manures and may bias quantitative as-
sessments (Cade-Menun, 2011; Turner et al., 2003d).
Some characteristics of P in NaOH-EDTA extracts of selected pas-
ture/grassland soils analysed using 31P NMR are presented in Table 1.
Collectively, phosphomonoesters and diesters predominate and
comprise ~25% of the total P in pasture soils. The most prevalent
phosphomonoester is myo-IP6 (Fig. 2) (Cosgrove, 1980; Turner et al.,
2002a), which contains six phosphatemoieties and exists as nine possi-
ble stereoisomers, although only four of these appear to occur in the en-
vironment (Turner, 2007). The myo-IP6 stereoisomer (also termed
phytic acid (Shears and Turner, 2007)) is the most common form iden-
ti?ed in soil, followed by the scyllo-, neo-, and D-chiro- forms (Turner,
2007; Turner et al., 2012). Phytic acid is synthesised by plants and is
stored primarily in seeds where it can represent 60% to 80% of total
plant P. Seeds of major cereal crops typically contain between 3.0 and
4.0 mg P/g dryweight (Raboy, 1997), somyo-IP6 enters the soil through
direct deposition of plant material (including seeds) (Noack et al.,
2012), and from animal faeces, especially non-ruminant animals that
consume feed containing IP6 (Maguire et al., 2004). Non-ruminant ani-
mals do not produce gut phytase (i.e. phosphatase enzyme required for
speci?c hydrolysis of IP6) so a large proportion of IP6 in the diet may be
excreted in the faeces (Lei and Porres, 2007; Leytem et al., 2004; Turner,
2004; Turner and Leytem, 2004). Nonetheless the origins of phytate in
soil remain to be more fully elucidated. For example, the presence of
hexakisphosphates of scyllo-, neo-, and D-chiro-inositol stereoisomers
in soil, which do not occur in higher plants, suggests that direct synthe-
sis by microorganisms may be a signi?cant source of inositol phos-
phates in some soils (Anderson, 1980; Turner, 2007).
Fromaproduction standpoint the considerable variability in organic P
forms and their concentration in different soils and across farming sys-
tems (see Table 1), are both an opportunity and a concern. Moreover,
the signi?cance of organic P in plant nutrition is not re?ected in agronom-
ic soil tests which commonly involve dissolution and extraction of inor-
ganic P and thus do not take adequate account of biological processes
(Condron and Newman, 2011; Coventry et al., 2001; Murphy et al.,
2009). Therefore, accessing the potential value of organic P as a ?fertiliser?
resource requires a more comprehensive understanding of organic P
transformations in soil and factors that affect its mineralisation, along
with possibly new methods of assessing its role in plant nutrition in dif-
ferent pastoral systems. The variability in organic P between otherwise
similarly managed sites also suggests that rates of organic P
mineralisation varywidely and could bemanipulated to release inorganic
P for plant production.
3. Factors affecting organic phosphorus species and concentrations
in pasture soils
Forms of organic P in soil are affected, or at least correlated with a
wide range of soil geochemical, physical and climatic factors, including
precipitation and temperature, which have major impacts on biological
processes (Harrison, 1987). The proportion of phosphodiesters extract-
ed from bulk soil and the clay fraction increased with mean annual pre-
cipitation and temperature in a study of pasture/grassland soils of
western USA (Sumann et al., 1998). Phosphomonoester proportions in
13D.M. Nash et al. / Geoderma 221?222 (2014) 11?19bulk soil, on the other hand, decreased in the clay fraction with in-
creased mean annual temperature and with increased mean annual
precipitation. In another study investigating primarily irrigated, arable
soils from the western USA, temperature was negatively correlated
with both the concentrations and proportions of phosphomonoesters
and diesters (Turner et al., 2003a). The comparative lack of relationships
with precipitation may in this case in part re?ect irrigation.
