Linking Carbon Saturation Concepts to Nitrogen Saturation and Retention
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Ecosystems
DOI: 10.1007/s10021-011-9501-3
2011 Springer Science+Business Media, LLC
Linking Carbon Saturation Concepts
to Nitrogen Saturation and Retention
Michael J. Castellano,1,2* Jason P. Kaye,2 Henry Lin,2 and John P. Schmidt3
1
Department of Agronomy, Iowa State University, 2101 Agronomy Hall, Ames, Iowa 50011, USA; 2Department of Crop and Soil
Sciences, The Pennsylvania State University, 116 ASI Building, University Park, Pennsylvania 16802, USA; 3USDA-ARS PSWRMU,
University Park, Pennsylvania 16802, USA
ABSTRACT
Recent advances in soil C saturation concepts have decreases in proportion to the amount of C already
increased our understanding of soil C storage and present in the fraction; C inputs not stabilized in
mineralization without explicit links to N retention MAOM are susceptible to rapid mineralization. We
and saturation theories. Here, we exploit soil tex- demonstrate that NH4-N stabilization in MAOM is
ture and organic matter (OM) gradients in a similar to C stabilization in MAOM and associated
Maryland, USA hardwood forest to test hypotheses with nitrification, thereby extending soil C satura-
that link soil organic C saturation with soil 15N tion theory to mineral N and linking it with N
retention and nitrification. At our site, mineral- retention and saturation theories. These data and
associated OM (MAOM) N concentrations in the concepts complement N saturation models that
silt + clay particle fraction (g MAOM-N g silt + emphasize vegetation type, N input levels, and
clay-1) were negatively correlated with the fraction microbial turnover. Incorporating the OM reten-
of NH4-N transferred to MAOM during a 3-day in tion capacity of fine mineral particles into N satu-
situ incubation (R = -0.85), but positively corre- ration theory can improve predictions of N
lated with potential net nitrification (R = 0.76). saturation rates and resolve inconsistent relation-
Moreover, the fraction of NH4-N transferred to ships between soil organic matter, texture, N min-
MAOM was negatively correlated with potential eralization, and N retention.
net nitrification (R = -0.76). Due to physico-
chemical stabilization mechanisms, MAOM is Key words: soil; texture; net; nitrification; par-
considered to be resistant to mineralization. Carbon ticulate organic matter; nitrogen retention; gross
saturation theory suggests that the proportion of nitrogen mineralization; gross nitrification.
new C inputs that can be stabilized in MAOM
INTRODUCTION and nitrate leaching (Vitousek and others 1997).
Ecosystem models typically include soil texture as
Soil nitrogen (N) mineralization can impact eco- one of several mechanistic controls on N mineral-
system processes including primary productivity ization (for example, Parton 1994). In these mod-
els, N mineralization and sand are positively
correlated because silt and clay particles stabilize
Received 18 July 2011; accepted 7 October 2011 potentially mineralizable soil organic matter
Electronic supplementary material: The online version of this article (SOM). However, empirical work has demon-
(doi:10.1007/s10021-011-9501-3) contains supplementary material, strated that N mineralization and sand concentra-
which is available to authorized users.
tion can be positively, negatively or not correlated
Author Contributions: MJC and JPK analyzed the data and wrote the
manuscript. MJC, JPK, HL, and JPS designed the study. (for example, Reich and others 1997; Cote and
*Corresponding author; e-mail: castellanomichaelj@gmail.com others 2000; Giardina and others 2001; BechtoldM. J. Castellano and others
and Naiman 2006). Negative correlations between
N mineralization and sand are often attributed to
the tendency for sand to be negatively correlated
with total SOM. This interpretation is based on the
widespread positive correlation between SOM and
N mineralization (Booth and others 2005).
These inconsistent relationships between soil
texture and N mineralization can be reconciled by
incorporating C saturation theory (Stewart and
others 2007) into conceptual models of N retention
and saturation (Vitousek and others 1997; Aber
and others 1998). Carbon saturation theory is
based on the capacity of silt and clay particles to
physico-chemically stabilize C, protecting it from Figure 1. Responses of soil organic matter and nitrogen
mineralization (Sollins and others 1996). Carbon dynamics (y axis) to increasing concentrations of mineral
saturation theory demonstrates that the capacity of associated organic matter (MAOM) nitrogen in the
silt and clay to stabilize C is limited; when the silt + clay soil fraction (g MAOM-N g silt + clay-1; x axis).
