Introduction
Phosphorus (P) is a key nutrient in animal, microbial and plant nutrition
and “bears light” to terrestrial ecosystem functioning, regulating primary
and secondary productivities (Walker and Adams, 1958; Vitousek et al., 2010).
Phosphorus input into a young ecosystem derives predominantly from the
weathering of parent material, with some systems receiving P input from
eolian deposits (Chadwick et al., 1999). Once P has been dissolved from
primary minerals, plants and microorganisms access it from the soil
solution. This P is then recycled through soil as organic and inorganic P
(Pi and Po, respectively) compounds (Noack et al., 2012; Damon et
al., 2014), which are similarly subjected to a new cycle of physico-chemical
and biological reactions. Each iteration of this cycle alters the form and
bioavailability of the P, leading to decreasing levels of bioavailable P
compounds (McDowell et al., 2007). In the absence of “fresh” P inputs, this
results in severe P limitations to ecosystem productivity (Walker and Syers,
1976). The five state factors of soil formation (time, parent material,
climate, topography and biota) determine the rate at which the cycle is
completed (Jenny, 1941). Therefore, a better understanding of the role of the
five state factors as drivers of soil P composition is crucial to
quantifying the relative abundance and form of both Pi and Po
pools.
In soils, Pi and Po pools are each composed of specific P
compounds (species) (Newman and Tate, 1980; Tate and Newman, 1982). The main
Po compound categories are: (i) orthophosphate monoesters (single ester
linkage to orthophosphate) such as inositol hexakisphosphates, (ii) orthophosphate diesters (two ester linkages to orthophosphate) such as
ribonucleic acid, deoxyribonucleic acid, lipoteichoic acid, phospholipid
fatty acids, and (iii) phosphonates. Inorganic P compounds include
orthophosphate, polyphosphate and pyrophosphate. Specific phosphatase
enzymes are required to transform the different Po and Pi forms
into orthophosphate, which is the P compound directly taken up by plants and
microbes (Jackman and Black, 1952). As with most enzymes, the activities of
soil phosphatases are very sensitive to the hydrogen potential (pH) with
specific enzyme optima (Frankenberger Jr. and Johanson, 1982). However, other
soil variables are also involved in regulating Pi and Po
transformations. For example, inositol hexakisphosphates bind strongly to
metal oxides and other soil components, which strongly limits their
bioavailability (Turner et al., 2007). Similarly, in pH lower than 5 (the
isoelectric point of DNA), amino group protonation of adenine, guanine and
cytosine bases in the DNA molecule can cause adsorption of positively
charged DNA to the negatively charged clay surface (Cai et al., 2006). As a
result, many soil properties regulate soil P composition but their relative
importance across contexts is unclear.
Conceptual diagrams of the changes in soil P fractions (a) and
NaOH-EDTA extractable P compounds (excluding orthophosphate) (b) with
time (redrawn from Walker and Syers, 1976, with permission from Elsevier,
and McDowell et al., 2007, with permission from John Wiley and Sons,
respectively).
The absolute and relative abundances of Po and Pi forms and
compounds are likely related to ecosystem development and soil weathering
(McDowell et al., 2007; Turner et al., 2007), as conceptualized by the Walker
and Syers model for P fractions (e.g., Walker and Syers, 1976; Yang and Post
2011) (Fig. 1, upper panel). As soils undergo pedogenesis, ecosystem
productivity progresses from nitrogen (N) to P limitation with ecosystem
productivity peaking at the N–P colimitation intermediate stage of
pedogenesis (Walker and Syers, 1976; Turner and Condron, 2013). Parallel
changes occur in soil properties including a decrease in total exchangeable
bases, soil acidification, and an increase in Al and Fe oxide concentration
(Albrecht, 1957; Walker, 1965). As a result, some Po and Pi
compounds increasingly react with the mineral surface and progressively
become occluded P (Yang and Post, 2011). Subsequently, the complexity in
Po and Pi composition increases during ecosystem development
(McDowell et al., 2007, Fig. 1, bottom panel). The degree of soil
weathering is inherently linked to the state factor “time”, as demonstrated
along many chronosequences (e.g., Turner and Laliberté, 2015), but it can
be altered through other state factors (Albrecht, 1957), such as along
climosequences (e.g., Feng et al., 2016) or toposequences (e.g., Agbenin and
Tiessen, 1995). Along a climosequence, precipitation increased both base
cation leaching and the degree of soil weathering, whereas potential
evapotranspiration decreased these processes (Feng et al., 2016). While the
Feng et al. (2016) study evaluated the mineral-P associations described by
the Hedley P fractions, rather than P speciation, it illustrates the
opposing effects of various climatic factors on edaphic factors of interest.
The parent material has distinct effects on soil properties, all other state
factors otherwise being equal. Some of these effects are direct effects,
such as the total P concentration of the initial geologic material. However,
other factors may be more indirect. Parent materials can differ in total
exchangeable base concentration and mineral composition. Variations in
mineral composition can lead to differences in soil pH, soil texture, and Al
and Fe oxides, all of which influence soil P cycling and P composition. For
instance, soil P retention potential is influenced by differential
absorption of Pi and Po to clay, soluble Ca content, as well as
Al and Fe oxhydroxides (Batjes, 2011). As such, the parent material state
factor is an essential consideration in describing soil P cycling. Most
importantly, we need to investigate the hierarchical nature of causal
effects between state factors, soil weathering, soil properties, and Po
and Pi composition.
Nuclear magnetic resonance spectroscopy (NMR) is a widely used method to
study Po and Pi compounds in ecosystems around the world (Kizewski et
al., 2011). This technique can be used for both qualitative and quantitative
estimates of P compounds in soil (Cade-Menun and Preston, 1996). The most
effective extractant for NMR analysis has been NaOH combined with the
chelating agent EDTA (Cade-Menun and Liu, 2014). This does not imply that
NaOH-EDTA is the best extractant for 31P NMR; however, because of its
widespread use, it is a good baseline for comparison (Cade-Menun and Liu,
2014). According to Cade-Menun and Preston (1996), NaOH can solubilize
Pi and Po compounds while EDTA chelates metallic cations to
increase P extraction efficiency from the soil. The NaOH-EDTA extraction
method is widely recognized to quantitatively extract P compounds from the
soil (Turner and Blackwell, 2013; Cade-Menun and Liu, 2014). However, there
are drawbacks of using NaOH-EDTA extractant for 31P NMR analysis.