(a) Lipoteichoic acid (b)
(d) Pyrophosphate
(g) Deoxyribonucleic acid (DNA)
Fig. 1. Chemical structures of selected phosphorus containing molecules found in soil. (a) Lipo
(f) unsubstituted inositol, (g) deoxyribonucleic acid (DNA), and (h) ribonucleic acid (RNA).The phosphodiester to monoester ratio is another soil metric that
may be affected by climatic factors. In one study, phosphodiester to
monoester ratios appeared to increase with mean annual temperature
(Sumann et al., 1998). In a climosequence of NewZealand tussock grass-
land soils correlation coef?cients suggested that the phosphodiester to
monoester ratio tended to decrease with temperature (Tate and
Newman, 1982). Some of the variability (i.e. b25%) observed in
 Phospholipid (c) Phosphonate
(e) Polyphosphate
(f) Unsubstituted inositol
(h) Ribonucleic acid (RNA)
teichoic acid, (b) phospholipid, (c) phosphonate, (d) pyrophosphate, (e) polyphosphate,
sph
14 D.M. Nash et al. / Geoderma 221?222 (2014) 11?19phosphodiester to monoester ratios between experiments may be
accounted for by the different extraction procedures (Cade-Menun
and Liu, 2013), between-year variation and the time of sampling as sea-
sonal trends in ratios have been observed in pasture/grassland soils
(Turner et al., 2003b).
There are limited data on the effects of climatic factors on the compo-
a)
b)
Fig. 2. The structure of a) phytic acid (myo-inositol hexakisition of different organic P classes. In an Irish study (Murphy et al., 2009)
using four non-basaltic soils, the concentration of myo-IP6 ranged from
97 to 185 mg P/kg of soil (representing 20 to 52% of the total organic
P) while scyllo-IP6 ranged between 23 and 99 mg P/kg (representing
12 to 17%). The ratio of scyllo- to myo-isomers varied from 0.23 to 0.85
(Murphy et al., 2009). Similar results were found in twenty nine temper-
ate pasture soils fromEngland andWales (Turner et al., 2005), where the
ratio of scyllo- to myo-isomers ranged from 0.31 to 0.79 (mean 0.45). In
these samples scyllo-IP6 concentrations ranged from 11 to 130 mg P/kg
of soil (representing 4 to 15% of the organic P extracted) and myo-IP6
concentrations ranged between 26 and 189 mg P/kg (representing 11
to 35% of the extracted organic P). In the future, the forms of inositol P
may prove to be important if the enzymes responsible for dephosphory-
lation show speci?city for particular structural isomers.
Plant species and soil mineralogy also affects organic P composition
in soil. In a Brazilian study, tropical pasture/grassland soils had a low
phosphodiester to monoester ratio (0.001) which was signi?cantly
higher where legumes were planted, (i.e. between 0.17 and 0.34, de-
pending on species) (Canellas et al., 2004). In Brazilian Oxisols (high
in Al and Fe oxides), the phosphodiester to monoester ratios on a
range of pastures varied from 0.20 to 0.28 (Chapuis-Lardy et al., 2001),
with the predominance of phosphomonoesters being attributed largely
to the soil mineralogy (i.e. N50% gibbsite, 13?20% goethite, 10?20% ka-
olinite and 6?8% haematite). In this case, the phosphodiester to mono-
ester ratio showed no clear trend as a result of land-use changing
from savannah to pasture (Chapuis-Lardy et al., 2001).
Based on the available data, it is dif?cult to draw any ?rm conclu-
sions regarding the effects of climate or plant species on organic P.
This is not surprising given the large range of reported data (see
Table 1), the likelihood that the primary effects of climate, soil andplant species along with soil formation are strongly interrelated, and
the lack of consistent analytical protocols between studies (Cade-
Menun and Liu, 2013). Clearly, further work measuring organic P spe-
cies in soils using standardised procedures, along with meta-analysis
of existing data sets, is required to identify factors that regulate organic
P composition across different environments.
