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Communications in Soil Science and Plant Analysis
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Interference by Iron in the Determination of
Boron by ICP-OES in Mehlich-III Extracts and Total
Element Digests of Tropical Forest Soils
Benjamin L. Turner, Aleksandra W. Bielnicka, James W. Dalling & Jeffrey A.
Wolf
To cite this article: Benjamin L. Turner, Aleksandra W. Bielnicka, James W. Dalling & Jeffrey
A. Wolf (2016) Interference by Iron in the Determination of Boron by ICP-OES in Mehlich-III
Extracts and Total Element Digests of Tropical Forest Soils, Communications in Soil Science and
Plant Analysis, 47:21, 2378-2386, DOI: 10.1080/00103624.2016.1228952
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Interference by Iron in the Determination of Boron by ICP-OES in
Mehlich-III Extracts and Total Element Digests of Tropical Forest
Soils
Benjamin L. Turnera, Aleksandra W. Bielnickaa, James W. Dallinga,b, and Jeffrey A. Wolfc
aSmithsonian Tropical Research Institute, Balboa, Ancon, Republic of Panama; bDepartment of Plant Biology,
University of Illinois, Urbana, Illinois, USA; cDepartment of Ecology and Evolutionary Biology, University of California
Los Angeles, Los Angeles, California, USA
ABSTRACT
Boron detection in soil extracts by inductively-coupled plasma optical-emission
spectrometry (ICP-OES) can be influenced by iron interference, particularly in
strongly weathered tropical forest soils. Boron concentrations in Mehlich-III
extracts of 230 soils under lowland tropical forest in Panama were markedly
overestimated at the most sensitive ICP-OES wavelength (249.772 nm) com-
pared to a less sensitive but interference-free wavelength (208.957 nm) due to
iron interference. Hot-water extracts contained insufficient iron to interfere in
boron detection, but boron quantification in total element digests was affected
strongly by iron interference at 249.772 nm. The relationship between boron
overestimation and iron was used to correct a database of 300 Mehlich-III
extractable boron measurements made at 249.772 nm for soils from a large
forest dynamics plot. We recommend that boron measurements in tropical
forest soils by ICP-OES should use the less sensitive 208.957 nm wavelength to
avoid interference by extracted iron.
ARTICLE HISTORY
Received 18 January 2016
Accepted 17 June 2016
KEYWORDS
Boron; CaCl2-extraction; ICP-
OES; Mehlich-III extraction;
total element digestion;
tropical forests
Introduction
Boron (B) is an essential micronutrient for plants, being involved in a number of physiological
processes (Marschner 2006). Boron is one of the most common micronutrient deficiencies,
particularly for plants growing on strongly weathered soils, while toxicity can also occur in
some circumstances, notably in semiarid soils (Nable, Bañuelos, and Paull 1997; Shorrocks 1997).
Although studied mainly in agriculture, it has been suggested that B is ecologically important in
tropical forests, where its availability is correlated with the distribution of tree species at local
scale (John et al. 2007; Steidinger 2015).
Boron availability has been assessed conventionally by extraction in hot water or
0.01 M calcium chloride (CaCl2) (Berger and Troug 1939; Berger and Truog 1944; Jeffrey and
McCallum 1988). However, B availability has recently been assessed by Mehlich-III extraction
(Redd et al. 2008; Shuman et al. 1992), a procedure applied widely as a multi-element extraction
for plant-available nutrients, including phosphorus, base cations (calcium, potassium, magnesium,
sodium), and micronutrients (e.g., copper, iron, manganese, zinc) (Mehlich 1984; Sims 1989).
Although it is primarily used in temperate agriculture, the procedure has also been employed in
ecological studies, including the evaluation of nutrient availability in tropical forests (Andersen,
Turner, and Dalling 2010; John et al. 2007; Turner et al. 2013).
CONTACT Benjamin L. Turner TurnerBL@si.edu Smithsonian Tropical Research Institute, Apartado 0843-03092, Balboa,
Ancon, Republic of Panama.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lcss.
This article not subject to US copyright law.