NaOH-EDTA does not extract all soil P and the highly alkaline environment
can potentially degrade some P compounds (Cade-Menun et al., 2006; Cade-Menun
and Liu, 2014). Additionally, the high pH of the NaOH-EDTA extraction
separates P species from the cations (e.g., Al, Fe, Ca) with which they were
associated in soil. There are other methods to study P dynamics (Frossard et
al., 2011), and soil P composition (Kruse et al., 2015), but none of these
methods is perfect individually. For example, while X-ray absorption near
edge structure (XANES), is a more preferred method for looking at
orthophosphate speciation (Hesterberg, 2010), and is a solid-state technique
that does not require extraction, P concentrations are often below the
detection limit. Therefore, XANES can only detect broad P species groups
(e.g., Fe-P, Ca-P), but cannot, for example, determine if DNA is sorbed to Fe
or Al. The most thorough studies of soil P use a combination of techniques
together, and not any single technique (e.g., Liu et al., 2013, 2015).
There is a lack of broader understanding of how soil P composition is
affected by different state factors of soil formation. Using a large-scale
comparative geographical approach, we aim to determine the causal paths
through which climate, parent material and time influence soil properties,
as well as their impact on Pi and Po pools and specific P
compounds. Combining the soil Pi and Po results obtained with
31P NMR using NaOH-EDTA from different studies, allows us to describe
the effect of state factors on soil P composition in natural ecosystems. We
hypothesize that the compounds comprising soil Pi and Po will be
modified by distinctive edaphic and climatic properties due to different key
ecological processes coupled with soil P cycling.
Methods
Dataset
A database search was conducted until 17 November 2017, to identify
published papers that accurately determined soil P compounds through
one-dimensional liquid state 31P NMR on NaOH-EDTA extracts. According
to McDowell et al. (2006) and Cade-Menun and Liu (2014), we consider as
accurate the papers that used an adequate delay time prior to the NMR
analysis, thus enabling the production of quantitative data on the NMR
instrument. We used two platforms and specific search terms for each one.
The first platform was the Web of Knowledge. The following terms were used:
“soil* phosphorus or P or 31P* nuclear-magnetic-resonance or NMR* naoh or
sodium hydroxide* edta or ethylenediaminetetraacetic” from which 129
results were obtained. The second platform was Google Scholar. The following
terms were used: “soil* phosphorus* “nuclear magnetic resonance”* naoh*
edta”, which yielded 2190 results (excluding patents and citations).
We followed pre-defined eligibility criteria to consider the papers and
then to select or reject them. The first criteria was that only native
growth media were considered (manure, pot soil, soil leachate, and sediment
samples were excluded). In studies focusing on changing natural conditions,
only the control (unchanged) samples were used, (e.g., litter removal in
Vincent et al., 2010 was excluded). Next we only considered studies where the
one-dimensional liquid state 31P NMR method was used with the following
features: (1) NaOH-EDTA extractor without pretreatment (0.5 or 0.25 M NaOH
and 0.1 or 0.05 M EDTA); (2) delay times > 2 s (i.e., quantitative
data, see Cade-Menun and Liu, 2014); (3) NMR features or explanations
according to 31P NMR principles (see Cade-Menun and Liu, 2014); and (4) total
NaOH-EDTA extracted P and total P. Top mineral and organic layers
were both considered. From selected papers we compiled the following data: total P, total NaOH-EDTA
P, and NaOH-EDTA organic P, as well as the Po compounds inositol
hexakisphosphates (myo-, scyllo-, neo-, and D-chiro-IHP, when available), deoxyribonucleic
acid (DNA), phosphonates, NaOH-EDTA inorganic P, and the Pi compounds
orthophosphate, pyrophosphate and polyphosphate. No duplicity was found in
the selected papers.
Site environmental properties
Soil texture, total C, total N and pH, short range ordered Al and Fe minerals
(poorly crystalline) estimated with oxalate extraction, climate
characteristics (mean annual precipitation and mean annual temperatures), as
well as soil age, when available, were also collected from the papers. When
the total C was unavailable, the organic C was assumed to be the total C.
This assumption only occurred for non-calcareous soils. Some variables were
unavailable for some results, and the number of experimental used for each
analysis is presented in the results section. Missing texture and total C
data (representing 12 sites and 1 site, respectively) were extracted from a
global soil dataset, SoilGrids, which is now at 250 m resolution (0–20 cm
topsoil, Hengl et al., 2017). The resulting dataset is available in
Sect. S1 int the Supplement (Deiss et al., 2018). We used the Whittaker's diagram (Whittaker, 1975) and
the “BIOMEplot” package (Kunstler, 2014) to determine the biomes of our
sites (Supplement S2).
Soil weathering stages were derived from the soil type according to Cross
and Schlesinger (1995) and Yang and Post (2011) as well as from
chronosequence positions. A low weathering stage was attributed to Entisol,
Mollisols and Inceptisols forming the first stages of chronosequences and
gleyed Acrisols. An intermediate weathering stage was attributed to Alfisol,
Aridisol, Mollisols and Inceptisols forming the intermediate stages of
chronosequences and orthic Acrisol. Finally, a high weathering stage was
attributed to Oxisol, Spodosol, Ultisol, and humic Acrisol.
Data analysis
Statistical analyses were conducted on R Version 3.1.0 (©2014 The
R Foundation for Statistical Computing) using mixed-regression models
including edaphic and climatic variables as continuous and categorical fixed
effects. The latitude, the percentage of P extracted with NaOH-EDTA and the
soil sampling depth were considered as random effects. Latitude was used to
control for the spatial auto-correlation (Maestre et al., 2005). The
percentage of P extracted with NaOH-EDTA was used because the NaOH-EDTA
extraction process varies according to soil characteristics and experimental
conditions (i.e., pretreatment, soil-to-solution ratio and soil
characteristics) (Cade-Menun and Liu, 2014, see Fig. S5). Sampling depth was
used because of potential differences in organic matter concentration along
the soil profile. The bivariate effects of latitude, percentage of P
extracted and sampling depth on the soil P composition are presented in
Supplement S3–S5.
We used variation partitioning and Venn diagrams (Legendre and Legendre,
2012) (the “vegan” package) to partition the total variation explained uniquely
by the matrix of either soil variables, climate variables or soil weathering
stages as well as the variation explained by the combined effect of these
matrices. The unique effect of soil, climate or soil weathering stages was
calculated as the adjusted r2 value (ra2) difference between
the full model and unique model. The joint effect of these matrices was
calculated as the difference between the summed ra2 of unique
models and the ra2 of the full model.
Structural equation modeling (path analysis, the “lavaan” package) was used to
explore how variations in Pi and Po compounds are driven by both
direct and indirect effects of key environmental drivers (soil, climate, and
parent material). We first established an a priori model that is based on our
knowledge and is presented in Supplement S10. Then, we tested for the most
parsimonious model among many alternative ones, i.e., the one that differed
the least from the observations and presented the greater p value. Parent
material, which was unknown for our sites, was considered as a latent
variable in the model to explain the remaining coordinated variations in
total P, clay, and pH variables that were not explained by climate and
weathering. As mainly determined by the biota, total C was not considered as
being constrained by the parent material.