osphate) and b) conformations of unsubstituted inositols.4. Stabilisation and mineralisation of organic phosphorus in soil
Mineralisation of organic P in soil is governed by a range of both abi-
otic and biotic factors (George et al., 2007b). Access to the substrate is a
primary requirement for organic P hydrolysis, with Al and Fe oxides
being particularly important. Inositol phosphates have greater af?nity
than orthophosphate for Fe oxides (Martin et al., 2004) as the high anion-
ic charge of inositol phosphates, particularly IP6, facilitates the formation
of strong electrostatic bonds (Menezes-Blackburn et al., 2012; Turner
et al., 2002a). Originally it had been thought that adsorption of IP6 onto
goethite (a common soil Fe oxide mineral) is through ligand exchange
at the surface, releasing OH? and H2O to the solution and formation of
an inner sphere complex (i.e. ions bind directly to the surface with no in-
tervening water molecules) (Ognalaga et al., 1994). However, a recent
study suggests that some of the phosphate groups may form an outer
sphere complex and that hydrogen bonding plays an important role in
binding of IP6 to goethite (Johnson et al., 2012). It would seem likely
that bonds between IP6 and soil constituents like goethite have features
of both inner and outer sphere complexes, with the balance dependant
on soil pH. This would explain why phosphomonoester concentrations
are commonly related to the Fe and Al content of forestry (Vincent
et al., 2012) and pasture (Murphy et al., 2009; Turner et al., 2003c) soils.
Model (i.e. laboratory) studies have shed further light on the reactions
ofmyo-IP6with other soil constituents (Celi et al., 1999). Goethite adsorp-
tion ofmyo-IP6 (0.64 ?mol P/m2) was greater than for phyllosilicatemin-
erals (illite 0.38 ?mol P/m2; kaolinite, 0.27 ?mol P/m2). The stronger
adsorption of myo-IP6 to Fe oxides in that study was attributed to the
electrostatic association of four of the six orthophosphate groups, rather
than two in the cases of illite and kaolinite. The study suggested that IP6
ted
36)
03)
8)
43)
92)
90)
53)
43)
38)
15D.M. Nash et al. / Geoderma 221?222 (2014) 11?19changed the surface properties of themineral. Net negative charge on the
surface increased and dispersion of the particles was enhanced, presum-
ably as a result of the unattached phosphate groups in IP6 modifying the
electrochemical properties. This suggests that IP6 may, in some soils, in-
crease colloidal P concentrations in soil water and leachate. Calcite
(CaCO3) is another common soil mineral that has been shown to adsorb
and retain IP6 (17.8 ?mol P/m2) (Celi et al., 2000).
Similarmodel studies suggest that the electrolyte solution surround-
ing goethite also affects myo-IP6 and orthophosphate adsorption. IP6
and orthophosphate adsorbed onto goethite decreased with increasing
pH in the presence of KCl, but increased with a CaCl2 solution (Celi
et al., 2001). Compared to myo-IP6, a less pronounced decrease in ad-
sorption of orthophosphate with increased pH in a KCl solution was at-
tributed to the higher negative charge on myo-IP6 and the tendency for
phosphate groups on myo-IP6 to be less effective in neutralising the hy-
droxyl groups released during adsorption. It was suggested that precip-
itation of Ca saltsmay partly contribute to ?apparent? adsorption of both
Table 1
Selected characteristics of phosphorus in NaOH-EDTA extracts of selected grassland.
Site location Number
of sites
Vegetation
classi?cation
Sampling
depth (mm)
Total P
(mg/kg)
P extrac
(mg/kg)
Canada 3 Grassland
(native) a
0?150 956
(721?1318) b
696
(410?11
Australia 12 Pasture 0?100 1262
(257?2526)
856
(252?19
Australia 3 Pasture 0?100 984
(545?1299)
695
(360?94
New Zealand 5 Pasture 0?150 1533
(938?2405)
1085
(411?22
New Zealand 24 Pasture 0?75 1268
(225?2770)
870
(128?22
New Zealand 3 Pasture 0?75 1433
(460?2910)
1207
(417?22
Ireland 25 Grassland
(deforested)
0?20 1428
(616?2580)
718
(355?23
North America 18 Grassland 0?100 1292
(337?1838)
670
(393?10
England and
Wales
29 Pasture 0?100 939
(376?1981)
686
(330?15
Weighted Mean 1216 776
a Additional descriptor.
b Range.myo-IP6 and orthophosphate above pH 5 (Celi et al., 2001). As both Ca
and K salts in this study were used at similar concentrations (0.01 M),
the effects of an interaction between the charge cloud surrounding the
solid phase (i.e. electrical double layer) (van Olphen, 1977) may also
warrant consideration.