COMMUNICATIONS IN SOIL SCIENCE AND PLANT ANALYSIS
2016, VOL. 47, NO. 21, 2378–2386
http://dx.doi.org/10.1080/00103624.2016.1228952
Although there are a number of procedures for the detection of B in soil extracts (Keren
1996), it is now most commonly determined by inductively-coupled plasma optical-emission
spectrometry (ICP-OES, Thermo Fisher Scientific, Waltham, MA), which allows multiple ele-
ments to be determined simultaneously on the same extract (Redd et al. 2008; Shuman et al.
1992; Taber 2004). In most cases, each element can be determined at a number of wavelengths,
which vary in sensitivity. There are four wavelengths available for determination of B by
ICP-OES in environmental samples, but the two most sensitive wavelengths suffer from iron
(Fe) interference (Sah and Brown 1997). In particular, the most sensitive wavelength of
249.772 nm is subject to interference from the Fe signal at 249.782 nm (Sah and Brown 1997;
Taber 2004). This interference is assumed to be negligible for the analysis of water or 0.01 M
CaCl2 extracts, which contain little Fe (Jeffrey and McCallum 1988). However, it is an important
limitation for analysis of Mehlich-III extracts, which can contain considerable concentrations of
Fe due to the acidic nature of the extraction solution (pH 3). This is likely to be a particular
problem for strongly weathered soils, typical of much of the lowland tropics, because they contain
high concentrations of acid-soluble Fe oxides. It therefore seems likely that an alternative
wavelength should be used to quantify B in Mehlich-III extracts of tropical forest soils. The
problem is also likely to apply to other soil analyses in which B is determined by ICP-OES in
solutions containing sufficient concentrations of Fe. For example, acid digestions for total
elements are typically analyzed by ICP-OES, raising the possibility that total soil B might be
routinely overestimated if detection occurs at 249.772 nm.
We assessed the potential interference by Fe in the quantification of B in Mehlich-III extracts,
hot-water extracts, and total element digestions of lowland tropical forest soils. First, we determined
B by ICP-OES at two wavelengths in Mehlich-III extracts of 230 soils under lowland tropical forest
in Panama, and compared the overestimation with the concentrations of Fe in the extracts. Using an
equation derived from this analysis, we corrected data for 300 soils used to develop a B availability
map for a large forest dynamics plot on Barro Colorado Island (BCI), Panama, where B has been
proposed to be of particular importance in structuring tree species distributions. Using the corrected
data, we present revised B availability data for the plot for use in future studies. Finally, we quantified
hot-water extractable B and total B in soils from long-term nutrient experiments in lowland tropical
forest on Gigante Peninsula, close to BCI. By determining B by ICP-OES at wavelengths with and
without Fe interference, we estimated the potential overestimation of total B in strongly weathered
tropical forest soils.
Methods
Sampling locations
We quantified Mehlich-III extractable B in soils from a network of 80 lowland tropical forest census
sites in Panama (Condit et al. 2004, 2013; Engelbrecht et al. 2007; Pyke et al. 2001; Turner and
Engelbrecht 2011) and 150 locations associated with long-term (25 year) litter fall measurements on
BCI and San Lorenzo National Park, Panama (Harms et al. 2000; Wright and Calderón 2006; Wright
et al. 1999, 2008). The forest census sites spanned a strong rainfall gradient (approximately
1800–3200 mm y−1) and a wide range in soil properties, including pH (3.3–7.0), organic carbon
(2%–10%), and readily-exchangeable soil phosphorus (P) (<0.1 to >20 mg P kg−1). Each soil sample
was a composite of either five soil cores (for transects and 40 × 40 m plots), 13 cores (for 1 ha forest
dynamics plots), or 25 cores (for larger 6–50 ha plots). Soils from sites associated with long-term
litter fall measurements were sampled using a single soil core (6.25 cm diameter × 10 cm depth)
taken directly adjacent to each of the litter traps. All soil samples were taken from the surface soil
(0–10 cm depth), because this zone integrates the nutrient cycle and contains the majority of the fine
roots. Samples were air-dried (10 days at approximately 22 °C), crushed to break up large aggregates,
and sieved <2 mm to remove roots and stones.