Different units were used across statistics to analyze soil P composition.
The bivariate relationships (Figs. 2, 3, and 5, and Figs. S3–S8)
considered are as follows: (i) total Pi or Po concentration in NaOH-EDTA extracts
(mg kg-1 soil), (ii) proportion of total Pi or Po as
percentage of total NaOH-EDTA P (% of NaOH-EDTA P), and (iii) proportion
of soil P compounds as percentage of their respective pools (% of
NaOH-EDTA Pi or Po). In contrast, in both Venn diagrams (Fig. 6)
and structural equation modeling (Fig. 7) soil P compounds were in mg kg-1. In bivariate relationships, we used percentages was to compare
the relative composition of P along environmental variables that are linked
with the weathering of soils. In both the Venn diagram and path analysis,
the objective was to explain soil P composition either partitioning the
variation among state factors or accounting for the causal structure of
environment. For that, we used the mg kg-1 unit so that the
distribution of our variables was not constrained as a proportion.
Relationship between edaphic properties and soil inorganic
phosphorus (P) composition in NaOH-EDTA extract from soil mineral and
organic layers on natural ecosystems. Note that the reported total P is the
one obtained by digestion and usually comprise the residual P non-recovered
by the NaOH-EDTA extractant. Regression models (n=80 mineral layer and n=20 mineral layer):
mineral layer, log(total Pi mg kg-1)=-1.62+1.28pH-0.11pH2, r2=0.33; mineral layer, total
Pi (%) =7.21+7.12 pH, r2=0.34; mineral layer,
orthophosphate =79.7+2.00 pH, r2=0.11; mineral layer,
pyrophosphate =20.8-2.23 pH, r2=0.11; mineral layer,
log(total Pi mg kg-1)=1.68+0.028-0.00041 clay2,
r2=0.23; mineral layer, log(total Pi mg kg-1)=1.22+0.46 log(total C), r2=0.32; mineral layer, total Pi (%) =68.0-17.2 log(total C), r2=0.14; mineral layer,
orthophosphate =97.6-4.74 log(total C), r2=0.08; organic
layer, orthophosphate =348.0-113.4 log(total C), r2=0.30;
mineral layer, pyrophosphate =1.85+4.74 log(total C), r2=0.08; organic layer, pyrophosphate =151.7-53.0 * log(total C), r2=0.34; organic layer, polyphosphate =-446.4+184.4 log(total C),
r2=0.45; mineral layer, log(total Pi mg kg-1)=-0.63+0.97 log(total P), r2=0.73; organic layer, log(total Pi mg kg-1)=-0.50+1.00 log(total P), r2=0.27; organic layer,
log(total Pi mg kg-1)=2.51-0.00033 C : P ratio, r2=0.26; mineral layer, total Pi (%) =77.0-7.88 * log(C : P ratio),
r2=0.33; mineral layer, orthophosphate =94.0-0.0353 C : P ratio,
r2=0.37; organic layer, orthophosphate =91.1-0.00057 C : P ratio, r2=0.19;
mineral layer, pyrophosphate =5.04+0.0359 C : P ratio, r2=0.37; organic layer, polyphosphate =-20.5+0.079 C : P ratio, r2=0.31.
Results
Our search resulted in 100 native vegetation outcomes from 13 references
(Supplement S1) (Backnäs et al., 2012, n=1; Celi et al., 2013, n=4;
Doolette et al., 2017, n=5; Li et al., 2015, n=1; McDowell and Stewart,
2006, n=4; McDowell et al., 2007, n=26, Turner and Engelbrecht, 2011,
n=19; Turner et al., 2003, n=1, Turner et al., 2007, n=8; Turner, 2008b,
n=1; Turner et al., 2014, n=20; Vincent et al., 2013, n=8; Vincent et
al., 2010, n=1). Most of the papers were excluded (from more than 2000
papers found during the search) because they failed to meet the eligibility
criteria including land use (e.g., crop, pasture, planted forest or
wetlands) and 31P NMR features. The results selected were from the
following countries: Australia (n=5), Finland (n=1), Italy (n=1), New
Zealand (n=59), Panama (n=21), Russia (n=4), Sweden
(n=8), and the United States of America (n=1). These results comprised
most of the global biomes classified according to Whittaker's diagram
(Whittaker, 1975), except for the subtropical desert, tundra, and temperate
rain forest. The six chronosequences studies (5 in New Zealand, 1 in Sweden;
5 on A layer, 2 on O layer) were the most important contributors to the data
(45/74 sites on A layer, 18/20 sites on O layer).
Relationship between edaphic properties and soil organic
phosphorus (P) composition in NaOH-EDTA extract from soil mineral and
organic layers on natural ecosystems. Note that the reported total P (x axis) is the one obtained by digestion and usually comprise the residual P
non-recovered by the NaOH-EDTA extractant. Regression models: mineral layer,
log(total Po mg kg-1)=-3.61+2.27pH-0.22pH2,
r2=0.34 (n=80); mineral layer, total Po (%) =93.4-7.24 pH, r2=0.35 (n=80); mineral layer, inositol hexakisphosphate
(IHP) =66.4-6.94 pH, r2=0.16 (n=52); mineral layer, DNA =27.8-3.63 pH, r2=0.19 (n=64);
organic layer, DNA =27.4-3.83 pH, r2=0.10 (n=20); mineral layer, log(total Po
mg kg-1)=1.75+0.035clay-0.00049 clay2, r2=0.16 (n=80); mineral layer, log(total Po mg kg-1)=31.2+120.5 log(total C), r2=0.60 (n=80); mineral layer, total Po (%)
=32.4+17.0 log(total C), r2=0.12 (n=80); mineral layer,
log(total Po mg kg-1)=-0.55+0.97 log(total P), r2=0.29 (n=80); organic layer, log(total Po mg kg-1)=-0.89+1.19 log(total P), r2=0.68 (n=20); mineral layer, DNA =34.2-9.27 log(total P), r2=0.18 (n=64); organic layer, log(total
Po mg kg-1)=2.69-3.58×10-4 C : P ratio, r2=0.48 (n=20); mineral layer, total Po (%) =23.0+7.90 * log(C : P
ratio), r2=0.33 (n=80); mineral layer, DNA =6.53+0.029 C : P ratio, r2=0.34 (n=64).
In the compiled data, 80 % of results were from mineral layers and the
remaining 20 % from organic layers; 39 % did not contain inositol
hexakisphosphates results (including all tropical regions), and 12 % of
DNA results were absent (including both non-tropical and tropical regions).