Compared to adsorption, desorption of inositol phosphates from soil
minerals has received less attention. Desorption of myo-IP6 from goe-
thite has been shown to increasewith pH and the number of desorption
cycles (Martin et al., 2004). The amount of myo-IP6 desorbed from goe-
thite was less than 20% of the desorption of orthophosphate, whichwas
attributed to the strength of the bonds and the high negative charge of
the complex, hindering the in?uence of other cations and/or ligands.
The same study examined the effects of organic anions, which may be
released by plant roots and are commonly assumed to in?uence the de-
sorption of orthophosphate in soil (Ryan et al., 2001). After ?ve extrac-
tion cycles, 2.6% of the initially adsorbed IP6 was extracted with citrate
and 4.5% with a KCl solution. The equivalent data for orthophosphate
were 32.7% and 20.6%, respectively. It was concluded that the effects
of the organic anions were negligible.
Solid phase adsorption of inositol phosphates is commonly reported
to hinder their degradation in soils (Madrid and Diaz-Barrientos, 1998;
Turner et al., 2002b). For example, in model studies, IP6 adsorbed to
iron oxides (goethite and haematite) and montmorillonite appeared to
be inaccessible for enzyme hydrolysis and subsequent release of ortho-
phosphate (Giaveno et al., 2010). However, in the same study, enzymeswere able to hydrolyse some of the IP6 adsorbed to two soil clays. The lat-
ter results were attributed to the heterogeneous surface of the clays and
organic matter that may hinder the perfect arrangement of IP6 on those
surfaces (Rao et al., 1996, 2000). The relatively high organic matter con-
centrations in the surface of some pasture soils, and associated soil organ-
ic matter turnover, would therefore be expected to enhance the
hydrolysis of IP6 in ?eld studies (Nash et al., 2007a; Nash et al., 2007b).
When in solution, enzyme hydrolysis of inositol phosphates can be
in?uenced by complexation reactions. For example, reaction of myo-
IP6 with Fe2+ occurs more quickly in solution and provides greater
protection from enzyme hydrolysis than Fe3+-myo-IP6 complexes
(Heighton et al., 2008). The formation of such complexes has received
little study, but is likely to depend strongly on soil pH and soil redox po-
tential, which affect the concentrations and relative proportion of Fe2+
and Fe3+ in soil water, and P adsorption (Bohn et al., 1985). It follows
that periodic change in soil oxic/anoxic conditions, such as would
occur in irrigated or occasionally waterlogged soils and subsoils, could
2+
Phosphomonoester
P (mg/kg)
Phosphodiester
P (mg/kg)
Phosphodiester/
monoester (%)
Reference
307 (202?474) 43 (12?94) 14.0 Condron et al. (1990)
272 (149?496) 18 (4?39) 6.6 Doolette et al. (2011)
209 (154?294) 15 (11?19) 7.2 Dougherty et al. (2007)
442 (209?848) 18 (7?43) 4.1 McDowell and
Stewart (2006)
349 (55?613) 19 (5?62) 5.4 McDowell et al. (2005)
423 (204?613) 27 (6?62) 6.5 McDowell and
Stewart (2005)
339 (30?653) 16 (0?53) 4.7 Murphy et al. (2009)
350 (196?653) 19.6 (0.5?53) 5.6 Sumann et al. (1998)
343 (154?751) 39 (11?109) 11.4 Turner et al. (2003c)
345 27 7.8affect the formation of Fe -myo-IP6 complexes and stabilisation of or-
ganic P in soils, intermittently facilitating the release of plant available P
from myo-IP6.