capacity of silt and clay to stabilize C becomes sat- The absolute capacity for MAOM-N storage (g MAOM-N g
urated, additional C inputs cannot be stabilized and soil-1) is finite (Hassink 1997). However, as MAOM-N
remain available for rapid mineralization. concentrations increase, the amount of N inputs trans-
Because soil C and N concentrations are well ferred to MAOM are expected to decrease exponentially.
correlated with strong biological links and consis- Nitrogen inputs that are not transferred to MAOM should
tent stoichiometry (Cleveland and Liptzin 2007), C remain in actively cycled POM fractions. As POM accu-
mulates, the C/N ratio of POM is expected to decrease due
saturation concepts should apply to N stabilization
to carbon mineralization. Accordingly, microbial N de-
and mineralization. The absolute capacity of mands are increasingly met and N mineralization in-
silt + clay to stabilize C and N is a positive linear creases. Although not indicated in this model, net
function of silt + clay content (Hassink 1997). nitrification could continue to increase after MAOM-N
However, the proportion of new organic matter saturation as a result of continued N inputs, soil acidifi-
(OM) inputs that can be stabilized by the silt + clay cation, and vegetation mortality.
fraction decreases in proportion to the amount of
OM already present in the fraction. Accordingly, as which occurs primarily through rapid microbial
the capacity for silt + clay to stabilize OM ap- transformation of inorganic N to DON (Norton and
proaches zero (that is saturation), the efficiency of Firestone 1996; Zogg and others 2000; Kaye and
OM stabilization decreases and a greater proportion others 2002). However, as MAOM accumulates,
of OM inputs accumulate in the easily mineraliz- MAOM concentrations in the silt + clay fraction
able particulate OM (POM) fraction (Stewart and will increase toward saturation and the efficiency
others 2007). of DON adsorption in MAOM should decrease. In
These recent advances in C saturation theory response, N and C inputs that are not retained in
combined with the tight coupling of C and N in stable MAOM pools will accumulate in the actively
SOM (Kaye and others 2002; Cleveland and Liptzin cycling labile POM fraction (Six and others 2002).
2007; Stewart and others 2007) led us to develop a As N and C concentrations in unprotected POM
new conceptual model linking OM concentrations increase, gross N and C mineralization should in-
in the silt + clay mineral-associated OM (MAOM) crease, but POM C/N ratios should decrease be-
soil fraction and N retention capacity (Figure 1). cause N mineralization is partially balanced by N
The premise of this model is that soil texture and immobilization (while mineralized C is lost from
OM inputs should interact to affect MAOM con- the soil system as CO2). High gross N mineralization
centrations in the silt + clay fraction and thus the and lower POM C/N ratios should satisfy plant and
capacity for N retention and the rate of N satura- heterotrophic N demand, leading to a final predic-
tion. When MAOM concentrations in the silt + tion, that net nitrification should increase with
clay fraction are low and far from saturation, N increasing MAOM concentrations on silt + clay
retention in MAOM is expected to be high. In this particles.
case, dissolved organic N (DON) compounds are To test our conceptual model, we measured OM
rapidly adsorbed to silt + clay soil particles, inhib- fractions, gross and net ammonification and nitri-
iting N mineralization. This mechanism also applies fication, and 15NH4-N transfer to SOM in A and B
to the retention of inorganic N inputs in SOM, soil horizons across a 0.5 ha temperate forest.Nitrogen Saturation and Soil Texture The site was well suited for isolating the effect of Soil texture was determined with the method MAOM on N cycling because it exhibits broad soil presented by Kettler and others (2001) on one texture gradients, but other factors that may affect composite sample derived from the five soil cores at N cycling (climate, vegetation, and stand age) did each location. Bulk density was measured for each not vary. Based on predictions of C saturation core using the mass of dry soil (90%). We one K15NO3-injected soil core were extracted for located eight sample locations in the upland, eight NH4 and NO3 by reciprocal shaking for 2 h in a 2 M locations on the hill slope, and eight locations to- KCl solution at a m/v ratio of 1:5 (soil:KCl). After ward, but above, the floodplain (n = 24). Our soil shaking, the solution was filtered (pre-leached samples did not exhibit redoximorphic features or Whatman 1) for determination of NH4-N and NO3- alluvial discontinuities. In August 2008, we ex- N on a flow injection spectrophotometer. The tracted five replicate A horizon cores at all 24 sample remaining 15NH4Cl-injected soil core and K15NO3- locations and five replicate B horizon soil cores at 16 injected soil core were left in the field to incubate of the 24 locations using a 15 cm deep 