COMMUNICATIONS IN SOIL SCIENCE AND PLANT ANALYSIS 2379
Mehlich-III extraction and ICP-OES spectrometry
Boron was extracted from soils in Mehlich-III solution (Mehlich 1984). Briefly, air-dried soils
(sieved <2 mm) were shaken for 5 min in Mehlich-III solution (0.2 N acetic acid, 0.25 M
ammonium nitrate (NH4NO3), 15 mM ammonium fluoride (NH4 F), 13 mM nitric acid
(HNO3), and 1 mM ethylenediaminetetraacetate (EDTA) in a 1:10 soil to solution ratio. The
samples were centrifuged (8000 × g, 10 min) and the supernatant decanted. Boron and Fe were
determined in the extracts by ICP-OES (Optima 7300DV, Perkin Elmer Inc., Shelton, CT, USA).
All measurements were performed at two wavelengths: 249.772 and 208.957 nm. The former
wavelength is most sensitive (greater signal intensity at a given concentration) but is subject to
interference from Fe, while the latter wavelength is less sensitive but does not suffer from Fe
interference (Taber 2004). All extractions and preparation of standards were conducted in plastic
to avoid B contamination from borosilicate glassware.
Correction of boron concentrations on the Barro Colorado Island 50 ha forest dynamics plot
Boron was determined previously on the 50 ha forest dynamics plot on BCI by Mehlich-III
extraction and ICP-OES using the 249.772 nm wavelength (John et al. 2007). Using these data,
John et al. (2007) found that B availability was significantly associated with the spatial distribution of
a large number of tree species, while Steidinger (2015) used niche breadth analysis to further suggest
that B might be the most important nutrient structuring tree species distributions in this tropical
forest. Using the relationship between Mehlich-III extractable Fe and the difference between the two
measures of Mehlich-III extractable B for the 230 soils, we calculated corrected values for B in the
300 analyses of BCI soils from the original John et al. (2007) dataset. The corrected values were
calculated using the following equation:
[Corrected Mehlich-B] = [Mehlich-B measured at 249.772 nm]—[Mehlich-Fe] × 0.00256, where
Mehlich-B is in mg B kg−1, Mehlich-Fe is in mg Fe kg−1, and 0.00256 is the slope of the regression
model between Fe and the difference between B determined at 249.772 and 208.957 nm (see below).
Using these corrected data, we used kriging as described previously (John et al. 2007) to re-estimate
B concentrations at the 20 × 20 m quadrat scale across the 50 ha forest dynamics plot.
Total soil boron
We measured total B in soils from all 15 plots in a long-term litter manipulation experiment in
lowland tropical forest on Gigante Peninsula, part of the Barro Colorado Natural Monument,
Panama. The experiment is described in detail elsewhere (Sayer et al. 2012; Vincent, Turner, and
Tanner 2010). Soils are Oxisols developed on Miocene basalt. Briefly, we sampled soil to 1 m in five
increments (0–5, 5–10, 10–20, 20–50, and 50–100 cm) at four locations in each of the 15 plots in the
experiment. Soils were air-dried and sieved as described above, and then ground to a fine powder in
a ball mill. Samples (100 mg) were digested in 2 mL of concentrated nitric acid for 6 h under
pressure at 180 °C in PTFE vessels (PDS-6 Pressure Digestion System, Loftfields Analytical Solutions,
Neu-Eichenberg, Germany). The digests were diluted to 50 mL with deionized water and B and Fe
were determined by ICP-OES as described above.
Hot 0.01 M Cacl2 extraction of plant-available boron
We quantified plant-available B by extraction in hot 0.01 M CaCl2 (Jeffrey and McCallum 1988) in
surface soils (0–5 cm) from 36 plots in a long-term nutrient addition experiment on Gigante
Peninsula, Panama. This procedure is equivalent to hot-water extractable B (Berger and Troug
1939; Berger and Truog 1944), but the inclusion of CaCl2 eliminates problems from silicate clays.