Like inositol phosphates, phosphodiesters such as DNA can be
protected fromdephosphorylation by adsorption to the soil matrix. Fac-
tors such as mineralogy of the solid phase, pH, ionic strength and com-
position, and molecular size of the DNA affect that process (Levy-Booth
et al., 2007; Ogram et al., 1988). For example, at pH N 5DNA is negative-
ly charged and its adsorption to minerals such as montmorillonite is fa-
cilitated by cation bridging that is not necessary at lower pHwhen DNA
is positively charged (Greaves andWilson, 1969). Notably, pH 5 iswith-
in the range encountered in pasture soils (Crawford et al., 1994). The lo-
cation of DNA and RNA adsorption on monmorillonite, and therefore
susceptibility to biodegradation, has also been shown to depend on so-
lution pH (Greaves andWilson, 1969, 1970). In general DNA adsorption
to 2:1 phyllosilicates (e.g. montmorillonite) appears stronger than to
1:1 phyllosilicates (e.g. kaolinite) (Cai et al., 2006). A detailed discussion
of DNA and RNA adsorption is outside the scope of this review but
readers are referred to reviews which comprehensively examine this
complex topic (Pietramellara et al., 2009; Trevors, 1996; Yu et al., 2013).
5. Reaction of phosphatases and their ef?cacy in soil
While availability of the substrate is critical for the transformation of
organic P to orthophosphate, so too are the phosphatase enzymes in soil
16 D.M. Nash et al. / Geoderma 221?222 (2014) 11?19which catalyse dephosphorylation reactions. In P de?cient environments,
selection pressure commonly results in the proliferation of free-livingmi-
croorganisms and symbiotic associations with mycorrhizal fungi that
have potential to mobilise and mineralise otherwise unavailable organic
P (and solubilise inorganic P) (Mander et al., 2012; Richardson and
Simpson, 2011). For example, in pot trials ryegrass (Lolium perenne L.)
was grown using orthophosphate and myo-IP6 adsorbed to goethite as
sources of P (Martin et al., 2004). Compared to plants receiving no P, P
concentrations in the shoots increased by factors of approximately 3
and 6wheremyo-IP6 and inorganic P adsorbed to goethitewere supplied,
respectively. These results suggest that some myo-IP6 was converted to
orthophosphate probably through microbial processes.
Dephosphorylation of IP6 is generally attributed to microorganisms,
but plant roots also appear to possess limited phytase activity
(Richardson, 1994). In model studies, pasture grasses (Danthonia
richardsonii, Phalaris squatica) and legumes (Medicago polymorpha,
M. sativa, Trifolium repens, T. subterraneium) were grown in sterile agar
or sterile sand-vermiculite media (Richardson et al., 2001). Under
these conditions plants utilising myo-IP6 contained only 20?34% of the
total P content, and had restricted growth, when compared to plants
supplied with an equivalent amount of orthophosphate. This effect was
even more pronounced for the sand-vermiculite treatments where
total P uptake from plants utilising IP6 was only 5 to 10% of those receiv-
ing orthophosphate. Phosphorus uptake from themyo-IP6 treatments in-
creased by 3.9 to 6.8-fold when plants were inoculated with soil
microorganisms, although the response depended on the growth medi-
um used and the source of the inoculant. Impaired myo-IP6 availability
was suggested as the major reason why the sand-vermiculite medium
was the least responsive to inoculation. A similar mechanism would ex-
plain the lack of a response to P additions where a Fe-IP6 complex was
added to the soil under exotic pasture in Australia (Taranto et al.,
2000). By comparison, in this study P supplied as RNA was mineralised
within two months, which is consistent with RNA being less tightly
held on adsorption sites and thus more labile than myo-IP6.