9 5 cm for 3 days, and then extracted using the above diameter soil core sampler and butyrate liners (that procedure. We followed laboratory procedures and is, 200 soil cores sampled in butyrate liners). Sam- calculated gross N cycling rates as well as NH4 and pling of B horizons at the eight locations at the bot- NO3 consumption rates as presented in Hart tom of the catena was prevented due to deep (1994). Unlike isotope dilution estimates of gross (>30 cm) B horizons that exhibited redoximorphic mineralization, gross consumption rates are sus- features. Soil from the 200 cores was used for all ceptible to overestimation due to substrate stimu- analyses. In a few locations, the A horizon was less lation (Hart 1994). Solid sample C and N than 15 cm deep; in these situations we removed the concentrations and N isotope ratios were deter- B horizon portion from the soil core prior to analyses. mined at the University of California Stable Isotope
M. J. Castellano and others
Facility (Davis, CA) with an elemental analyzer Dried samples were analyzed for total C, total N, and
15
interfaced to an isotope ratio mass spectrometer. N APE. The amount of 15NH4-N that was retained
Total soil C and N are the means of the five soil core in MAOM and POM insoluble soil organic N frac-
samples from each sample location. Isotope ratios tions after the 3-day incubation was determined
of NH4-N and NO3-N were also determined by this with two-pool mixing models. 15NH4-N that was not
procedure after diffusion of each species to acidified recovered in OM was determined by subtraction and
filter paper (Stark and Hart 1996). Using 2 mm includes 15N tracer extracted in the KCl (organic and
sieved, air-dried soil from the ambient cores that inorganic) or lost to gaseous species during the
were not amended with N, we conducted a 30-day incubation (although significant denitrification was
potential net N mineralization analysis in the lab unlikely due to aerobic soil conditions).
(Binkley and Hart 1989). We extracted one sub- We compared the amount of 15NH4-N transferred
sample for NH4 and NO3 with the above procedure to POM and MAOM with three procedures. First,
at the onset of the incubation. A paired subsample using the 15NH4-N tracer as a label for the NH4 pool
was maintained at 60% water holding capacity and over the 3-day incubation, we compared the frac-
20C for 30 days and then extracted for NH4 and tion of the total 15 min-extracted NH4-N pool that
NO3. The net change in NH4 and NO3 concentra- was transferred to POM and MAOM on the third
tions from 0 to 30 days were interpreted as day. We compared fractions because a different
potential net ammonification and nitrification. mass of 15NH4-N was added to A and B horizons to
Subsamples of 2 M KCl-extracted soil from the permit detection of the isotope while minimizing
3-day 15NH4Cl field incubations were physically potential effects of the added NH4 on N cycling.
fractionated into sand-associated POM and silt + This measure provides a comparison of the effi-
clay MAOM (Moran 2006) so that we could ciency of NH4 transfer to POM and MAOM because
determine the concentration of POM- and MAOM- NH4 concentrations are chronically lower in B
associated C and N as well as the 15 min to 3-day horizons (50%) and vary across the texture and
transfer of NH4 (via the 15 min 15NH4Cl label) to OM gradients at our site (unpublished data). Sec-
POM and MAOM insoluble organic N pools. The ond, we calculated the absolute mass of NH4-N per
POM and MAOM-NH4 sinks represent fractions of mass of dry soil (NH4-N kg-1 dry soil) transferred to
NH4 consumption. We selected soils from the 3-day POM and MAOM pools between the 15 min and 3-
15
NH4Cl field incubations from the four 15N addi- day extractions. Third, we calculated the absolute
tions because (1) time did not permit analysis of mass of NH4-N per mass of MAOM-N (NH4-N kg-1
15
KNO3 and 15NH4Cl, (2) NH4 is more rapidly MAOM-N) transferred to POM and MAOM pools
transferred to SOM than NO3 in our own pre- between the 15 min and 3-day extractions to iso-
liminary data and in many other N retention late the effect of MAOM on N retention. Although
studies, and (3) 15NH4 transfer generally increases we added 0.15 mg more to NH4-N to the A hori-
from minutes to days because it is biologically zons, this difference represented a small fraction of
mediated (Fitzhugh and others 2003; Fricks and total NH4-N pools (Nitrogen Saturation and Soil Texture
linear and nonlinear exponential correlations; for correlated with soil properties when calculated as:
these analyses, we only used log10 transformation (1) a fraction of the total NH4-N pool extracted at
when required to meet analytical assumptions. The 15 min; (2) the absolute mass of the NH4-N pool
model fit with the lowest total corrected sum of extracted at 15 min per kg of dry soil; and (3) the
squares is reported and untransformed data are absolute mass of the NH4-N pool extracted at
displayed. Dependent variables included POM 15 min per kg of MAOM-N.