The fertilization experiment is described in detail elsewhere (Turner et al. 2013; Wright et al. 2011;
2380 B. L. TURNER ET AL.
Yavitt et al. 2011). Soils were air-dried and sieved (<2 mm) prior to extraction. Soils were mixed with
0.01 M CaCl2 in a 1:2 soil to solution ratio and placed in a shaking water bath at 85 °C. After 20 min,
samples were centrifuged (8000× g, 10 min) and the supernatant decanted. Boron and Fe were
determined in the extracts by ICP-OES as described above (Optima 7300DV, Perkin Elmer Inc.,
Shelton, CT, USA).
Results
Comparison of Mehlich-III extractable boron determined at two wavelengths
Mehlich-III extractable B concentrations in soils from the 80 forest census plots, plus an additional 150 sites
on BCI and San Lorenzo National Park (i.e., a total of 230 sites), were determined at both 249.772 nm and a
slightly less sensitive line at 208.957 nm that is not influenced by Fe. The two measures of B were strongly
corrected (Pearson Product-Moment Correlation, r = 0.845, p < 0.0001) (Figure 1a). However, concentra-
tions determined at 249.772 nmwere much greater, indicating a considerable overestimation of extractable
B. The relationship was described by the equation: [Mehlich-B at 208.957 nm] = 0.866 ± 0.036 ×
[Mehlich-B at 249.772 nm)–0.260 ± 0.033 (n = 230, R2 = 0.72, p < 0.001).
The difference in B concentrations measured at the two wavelengths was correlated strongly
with the Mehlich-III extractable Fe concentration in the extracts (Figure 1b), confirming the
influence of Fe on B concentrations at 249.772 nm. We calculated regression models with a
calculated intercept (Y = 0.00221x + 0.0617; R2 = 0.95, p < 0.001) and with the intercept passing
through the origin (Y = 0.00256x; R2 = 0.98, p < 0.001). As extracts containing zero Fe should
yield zero B interference, we used the latter relationship to re-calculate Mehlich-III extractable B
concentrations in the BCI dataset (see below).
Correction of boron concentrations on Barro Colorado Island
Mehlich-III extractable B concentrations of kriged data reported by John et al. (2007) and used subsequently
by Steidinger (2015) were reported to be between 0.23 and 2.86 mg B kg−1 (mean 0.96 ± 0.013 mg B kg−1).
Figure 1. (a) Mehlich-III extractable boron (B) in soils from 230 sites under lowland tropical rainforest in Panama determined by
inductively-coupled plasma optical-emission spectrometry at two wavelengths (208.957 and 249.772 nm). The 1:1 line is shown as
a dashed line, demonstrating the markedly greater values measured at 249.772 nm. The relationship is described by the
equation: Y = 0.866 ± 0.036 x–0.260 ± 0.033 (n = 230, R2 = 0.72, p < 0.001). (b) The difference between Mehlich-III extractable
B determined at the two wavelengths plotted against Mehlich-III extractable iron (Fe), showing the effect of Fe interference in B
determination at 249.772 nm. The two lines are the regression models with a calculated intercept (Y = 0.00221 ± 0.00005
x + 0.0617 ± 0.008; R2 = 0.90, p < 0.001) and with the intercept passing through the origin (Y = 0.00256 x; R2 = 0.98, p < 0.001).
COMMUNICATIONS IN SOIL SCIENCE AND PLANT ANALYSIS 2381
Thesemeasurements weremade by ICP-OES at 249.772 nm, andwere therefore affected by Fe interference.
Using the empirical relationship betweenMehlich-Fe and the overestimation ofMehlich-III extractable B at
249.772 nm described above from our analysis of 230 soils under lowland tropical forests in Panama
(Figure 1b), we corrected the original values for the BCI 50 ha plot by subtracting the apparent B caused by
Fe interference. Kriging these corrected values yielded Mehlich-III extractable B for the BCI 50 ha plot
between 0.08 and 2.09 mg B kg−1 (mean 0.58 ± 0.009 mg B kg−1). The kriged values using the original and
revised data were well correlated (Figure 2) and the maps of B availability across the plot were similar
(Figure 3). The original and corrected data are in an online supplementary file.