Mineralisation of different inositol phosphate isomers was studied
in a 10-monthpot trial usingperennial ryegrass (Lolliumperenne) grow-
ing in seven pasture/grassland soils from New Zealand (Chen et al.,
2004). The inositol phosphate concentrations in soil were 142 to
598 mg P/kg and under ryegrass, myo-IP6 decreased in four soils (3 to
20% of inositol phosphates) and increased in three soils (6 to 12%)
whereas scyllo-IP6 decreased in three soils (1 to 5%) and increased in
three soils (2 to 8% (Chen et al., 2004; Turner et al., 2005)). Importantly,
the changes in concentrations of the two isomers were not consistent
across soil types. For example, in the Stratford soil, myo-IP6 decreased
by 18%while the scyllo-IP6 increased by 8%. These results may be attrib-
uted to; (a) speci?c enzyme activities (i.e., compared to myo-IP6, scyllo-
IP6 has been shown to be more resistant to enzyme hydrolysis by
phytase (Cosgrove, 1966; He et al., 2011)); and (b) an interaction be-
tween root exudates, pH in the root zone, and soil properties, which
may affect the abiotic stabilisation of the isomers differently. These fac-
tors could similarly contribute to the varying scyllo-IP6/myo-IP6 ratios
(0.29?0.69) measured in 29 lowland permanent pasture soils from En-
gland and Wales (Turner et al., 2005).
Soil pH appears to be a particularly important factor affecting ef?ca-
cy and biochemical availability of dephosphorylating enzymes in soil.
For example, phytases from Aspergillus niger and Peniophora lycii, with
differing biochemical properties including isoelectric point, were
added to suspensions of three contrasting soils (Spodosol, Oxisol,
Al?sol) and their activities for dephosphorylation of IP6 monitored
(George et al., 2007a). At pH 7.5 both phytases remained in solution,
but at pH 5.5 A. niger phytase was rapidly adsorbed to the soil solid
phase. The result was that P. lycii phytase, which had the more acidic
isoelectric point, stayed in the solution andwasmore effective at hydro-
lysing IP6 added to the soil and endogenous organic P. In a companion
experiment, the activity of A. niger phytase in solution declined rapidly
(i.e. b10 min.) when added to a soil suspension. The activity lost fromsolution phase was largely recovered (57 to 86%) on the soil solid
phase, although this recovery variedwith soil type (Al?sol N Spodosol N
Oxisol) (George et al., 2005). Two other observations from this study
were also notable. First, that soil taken from the rhizosphere slowed
the rate at which phytase activity declined by 2?4 times, suggesting
that rhizosphere soils (which in this study lowered soil pH by
~0.5 units) may modify phytase adsorption and thus the release of or-
thophosphate from organic P. Second, by increasing the soil suspension
pH from 5.5 to 7.5, 5 to 50% of the activity of phytase that had been
adsorbed to the soil solid phase for up to 28 days could be desorbed.
This suggests that adsorption of phytase to the solid matrix may be an
important mechanism for the protection and longer-term persistence
of activity in ?eld soils. However, in that context, it would also be impor-
tant to consider the effects of pH on the structure of the phytase (i.e.
charge density) and the concentrations of Fe and Al in solution.
The ef?cacy of phosphatases is also in?uenced by companion cations
and other possible soil water constituents. For example in model studies
representatives of three classes of phytase, some ofwhichmay be present
in soil (Lim et al., 2007) (histidine acid phosphatases (HAPs), ?-propeller
phytases (BPPs), purple acid phosphatases (PAPs)), were unable to hy-
drolyse Al3+, Fe2+, Fe3+, Cu2+ or Zn2+ salts of IP6, but were able to hy-
drolyse Ca2+, Mg2+ and Mn2+ salts (Tang et al., 2006). Additionally,
hydrolysis of Ca-IP6 was prevented when Al3+, Fe2+, Fe3+, Cu2+ or
Zn2+ ions were present. In a second part of the study, IP6 was adsorbed
to Al precipitates.When organic anions were added to desorb IP6, hydro-
lysis occurred with effectiveness in the order citrate N oxalate N malate.
Itwas notable that excessive concentrations of organic acids inhibited en-
zyme activity with PAP beingmore resistant to this inhibitory effect than
HAP.