The P extracted with NaOH-EDTA on mineral layers averaged 55 % (2 % to
98 % range), and on organic layers averaged 73 % (57 % to 94 % range) of
total soil P. The average sample depth was 12.2 cm (2 to 42 cm range) for
mineral layers and 11.0 cm (1 to 28 cm range) for organic layers.
Edaphic properties
All surveyed edaphic properties affected soil Pi and Po pools and
compounds. These results are summarized in Figs. 2 and 3. Total
Pi (Fig. 2a) and Po (Fig. 3a) concentrations in NaOH-EDTA
extracts (mg kg-1 soil) both had a quadratic response to soil pH, with
higher values occurring at an intermediate pH, although this effect was
constrained to the mineral layers. In mineral layers, the proportion of
NaOH-EDTA P in the form of Pi increased with pH (Fig. 2b), whereas
the proportion in the form of Po decreased with pH (Fig. 3b).
However, there was no pH effect on either pool in the organic layers. The
distribution of compounds in both Pi (Fig. 2c–e) and Po (Fig. 3c, d) pools responded dynamically to the pH. In the Pi pool (% of
Pi) of mineral layers, orthophosphate decreased, and pyrophosphate
accounted for the remaining Pi as the pH decreased. The pH had no
effect on these Pi compounds in the organic layer (even though there is
an apparent trend, these relationships became insignificant after including
sampling depth as a random effect on models; Supplement S3 shows the sampling
depth effect over the soil P composition). In the Po pool (% of
Po), both inositol hexakisphosphates (mineral layer) and DNA (mineral
and organic layers) proportions increased as the pH decreased. Phosphonates
response to edaphic properties (insignificant) is presented in Supplement S6.
Both total Pi and Po concentrations in NaOH-EDTA extracts (mg kg-1 soil)
responded quadratically to the clay concentration, with
higher values occurring at intermediate textural classes (Figs. 2f and 3e). Clay impacted on neither Pi and Po, nor on the proportions of
P compounds (% of NaOH-EDTA P) (Figs. 2g–j and 3f–h).
Total Pi and Po concentrations in NaOH-EDTA extracts (mg kg-1 soil)
increased as the soil C concentration increased in mineral layers,
whereas in organic layers there was no C concentration effect on Pi and
Po concentrations (Figs. 2k and 3i). As a percentage of NaOH-EDTA P,
Pi decreased and Po increased (% of NaOH-EDTA P) as the soil C
concentration increased in mineral layers, and there was no C concentration
effect on either Pi and Po proportions in organic layers (Figs. 2l and 3j). In the Pi pool (% of Pi) of mineral layers,
orthophosphate and pyrophosphate proportions decreased and increased,
respectively, as the soil C concentration increased (Fig. 2m–o). As the
soil C concentration increased in the organic layer, orthophosphate
decreased, at a greater extent when compared to the mineral layer,
pyrophosphate decreased (in contrast to the mineral layer, in which it
increased), while the polyphosphate proportion increased, and gradually
dominated the Pi pool at greater soil C concentrations. In the Po
pool (% of Po), there was no C concentration effect on the soil
Po composition (phosphonates, inositol hexakisphosphates and DNA) in
either mineral or organic layers (Fig. 3k–l; Supplement S6).
Both total Pi and Po concentrations in NaOH-EDTA extracts (mg kg-1 soil) from both mineral and organic layers increased as the
total soil P concentration increased (Figs. 2p and 3m). Only the DNA
compound from the Po pool (% of Po) in the mineral layer was
affected by the total soil P concentration (Fig. 3p). As the total soil P
concentration increased, the DNA proportion in the Po pool decreased.
It is important to note that the reported total P (x axis on Figs. 2 and 3) is the one obtained by digestion and it includes both the extracted P and
the residual P. The recovery of the total P by NaOH-EDTA extraction varies
depending on soil characteristics and laboratory procedures (Cade-Menun and
Liu, 2014).
Total Pi and Po concentrations in NaOH-EDTA extracts (mg kg-1 soil) were
only affected by the soil C : P ratio in organic layers.
Total Pi and Po concentrations both decreased as the soil C : P ratio
increased (Figs. 2u and 3q). As a percentage of NaOH-EDTA extract (% of
NaOH-EDTA P), Pi decreased while Po increased, both exponentially,
as the soil C : P ratio increased in mineral layers (Figs. 2v and 3r). As
proportions in the Pi pool (% of Pi) of mineral layers,
orthophosphate decreased and pyrophosphate increased as the soil C : P ratio
increased (Fig. 2w–x). In the Pi pool (% of Pi) of organic
layers, proportions of orthophosphate decreased and polyphosphate increased,
gradually dominating the Pi pool as the soil C : P ratio increased (Fig. 2w–y).
In the Po pool (% of Po), the DNA proportion increased
as the soil C : P ratio increased, only in the mineral layer (Fig. 3t).
Climatic properties
Climatic properties affected soil Pi and Po pools and their
composition only through the mean annual precipitation. These results are
summarized in Supplement S7 and S8. The mean annual temperature, ranging
from -0.4 to 27 ∘C, did not promote any change in the soil
P composition in natural ecosystems. There was no effect of climatic
variables on total Pi and Po concentrations in NaOH-EDTA extracts
(mg kg-1 soil) (Supplement S7a and S8a). As a fraction of the
NaOH-EDTA extract (% of NaOH-EDTA P), Pi decreased and Po
increased as the precipitation increased (Supplement S7b and S8b). As the
precipitation increased, proportions of orthophosphate decreased and
pyrophosphate increased as compounds of the Pi pool (% of Pi)
(Supplement S7c–d).
Soil weathering stages
Soil weathering stages determined from the soil type and
chronosequence positions affected soil age and C : P ratios following an
expected effect of pedogenesis (Fig. 4). As soil weathering stages
increased, the soil age and C : P ratio also increased. Pi and
Po pools were both affected by the soil weathering stage (Fig. 5). Total
Pi and Po in NaOH-EDTA extracts (mg kg-1 soil) were more
concentrated in soils at moderate weathering stages when compared to low and
high weathering stages (n=79, Fig. 5a, f). As percentages in the Pi
pool (% of Pi), orthophosphate decreased and pyrophosphate increased
as the soil weathering stage increased (n=79 for all Pi compounds,
Fig. 5c–d). In the Po pool (% of Po), the DNA (n=64)
proportion was greater in more weathered stages, and there was no effect of
weathering stages on phosphonates (n=79) and inositol hexakisphosphates
(n=52) proportions (Fig. 5h–j). Using available data (n=49), we
observed no effect of soil weathering stages on short range ordered (poorly
crystalline) Al and Fe minerals estimated with oxalate extraction
(p > 0.1, Supplement S9).
Soil weathering stage relationship with soil age (n=33) and C : P ratio (n=78) on natural ecosystems.