Total soil boron
Total B determined by nitric acid digestion and ICP-OES detection at 208.957 nm in soils to 1 m
depth in a litter manipulation experiment on Gigante Peninsula varied between the detection limit
(4.5 mg B kg−1) and 27.8 mg B kg−1 (mean 9.4 mg B kg−1). In contrast, total B determined in the
digests at 249.772 nm was between 198.6 and 325.5 mg B kg−1 (mean 288.0 mg B kg−1), approxi-
mately 30-fold greater than values determined at 208.957 nm (Figure 4a). The two measures were
not correlated significantly.
The difference in total B concentrations measured at the two wavelengths (184–320 mg B kg−1)
was correlated strongly with the total Fe concentration in the digests (Figure 4b), further confirming
the influence of Fe on B concentrations at 249.772 nm. As for Mehlich-III extractable B, we
calculated regression models with a calculated intercept (Y = 0.00234 ± 0.00001 x + 2.34 ± 1.123;
n = 75, R2 = 0.88, p < 0.001) and with the intercept passing through the origin (Y = 0.00256 x; n = 75,
R2 = 1.00, p < 0.001). For both relationships, the regression models are virtually identical to those
derived for Mehlich-III extractable B. For regression through the origin, the slope for total B is
identical to the slope for Mehlich B (0.00256).
Figure 2. Relationship between the original Mehlich-III extractable boron (B) concentrations determined by inductively-coupled
plasma optical-emission spectrometry at 249.772 nm for the Barro Colorado Island large forest dynamics plot and the corrected
concentrations accounting for iron interference. The relationship is described by the equation: Y = 0.613 ± 0.008 x–0.008 ± 0.008;
n = 1250, R2 = 0.84, p < 0.0001. The values are from Kriged estimates at the 20 × 20 quadrat scale.
2382 B. L. TURNER ET AL.
Original data
High : 2.86 mg B kg-1
Low : 0.23 mg B kg-1
Corrected data(b)
(a)
High : 2.09 mg B kg-1
Low : 0.08 mg B kg-1
Figure 3. Maps of boron (B) availability as determined by Mehlich-III extraction plotted using (a) the original data determined by
inductively-coupled plasma optical-emission spectrometry at 249.772 nm (i.e., including interference by iron) and (b) the revised
data corrected for iron interference. The maps also show topographic contours at 5 m intervals.
Figure 4. (a) Total boron (B) in soils in five depth increments to 1 m in 15 plots in a litter manipulation experiment under lowland
tropical rainforest in Panama, determined by inductively-coupled plasma optical-emission spectrometry at two wavelengths
(208.957 and 249.772 nm). The dashed line shows the detection limit for measurement at 208.957 nm (4.5 mg B kg−1). The
plot demonstrates the markedly greater B values measured at 249.772 nm. (b) The difference between total B determined at the
two wavelengths plotted against total iron (Fe), showing the effect of Fe interference in B determination at 249.772 nm. The two
lines are the regression models with a calculated intercept (Y = 0.00234 ± 0.00001 x + 2.34 ± 1.123; n = 75, R2 = 0.88, p < 0.001)
and with the intercept passing through the origin (Y = 0.00256 x; n = 75, R2 = 1.00, p < 0.001). As zero Fe should yield zero
interference, the latter relationship is considered to be the most accurate.
COMMUNICATIONS IN SOIL SCIENCE AND PLANT ANALYSIS 2383
Hot 0.01 M Cacl2 extraction of plant-available boron
Measurements of plant-available B in hot 0.01 M CaCl2 extracts with detection by ICP-OES spectro-
metry at two B wavelengths were similar and strongly correlated (Figure 5). There was therefore no
evidence of Fe interference. The relationship was described by the equation: [CaCl2-B at
249.772 nm] = 0.966 ± 0.012 × [CaCl2-B at 208.957 nm] + 0.006 ± 0.002; n = 36, R
2 = 0.995,
p < 0.001). Iron concentrations in the extract were <0.5 mg Fe kg−1, too low to cause interference in
B measurement at 249.772 nm. For example, 0.5 mg Fe kg−1 would produce a B signal of 0.001 mg B
kg−1, below the detection limit for B by this procedure.