There is need to develop a consistent procedure for quantitatively
comparing the potential of different microorganisms, especially those
in the rhizosphere of different plants, to release orthophosphate from
different sources. Orthophosphate release, especially from mineral
sources is routinely assessed under laboratory conditions by incubating
microbial isolateswith various forms of organic P or poorly soluble inor-
ganic P sources (Nautiyal, 1999; Schneider et al., 2010). By contrast, a
soil-based procedure to quantitatively assess P mobilisation potential
directly in soils has recently been proposed (Wang et al., 2012). The
technique involves incubation of rewetted soils with poorly available
P sources for up to 30 days with adequate addition of C and N to ensure
that P was limiting microbial growth. In one application, the technique
suggested that P mobilisation potential (measured as the sum of micro-
bial and resin P) of rhizosphere soil from different crops decreased in
the order faba bean (Vica faba L.) N chickpea (Cicer arietinum L.) and
lupin (Lupinus albus L.) N wheat (Triticum aestivum L.). Interestingly,
the use of speci?c inorganic P and organic P substrates provided some
indication of the mechanisms responsible for P mobilisation, although
the effects of high C and N concentrations on soil redox status and asso-
ciated properties were not investigated.
It is clear from the literature that there is only a limited understand-
ing of the mechanisms associated with transformation of organic P to
inorganic P at the process level, and that further work is needed using
soil-based studies, especially under ?eld conditions. In most studies,
the experimental conditions, by necessity, removed interactions be-
tween system components and it follows that while these studies indi-
cate what is possible, they do not necessarily indicate what is probable.
Such knowledge of organic P transformations is a prerequisite for un-
derstanding the potential contribution organic P mineralisation could
make to pasture productivity.
6. Implications for the P nutrition of pastures
The underlying purpose of this paper was to address whether the
production of intensive pasture systems can be sustained by greater ac-
cess to soil organic Pwhen fertiliser applications are reduced. For exam-
ple, in aMediterranean climate like south-eastern Australia, organic P is
phatase enzymes from their point of origin, be that the soil matrix or
an organism, to the site where decomposition andmineralisation actual-
17D.M. Nash et al. / Geoderma 221?222 (2014) 11?19likely to accumulate as pasturesmature, and remain high over summer.
Decomposition of detrital material is likely to be initiated by the follow-
ing seasonal (autumn) rains. This incidentally is whenmost inorganic P
fertilisers are applied (Nash and Hannah, 2011) and when organic N is
alsomineralised. Inmost years the commencement of seasonal rain (au-
tumn break) is relatively gentle (i.e. little runoff) and occurs before low
soil temperatures (i.e. b8 ?C) suppress microbial activity. It follows that
orthophosphate would be released for plant growth. As seasons prog-
ress, some of the organic P accumulated over winter would be convert-
ed to orthophosphate when temperatures rise in early spring. This may
coincide with waterlogging, anoxia and the conversion of Fe3+ to Fe2+,
releasing orthophosphate derived from inorganic sources into solution.
Waterlogging and anoxia are also likely to affect soil pH and the ef?cacy
of phosphorylating enzymes. As the soil dries in late spring/early sum-
mer, conversions of organic P to orthophosphate and plant growth are
both likely to decline due to a lack of soil water. Whether or not these
processes are in synchrony with plant demands and can thus sustain
pasture production, especially in improved pasture systems, and for
how long, is however unclear (refer http://www.dairyaustralia.com.
au/).
There seems little doubt that organic P can be a substantial reserve of
P that is plant available (Stutter et al., 2012). Phosphomonoesters and
diesters comprise at least 25% of total P (Table 1). Assuming an A
horizon depth of 150 mm and bulk density of 1.25 kg/dm3, soil
phosphomonoester and diester concentrations of 345 and 27 mg P/kg,
respectively, equate toN600 and N50 kg P/ha,more thanor comparable
with fertiliser application rates (Gourley et al., 2012). However, these
concentrations re?ect the balance between rates of decomposition
and production in the period leading up to sampling. While such analy-
ses can be useful for investigating paedogenesis, what is lacking is quan-
titative data relating to the release of orthophosphate from different
organic P species under ?eld conditions and in a timeframe consistent
with the needs of pasture plants.