Variation partitioning among edaphic, climatic, and weathering on the
soil P composition
The variation partitioning of ecosystem properties governing the soil P
composition (in mg kg-1 soil) was generally more pronounced for soil
variables (pH, clay concentration, and total P and C concentrations) than
climatic variables (precipitation and temperature) and soil weathering
(Fig. 6). For the total Pi concentration and its compounds
orthophosphate and pyrophosphate, the total variation explained by models
ranged from 46 % to 89 %, and they were mostly explained by soil
variables and combined effects of soil and weathering. Polyphosphates had a
poorly defined response to the variation partitioning of ecosystem
properties (< 0.01 % of the total variation explained).
Soil inorganic and organic phosphorus (P) composition in NaOH-EDTA
extract as influenced by weathering stages on natural ecosystems.
In the Po pool, the total variation explained by models ranged from 41 %
to 86 % (Fig. 6). The total Po, inositol hexakisphosphates and DNA
had their total variation mostly explained by soil variables, and to a lower
degree, but more pronounced for the DNA compound, by combined effects of
soil variables and weathering. In contrast, most of the variation in
phosphonates was explained by combined effects of climate and soil
variables, followed by uniquely soil variables.
Interdependences between environmental variables and soil P compounds
We used path analyses to explore the interdependences between edaphic and
climatic variables and how they relate to the soil Pi and Po
compounds (Fig. 7; Supplement S10). The parent material was used as a latent
variable (set by the pH) in both models (Pi and Po). Climate and
soil weathering drivers were independently related to soil variables (total
P, pH, clay and total soil C), and soil variables were considered direct
effects in the models. The most parsimonious path analysis model explained
up to 78 % of Po compounds variation and 89 % of Pi compounds
variation.
Variation partitioning among edaphic, climatic, and weathering
stages on soil inorganic and organic P composition in NaOH-EDTA extract on
natural ecosystems. Soil organic and inorganic P forms and compounds were in
mg kg-1, and the other variables followed units described on Figs. 2 and 3.
Following an expected effect of pedogenesis, the path analysis indicated
that the parent material (latent variable) was positively related to the
soil total P, clay and pH. Greater mean annual precipitation was negatively
related to the soil total P, pH, and it positively influenced soil total C,
while clay was negatively influenced by precipitation in the Po model
only. In the Po model, precipitation promoted soil weathering, whereas
in the Pi model, soil weathering was positively affected by
temperature. The mean annual temperature positively affected the clay and
pH. Soil weathering was negatively related to the soil pH, and positively
related to the soil clay and total C. In the Pi model only, soil
weathering negatively affected soil total P. There were also significant
direct and positive effects between soil total C and clay, and total P, in
both Pi and Po models, and there was a positive relationship
between soil total C and pH in the Po model only.
Path analysis describing the direct and indirect effects of the
main environmental predictors of soil inorganic and organic P compounds (mg kg-1)
in NaOH-EDTA extract as influenced by edaphic and climatic
drivers on natural ecosystems. Solid and dashed lines represent positive and
negative relationships, respectively. Soil organic and inorganic P compounds
were in mg kg-1, and the other variables followed units described on
Figs. 2 and 3.
In the Pi model, orthophosphate was negatively related to
precipitation, and it was positively influenced by soil total P and total C.
Pyrophosphate was positively influenced by precipitation, soil total P, and
total C. Polyphosphate was negatively influenced by temperature, and it was
positively related to soil pH. In the Po model, inositol
hexakisphosphates were negatively affected by precipitation and temperature,
but positively affected by the soil total P, total C and pH. In contrast,
total P and total C positively affected DNA, and there were no effects of
climatic variables over DNA. Phosphonates were negatively affected by
temperature and weathering, but positively affected by precipitation and
soil total C.
Discussion
Our results showed how soil Pi and Po compounds responded to
edaphic variables (Figs. 2 and 3), climatic variables (Supplement S7-S8), and soil weathering stages as a proxy for pedogenesis (Fig. 5) on a
wide geographical scale, including a variety of natural ecosystems. While
the soil P composition was primarily directly influenced by soil properties,
the impact of climate and weathering stage occurred mainly through indirect
paths and their influence on soil properties (Figs. 6 and 7). In addition,
soil Pi and Po compounds responded to different combinations of
explicative variables, which likely indicates that each P compound has
specific factors governing its presence, transformation and persistence in
ecosystems. This could be due to many factors including (i) the source of P
inputs, primarily by minerals, and then altogether with plant and microbe P
cycling; (ii) the presence of specific phosphatase enzymes that are required
to transform Pi and Po compounds into orthophosphate; and (iii) the
soil specific reactivity and P losses governed by physico-chemical
properties (e.g., clay, short-range ordered oxides and pH); and iv) the P
persistence induced by the increasing complexity of Pi and Po
compounds as pedogenesis evolves.
As time passes after the onset of pedogenesis, the ecosystem accumulates
organic matter up to a maximum, and then starts to decline. Along with this
decline, there are also changes in the chemical composition of organic
matter, in which the decaying degree (i.e., decomposition) of C element is
lower than the P, and concomitantly there is an increasingly acidic soil
environment (Walker, 1965). In addition, parent material supplies cations and
orthophosphate to young soils, whereas more weathered soils are
substantially changed from the parent material. Consequently, highly
weathered soils generally have higher C : P ratios, a lower pH and greater clay
concentration. The soil total P content depends on both weathering stages
and parent material, but generally decreases with increasingly weathered
soil orders (Yang and Post, 2011). Our data included soil orders ranging from
all three stages of soil weathering (low, intermediate and high), according
to Cross and Schlesinger (1995) and Yang and Post (2011). The soil
weathering stage classification also takes into account changes in the soil
P fractions, and generally follows the Walker and Syers (1976) conceptual
model: there is gradual decrease and eventual depletion of primary mineral P
(mainly apatite P), decrease of total P, increase and then decrease of total
Po and increase and eventual dominance of occluded P during the soil
development (Yang and Post 2011). In highly weathered soils, occluded P
increases through the encapsulation of the Pi and Po compounds
inside of Fe and Al minerals (McDowell et al., 2007; Turner et al., 2007).