Discussion
Determination of B in Mehlich-III extracts using the most sensitive wavelength (249.772 nm)
markedly overestimated extractable B due to interference by Fe in the extracts. The same over-
estimation occurred for total B in acid digests, and appears to be a particular problem for strongly
weathered tropical forest soils that contain high concentrations of extractable and total Fe. The
second most sensitive B wavelength (249.677 nm) also suffers from Fe interference, although this can
be corrected using a side line indexing procedure (Li-qiang and Zhu 1986). However, Fe interference
did not affect B determination in hot CaCl2 extracts, because Fe concentrations were too low to
cause a detectable change in the B signal by ICP-OES.
Several previous studies have used the 249.772 nm wavelength for soil B in Mehlich-III extracts, or
did not report the ICP wavelength used, suggesting that interpretation might have been influenced by
overestimation of Mehlich-III extractable B. For example, Redd et al. (2008) concluded that
Mehlich-III was unsuitable as an index of plant-available B because it yielded the weakest relationship
to crop yield of any of the extractants tested. However, the ICP wavelength was not reported, raising
the possibility that the results were influenced by overestimation of Mehlich-III extractable B due to Fe
interference. In contrast, Taber (2004) quantified Mehlich-III extractable B using a wavelength that did
Figure 5. Plant-available boron (B) in soils from 36 plots in a nutrient addition experiment in lowland tropical rainforest in Panama,
extracted in hot 0.01 M CaCl2 and determined by inductively-coupled plasma optical-emission spectrometry at two wavelengths
that are susceptible to iron interference (249.773, Y-axis) and that do not suffer from iron interference (208.957 nm, x-axis). The
relationship was described by the equation: [CaCl2-B at 249.772 nm] = 0.966 ± 0.012 × [CaCl2-B at 208.957 nm] + 0.006 ± 0.002;
n = 36, R2 = 0.995, p < 0.001).
2384 B. L. TURNER ET AL.
not suffer from Fe interference, and found that Mehlich-B correlated strongly with tissue B concen-
trations in three crop plants.
For both Mehlich-III extracts and total element digests, the regression models for Fe against
the difference in B measured at the two ICP wavelengths were virtually identical, indicating that
subtracting a B concentration equivalent to 0.00256 × Fe can be used to correct previous analyses
of soil B determined at 249.772 nm in any soil extracts or digests. This of course requires that Fe
was also measured in the sample, but routine multi-element detection in Mehlich-III extracts and
soil digests suggests that Fe concentrations will be available in most cases. As an example, we
applied this correction to a data set of Mehlich-III extractable B in a long-term forest dynamics
plot in Panama (John et al. 2007). The correlation between the original and corrected B
concentrations, and the similarity between the original and revised maps, indicate that the
original analyses (John et al. 2007; Steidinger 2015) are likely to remain unaffected by correction
of the B data. However, B concentrations in the corrected data set are much lower than the
original values, indicating that the possibility of B toxicity in this forest, postulated by Steidinger
(2015), is unlikely.
Conclusions
Determination of B availability using Mehlich-III extraction and total B by acid digestion in tropical
forest soils should be performed by ICP-OES detection at 208.957 nm to avoid interference from
Fe. Previous measurements of B that used the most sensitive B wavelength of 249.772 nm can be
corrected for Fe interference using a universal equation. Plant-available B extracted in water or
CaCl2 can be determined at the most sensitive wavelength (249.772 nm), because the extracts do not
contain sufficient Fe to cause a detectable increase in the B concentration.
Acknowledgments
We thank Dayana Agudo for laboratory support and S. Joseph Wright and Edmund Tanner for their contribution
through the field experiments and access to litter trap sites.
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