In forest systems 32P has been used to examine rates of organic P
mineralisation in vitro and to compare organic P mineralisation rates to
soil properties (Harrison, 1982a; Harrison, 1982b). Enzyme activity has
also been used to infer the potential contributions of organic P to forest
production (Achat et al., 2012). However, isotopic techniques are dif?cult
to apply in situ and for pasture production systems, which are potentially
susceptible to short-term (i.e.monthly or evenweekly) changes in ortho-
phosphate supply, using enzyme techniques the ?apparent? capacity of
organic P to sustain pasture productionmaywell be different from its ac-
tual ability. There is anecdotal evidence that organic P mineralisation
rates may be quite high (i.e. N100% p.a. for phosphodiesters) in pasture
systems but there appear few techniques that can provide direct, quanti-
tative measures of organic P mineralisation rates in situ.
A study in Pennsylvania (USA) investigated changes over several
years in cropping soils receiving dairy, swine and spent mushroom
compost as compared to adjacent pasture/grasslands (Dou et al.,
2009). Inorganic orthophosphate comprised 79 to 93% of NaOH-EDTA
extracted P while the equivalent range for unmanured soils was 33 to
71%, suggesting that addition of organic amendments does not neces-
sarily increase the proportions of organic P in soil. Phosphomonoesters
comprised 7 to 58% of extracted P whereas phosphodiesters were pres-
ent in only a small (b40%) number of samples and at low concentra-
tions. The latter results are in keeping with other studies (Hansen
et al., 2004; Koopmans et al., 2003) and are interesting given that
such amendments commonly have N4% and up to 10% phosphodiesters
(He et al., 2007; McDowell et al., 2008; Turner, 2004). Also of note, IP6
concentrations were similar in manured (52 to 116 mg P/kg) and
unmanured soils (53 to 137 kg P/kg), even when poultry manures,
that are typically high (i.e. N50%) in IP6 (Leytem et al., 2006; Maguire
et al., 2004) were applied, which suggests that manure-derived
phosphodiesters and IP6 were mineralised.
There is a pressing need to quantify phosphodiester and monoester
turnover rates on a more frequent basis in pasture soils. A hysteresis inly occurs; and (b) the role of themicrobial biomass and speci?cmicroor-
ganisms in the organic P mineralisation process. There are few studies
investigating the role of soilwater in facilitating organic Pmineralisation.
Similarly there has been little study of organic compounds that may
complex organic P and inorganic P in that transition phase, and affect ad-
sorption/desorption processes (Guppy et al., 2005a, 2005b; Singh and
Jones, 1976). This knowledge will be fundamental to provide land man-
agerswith the con?dence to lower fertiliser P application rates and to en-
sure that accessing organic P can adequately contribute to pasture
nutrition without unexpected side effects associated with P loss.
Combining new technologies such as those used in metabolomics
(i.e. the study of metabolites) to analyse temporal changes in soil
water constituents and quantitative PCR (i.e. quanti?cation of microbial
community structures), with established soil extraction/31P NMR and
enzymatic techniques, offers the possibility of studies simultaneously
addressing both the chemistry and biology of the decomposition and
mineralisation of organic P in soil. Further, where these techniques are
applied to the same system, data mining techniques (i.e. PCA, PLS-DA)
can be more easily used. While not demonstrating ?cause and effect?,
these data mining techniques demonstrate covariance that can be
used for generating hypotheses that are subsequently tested in model
systems. They are particularly useful in structured experiments where
through, for example, cultivation and the application of organic amend-
ments, a range of soil conditions have been established at one site,
minimising between plot variation that is attributable to climate, animal
management and soil type. Given the importance of P to agriculture and
receiving waters in agricultural catchments, we believe there is a com-
pelling argument for a more innovative, integrated and international
approach to investigating organic P transformations in pasture systems.
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