Even though most of the results were from New Zealand and Panama, our dataset
comprised several biomes according to the Whittaker's diagram (Whittaker
1975), including the temperate grassland desert, woodland shrubland,
temperate forest, boreal forest, tropical rain forest, tropical forest
savanna, intermediates between the temperate rain forest and boreal
forest, and tropical rain forest and temperate rain forest (Supplement S2);
however, quantitative data on the feedback between P compounds and
biological communities during pedogenesis is still incipient to conclusions
drawn from the influence of vegetation and organisms on the soil P
composition (Huang et al., 2017), especially for 31P NMR results. What
is clearer is how soil P availability shapes the ecosystem's overall primary
productivity, and to a lesser extent, soil food webs. In a global analysis,
Maire et al. (2015) demonstrated that the soil available P is a key
environmental dimension increasing leaf P content along with species'
maximum photosynthetic rates and lower stomatal conductance. However, this
trend is expected to gradually decline in more weathered soils due to a
lower P availability. Conducted at a narrower scale, Laliberté et al.'s
study (2017) showed that soil fertility (including P availability) strongly
shaped underground food webs, promoting changes such as a shift in dominance
from bacterial to fungal energy channels with increasing soil age.
Soil properties and the soil P composition
As soils aged, pyrophosphate and polyphosphate may have accumulated because
of the incorporation and stabilization of these compounds (biological
origin) into soil organic matter (Turner et al., 2007). The soil pH, total
carbon and C : P ratio, as well as weathering stage had a major influence on
the soil Pi pool composition. As the orthophosphate proportion
decreased in more weathered, acidic, organic-rich, and P-limited soil
environments (Figs. 2c, m, w and 5c), pyrophosphate and polyphosphate
proportions increased and dominated the Pi pool (Figs. 2d, n, o, and y and 5d).
Even though pyrophosphate and polyphosphate are Pi compounds, they have
a biological origin (Turner and Engelbrecht, 2011). Condensed forms of P
(including pyrophosphate and polyphosphates) are found in every bacterial,
archaeal, and eukaryotic cell, but in highly variable amounts (Kornberg et
al., 1999). Bünemann et al. (2008) found a positive relationship between
the proportion of fungi and the amount of pyrophosphate, and Reitzel and
Turner (2014) found a positive link between the pyrophosphate proportion and
soil microbial P. Polyphosphate can originate from ectomycorrhizal fungi
(Koukol et al., 2008), and there are some ectomycorrhizal fungi specialized
for P uptake in low P, acidified soil conditions (Wang and Qiu, 2006).
Ectomycorrhizal fungi convert the orthophosphate that they take up from the
soil into polyphosphates, and translocate the polyphosphate along fungal
hyphae, sometimes at a great distance from where the orthophosphate is taken
up (Bücking and Heyser, 1999; Plassard and Dell, 2010). Therefore, we
believe that pyrophosphate and polyphosphate dominated the Pi pool in
acidic, P-limiting (C : P ratio), and high organic matter (total C) soils
because of the microbial origin of these P sources, but much information is
still needed in regard to plant and microbial communities characterization
in studies of P forms. These organisms could have helped to deplete and
transform the bioavailable orthophosphate, turning it into more microbial
biomass derived P compounds as pedogenesis progressed in these environments.
Moreover, polyphosphates tend to occur in abundance only in soils where
decomposition is slowed, such as acidified soil conditions, or cold and wet
soils high in organic matter (e.g., Cade-Menun et al., 2000; Turner et al., 2004).
Studying wetland soils, Cheesman et al. (2014) found that
polyphosphates played a preeminent role in P-limited systems, predominantly
in acidic, high-organic-matter systems. Adding to that, pyrophosphate
hydrolysis was found to be more rapid with greater biological activity and
higher agricultural soil pH (Sutton and Larsen, 1964), and this may have
contributed to reducing the pyrophosphate proportion at a higher pH in
mineral soils (Fig. 2d). As the C concentration increased in organic
layers, polyphosphate dominated the soil Pi pool (Fig. 2m–o) possibly
because of its lesser lability when compared to orthophosphate and
pyrophosphate. Pyrophosphate is less polymerized and potentially more
susceptible to hydrolysis than polyphosphate. According to Savant and Racz (1972),
Subbarao et al. (1977) and Dick (1985), pyrophosphate is hydrolyzed
more rapidly than polyphosphate because pyrophosphate is an intermediate
product of polyphosphate hydrolysis until the final orthophosphate is
produced. These are also conditions under which ectomycorrhizal fungi are
found. These fungi produce hyphal mats in the forest floor, so an increase
in polyphosphates could reflect an increase in ectomycorrhizal hyphal mats.
However, caution must be taken when interpreting pyrophosphate and
polyphosphate hydrolyzation results from 31P NMR analysis on NaOH-EDTA
extracts. Polyphosphates can potentially degrade to pyrophosphates during
extraction and NMR analysis of P (Cade-Menun et al., 2006), so they cannot be
considered as fully distinct P forms. This potential degradation could be
one explanation to why the polyphosphates results were poorly explained by
the variation partitioning and structural equation models.
As time passes after the onset of pedogenesis, modifications in the soil
Po composition were possibly related to the acidifying environment in
soils, which may increase the charge of some Po compounds, and thus
increase sorption. Soil pH affects the sorption of Pi and Po
compounds by soils, but different P compounds respond differently to pH
changes (Shang et al., 1992). Shang et al. (1992) verified that sorption of
both orthophosphate and inositol hexakisphosphate by Al and Fe precipitates
generally decreased as pH increased, whereas there was little pH effect on
the adsorption of glucose 6-phosphate by both precipitates. Sorption of
inositol hexakisphosphate to minerals surface is often stronger than
orthophosphate, but both tend to be less sorbed to those minerals in neutral
to alkaline soils (Berg and Joern, 2006; Xu et al., 2017). The presence of
humic acids may affect the amount of inositol hexakisphosphate that sorbs on
the minerals surface at lower pH values, but it cannot displace inositol
hexakisphosphate from that surface (Ruyter-Hooley et al., 2016). Moreover,
another study founds that inositol hexakisphosphate sorption in soils was
unaffected by the presence of orthophosphate, β-D-glucose-6-phosphate
or adenosine 5′-triphosphate (Berg and Joern, 2006). Following a
similar pattern, the amount of DNA bound on clay minerals such as
montmorillonite, kaolinite, hydroxyl aluminum species and variable charge
soil colloids also increased by lowering pH of solution (Khanna and Stotzky,
1992; Cai et al., 2006, 2008; Wang et al., 2009; Saeki et al., 2010). At pH < 5, protonation of the amino groups of adenine,
guanine and cytosine occurs and causes the increase of net positive charge
of DNA and electrostatic attraction between negatively charged tetrahedral
silica layer on the clay surface and DNA (Cai et al., 2006). Therefore, the
increasingly acidic pH could have increased sorption, and thus
facilitated inositol hexakisphosphates (Fig. 3c) and DNA (Figs. 3d, 7) accumulation in those soils.
In our study, we found that there was an increasing proportion of inositol
hexakisphosphates and DNA in the Po pool (% of NaOH-EDTA P) as pH
decreased, and there was predominance of inositol hexakisphosphates in
acidic, more weathered soils (Figs. 3c–d, 5j). The hierarchy of investment
for P acquisition through enzymatic activity may also be a factor that
contributed to modifications in the soil Po composition as time
passes since the onset of pedogenesis. According to Turner et al. (2018),
Turner and Haygarth (2005) and Kunito et al. (2012), P limitation may
stimulate increased phosphoesterase synthesis as a way to increase
bioavailable P by the mineralization of Po. Fungi are well known for
their capacity to secrete acid phosphatases (Rosling et al., 2016), and are
usually the predominant microorganisms in acidic natural soils; while
alkaline phosphatase and phytase genes are distributed across a broad
phylogenetic range and display a high level of microdiversity (Zimmerman et
al., 2013). Plants and microorganisms that breakdown orthophosphate diesters
need a higher investment for the P acquisition (see Turner 2008a for the
conceptual model) than orthophosphate monoesters, as they require
hydrolysis by both phosphodiesterase and phosphomonoesterase to release
available phosphate, whereas orthophosphate monoesters require only the latter (Halstead, 1964; Skujins, 1967; Tabatabai and Bremner, 1969). Although
inositol hexakisphosphates are also classified as part of orthophosphate
monoesters, they need a higher investment in Po acquisition than other
orthophosphate monoesters and diesters because they can be strongly bounded
by metal oxides, clays and organic matter (Turner et al., 2007). Organic
acids are required to desorb inositol phosphates, so that phytases can
hydrolyze them to release a free orthophosphate. This would suggest that as
soil gets more weathered, inositol hexakisphosphates may accumulate more
than DNA, and the latter more than other Po compounds.
Nonetheless, inositol hexakisphosphates can persist as a main Po
compound up to a certain point of pedogenesis, and then decline in more
weathered soils. Some authors described that inositol hexakisphosphates
declined to lower concentrations in older soils of non-tropical regions
(Turner et al., 2007, 2014), and most tropical soils had
negligible inositol hexakisphosphates contribution (e.g., Turner and
Engelbrecht, 2011). In our results, soil weathering had no effect on inositol
hexakisphosphates concentrations in mg kg-1 in non-tropical
environments (Fig. 7), but in fact, all compiled results from tropical
soils had no inositol hexakisphosphates. Inositol hexakisphosphates have
been found in very weathered soils (e.g., Oxisols), but under agricultural
management that included well-known sources for that P compound such as
plant seeds (Turner, 2006; Smernik and Dougherty, 2007; Deiss et al., 2016).
These results suggest that inositol hexakisphosphates could occur in
tropical soils under native vegetation, but they are either being rapidly
turned into bioavailable compounds (by plants and microorganisms) or inputs
of inositol hexakisphosphates, which are abundant in seeds and pollen (Raboy,
2007), are lower in lowland tropical forests compared to temperate
ecosystems (Turner and Engelbrecht, 2011). In P limited soil environments,
the acquisition of inositol hexakisphosphates may be strongly improved by
root exudates, which may increase the solubility of these compounds in soil
(Gerke, 2015). In addition, mineralization of myo-inositol hexakisphosphate
by ectomycorrhizal fungi (Chen et al., 2004) may also have contributed to its
decline in more weathered, acidic soils, due to fungi predominance in these
environments. Finally we believe that other P compounds such as DNA (Figs. 3d, 5j) and pyrophosphate (Figs. 2d, 5d) will possibly prevail in more
weathered systems from tropical regions because they are intrinsic
components of the microbial biomass (Kornberg et al., 1999). Mature soils are
known for having the microbial P as the main component of the P pool (Turner
et al., 2013).
Soil clay concentration affected both soil total Po and Pi
concentrations (Figs. 2f and 3e), but had a minor association with soil P
compounds. Recent investigations have found that organic compounds
stabilization may be mainly driven by factors that are typically minor
constituents of the clay-sized fraction by mass, but highly reactive
components. Vogel et al. (2014) showed that organic matter is preferentially
stabilized in certain hot-spot zones (i.e., rough surfaces), and that only a
limited portion of clay-sized surfaces contributed to soil organic matter
stabilization. This concept was further tested for P by Werner et al. (2017)
who found that microscale spatial heterogeneity influences P accessibility
and bioavailability in soil aggregates, depending on soil substrate and
depth. They also found that in P-rich areas of soil aggregates, the P was
predominantly co-located with Al and Fe oxides, while in low-P topsoil
aggregates, most of the P was organically bound (Werner et al., 2017). Yang
et al. (2016) also showed that only limited portions of fine mineral
surfaces contributed to soil organic matter stabilization. So, these facts
could justify the minor association between bulk soil clay concentration and
soil P compounds as observed in our study.
Soil organic matter stabilization could be facilitated in more weathered
soils by the potential increase in amorphous Al and Fe oxides (Albrecht,
1957; Walker, 1965), and consequently more reactive surface area availability
to absorb and stabilize soil organic matter; however, we did not observe a
significant overall soil weathering effect on Al and Fe oxide concentrations
(Supplement S9), suggesting that it may depend on other factors such as the
parent material or specific soil orders. Moreover, contrasting soil organic
matter responses to short-range-ordered (amorphous) Fe or Al oxides have
been found in the literature. Some investigations found a pronounced role
promoted by Al oxides (Heister 2016; Kaiser et al., 2016), whereas others
found Fe oxides as the main soil organic matter stabilizing mechanism
(Wilson et al., 2013; Catoni et al., 2016; Deiss et al., 2017). Investigations
also found no apparent relationship between soil organic matter and both Al
and Fe oxides (Cloy et al., 2014; Rumpel et al., 2015; Vogel et al., 2015).
Therefore, we could not confirm the role of Al and Fe oxides as influenced
by soil weathering stages over the soil P composition.
Climate and the soil P composition
Climatic variables exerted an important role on the soil P composition but
to a lesser extent when compared to soil variables (Fig. 6). Contradicting
what we expected, our results showed that temperatures ranging from -0.4 to
27∘C had no effect on both Pi and Po pools and
their compounds (Supplement S7 and S8). It was expected that the soil
Po concentration would decrease with increasing temperatures because
higher temperatures are optimal for the breakdown of the soil Po
compounds by the microbial biomass through phosphatase enzymes release (Hui
et al., 2013). Hui et al. (2013) confirmed that greater maximum phosphatase
activity occurred at incubation temperatures > 25 ∘C when
compared to 20 ∘C, but no differences were observed among temperatures
greater than 25 ∘C (Hui et al., 2013). Therefore, phosphatase activity
may depend on the range and magnitude of temperatures; and our results
covered a greater range of markedly lower temperatures, which may reduce
microbial activity variability even more due to a slowdown in the
microorganisms' metabolism.
In contrast, precipitation affected several variables in the soil Pi
pool. This result was also expected based on the classic paper of Walker and
Syers (1976), which suggested that pedogenesis depends predominantly on the
volume of water leached through soil. In our results, the soil total P
concentration, pH (Fig. 7), and orthophosphate proportion (Supplement S7c)
were negatively related to precipitation. As precipitation increased
(Supplement 7c) and the soil was in a higher weathering stage (Fig. 5c), the
orthophosphate proportion (% of NaOH-EDTA P) possibly decreased because
of increased leaching. However, caution needs to be used when discussing
changes in orthophosphate extracted by NaOH-EDTA for 31P NMR, because
NaOH-EDTA will preferentially extract Po rather than orthophosphate. As
such, studies that analyzed the residual P after NaOH-EDTA extraction have
shown that it is mainly composed by orthophosphate (e.g., Cade-Menun et al.,
2005). Feng et al. (2016) evaluated P fractions along a climosequence and
observed that greater precipitation (in soils with no impeded drainage)
reduced the inorganic concentration of P linked to Ca, corresponding to a
marked decline in soil exchangeable Ca and suggesting an enhanced leaching
of P along with weatherable cations. Moreover, greater soil water
availability, and consequent greater primary productivity, may have
increased the demand for P in its bioavailable form and contributed to the
orthophosphate depletion.
As the orthophosphate percentage and concentration (Supplement 7c and Fig. 7, respectively) decreased following greater precipitation, the
pyrophosphate percentage and concentration (Supplement 7d and Fig. 7,
respectively) increased suggesting that this compound predominates under
these environmental conditions. As previously described, this may be due to
the incorporation of these compounds into recalcitrant soil organic matter
(Turner et al., 2007). Moreover, given the microbial origin of pyrophosphate
and its association with the microbial P biomass (Koukol et al., 2008; Turner
and Engelbrecht, 2011; Reitzel and Turner, 2014), pyrophosphate (Figs. 7, S7d) possibly mirrored the response of the total Po (Fig. S8b) to climatic variables, which may have resulted from greater soil organic
matter accumulation following greater productivity (i.e., plants and
organisms) in these ecosystems with greater water availability. Evaluating
the P budget of the whole ecosystem, Turner et al. (2013) demonstrated the
dominance of microbial P in mature soils. Wang et al. (2014) found that
greater Po concentrations were associated with increasing biomass
production (i.e., primary production and microbial biomass) because plants
and microbes incorporate P into biomass and return it to the soil. However,
it is important to note that the majority of P in plant biomass is as
orthophosphate (e.g., Noack et al., 2012) and not as Po compounds.
However, we believe that with higher orthophosphate inputs through plant
biomass, soil Po concentrations would increase altogether with
orthophosphate P concentrations, and also at expense of soil orthophosphate
due to the greater bioavailability to plants and organisms of that latter P
compound.
Changes in vegetation are expected to occur during pedogenesis, and climatic
variables may govern magnitudes of these alterations along with soil
changes. Vitousek et al. (1995) showed that as ecosystems develop, the
pattern of P concentration in plants leaves follows a non-linear response to
time, in which lower concentrations occur at either early or late stages of
pedogenesis, and a maximum is reached at an intermediate stage of
pedogenesis. In addition, precipitation can affect the magnitude of that
maximum response (intermediate stage), where the P concentration in plant
leaves is higher in mesic gradients when compared to more wet gradients
(Vitousek et al., 1995). Moreover, as described earlier, the soil available
P, along with other climatic variables, governs maximum photosynthetic
rates, but a trend that is expected to gradually decline in more weathered
soils, due to a lower P availability (Maire et al., 2015). Phosphorus
limitation can become sufficiently intense in the late stages of ecosystem
development (also known as the retrogressive phase) to cause a decline in
forest biomass, and productivity (Wardle et al., 2004). The exception seems
to be tropical forests (Turner et al., 2007), which exhibit very diverse tree
communities on old, infertile soils (Losos and Leigh Jr., 2004). Moreover, Turner
et al. (2018) showed that in lowland tropical ecosystems, P limitation
affects individual species, but species-specific P limitation does not
translate into a community-wide response, because some species grow rapidly
on infertile soils despite extremely low P availability.
Future research priorities
Many efforts have been made to explain soil P composition during
pedogenesis; however, a clear picture on how specific plant species, plant
functional traits, and their communities can influence the soil P
composition is still lacking, especially with results obtained with 31P
NMR. For example, why are inositol hexakisphosphates not found in tropical
soils under native vegetation, i.e., is it because the rapid turnover
promoted by plants and/or organisms, or exclusively due to
lack of inputs from plants? Do the changes in forest biomass and plant
species diversity as soil P turns scarcer contribute to soil P composition
in non-tropical environments, either by inputs or P compounds consumption,
or the soil per se governs both the soil P composition and vegetation dynamics?
Therefore, we point out that studies aiming to disentangle confounding
effects among soil biotic and abiotic components, climate and vegetation are
required to enable a better understanding of soil P composition in natural
ecosystems.
Moreover, studies coupling 31P NMR with other important techniques
(e.g., Liu et al., 2013, 2015) could contribute to a better
understanding of P cycling and composition in terrestrial ecosystems. A
clear understanding of how orthophosphate species such as Fe-, Al- and
Ca-phosphate (Hesterberg, 2010; Kizewski et al., 2011) respond to pedogenesis
could be elucidated with XANES (see Prietzel et al., 2013; Hashimoto and
Watanabe, 2014). Studies that quantify specific-P-related enzymes activity
(see Turner et al., 2018) in native vegetation soils could help understand if
the hierarchy of investment for the P acquisition actually contributes to
different degrees of accumulation of inositol hexakisphosphates and DNA as
pedogenesis progresses in non-tropical environments, and if phosphatases are
leading to a rapid turnover of inositol hexakisphosphates in tropical
environments. This can be achieved through determining the presence and
abundance of microorganisms and enzymes, and how these changes affect soil P
composition. Turnover, exchange kinetics, and mineralization rates could be
assessed using isotopic techniques (see Frossard et al., 2011), and enable a
separation between different sources of P compounds, and their dynamics in
soils, organisms and plants.
Finally, we expect future research to provide results of as many soil P
compounds as they can find rather than broad compound classes only (i.e.
orthophosphate diesters and monoesters), even when compound concentrations
are low (and describe when main soil compounds are not detected), which may
enable future analyses to avoid possible confounding effects of P compounds
inside functional groups (e.g., inositol hexakisphosphates and
orthophosphate monoesters) and to make a more precise correction for
potential degraded peaks occurring during the alkaline extraction and
reading process. We also urge researchers to determine variances or standard
errors for soils with distinctive properties. Then, as stated by Stewart (2010), future analyses could use the different information provided by
studies of different scopes and quality in a meta-analytical approach.