concentrations in the total soil, POM C/N ratio,
gross N mineralization, potential net N minerali- Paired Comparisons of Soil Horizons
zation, and the transfer of 15NH4-N to POM and
Paired t test comparisons between horizons
MAOM soil fractions after 3 days in situ field
(N = 16) demonstrated that sand, silt, and clay
incubation. Independent variables included sand
concentrations did not differ between the A and B
concentration, POM-C and N kg sand-1, POM-C
horizons. However, concentrations of total N,
and N kg soil-1, POM-C/N ratio, MAOM-C and
POM-N, and MAOM-N were all higher in the A
N kg silt + clay-1, MAOM-C and N kg soil-1, and
horizon (Table 1). Similarly, gross and net ammo-
MAOM-C/N ratio. We sampled A and B soil hori-
nification and nitrification as well as gross NH4
zons to maximize variation in soil properties so that
consumption rates were higher in the A horizon
we could identify widespread controls on N min-
(Table 1).
eralization and retention. Therefore, we focused
A smaller fraction of NH4-N was transferred to A
analyses on soil properties that continuously scaled
horizon MAOM compared to B horizon MAOM
N cycling processes across both horizons. Our
(Table 1), but the absolute mass of NH4-N trans-
sample sizes (40) coupled with the large number of
ferred to MAOM (mg NH4-N transferred to MAOM
independent variables precluded the use of multi-
kg-1 soil) did not differ between horizons. In con-
ple regression due to a loss of statistical power. We
trast, the fraction of NH4-N that was transferred to
report the probability of type I error to 0.00005 due
POM did not differ across horizons, but a larger
to the number of independent statistical analyses.
absolute mass of NH4-N was transferred to A hori-
zon POM (mg NH4-N kg-1 soil; Table 1). In both A
RESULTS and B horizons, a larger fraction and absolute mass
of NH4-N was transferred to MAOM than POM
Soil Properties and Scalars of Nitrogen (Table 1).
Dynamics
Sand concentrations ranged from 490 to 940 g kg-1
Nitrogen Dynamics Across Soil Horizons
soil and C/N ratios ranged from 15 to 26 (Table 1).
Across the 24 A horizon and 16 B horizon samples Sand concentrations were not correlated with total
(N = 40), total soil C and N were well correlated N or C/N ratios (Online Appendix). However, sand
(R = 0.98, P < 0.0001) as were MAOM-C and concentrations were positively correlated with
MAOM-N (R = 0.98, P < 0.0001) and POM-C and MOAM-N concentrations in the silt + clay fraction
POM-N concentrations (R = 0.98, P < 0.0001). (although the slope of this relationship differed
Therefore, we report all data in relationship to N between horizons; Figure 2A). Concentrations of
rather than C because N is the focus of this paper. MAOM-N in the silt + clay fraction were positively
Bulk density was not correlated with sand; there- correlated with concentrations of POM-N in the
fore, we report all N cycling processes as a mass of total soil (Figure 2B) and negatively correlated
N kg-1 soil (particles < 2 mm). with POM C/N ratios (although the slope of this
Gross and net ammonification and nitrification relationship differed between horizons; Figure 2C).
rates, when calculated per mass of MAOM-N Across soil horizons, total soil N, POM-N, and
(mg N kg-1 MAOM-N), were not correlated with MAOM-N were well correlated with gross and net
soil properties. However, gross and net ammonifi- ammonification and nitrification rates (Figure 3;
cation and nitrification rates, when calculated per Online Appendix). In contrast, correlations be-
mass of soil, were correlated with many soil prop- tween sand, C/N ratios and gross and net ammo-
erties (Online Appendix); thus, we report gross and nification and nitrification rates were relatively
net ammonification and nitrification rates per mass poor (Online Appendix). Within-horizon correla-
of soil rather than MAOM-N. In contrast to gross tions between sand, C/N ratios and gross and net
and net ammonification and nitrification rates, ammonification and nitrification rates were poor
NH4-N transferred to insoluble POM and MAOM when compared to across horizon correlations be-
pools at the end of the 3-day incubation was tween total soil N, POM-N, MAOM-N, and grossTable 1. Soil Properties
Soil properties Paired A and B horizon locations Unpaired A horizon locations
A horizon B horizon Paired t test A horizon
Minimum Maximum Mean Minimum Maximum Mean P Minimum Maximum Mean
Sand (g kg-1) 530 940 700 490 920 700 0.6604 670 930 770
M. J. Castellano and others
Silt (g kg-1) 10 370 210 40 370 200 0.2052 10 220 150
Clay (g kg-1) 30 160 90 40 140 100 0.3966 60 110 80
Bulk density (g cm-3) 0.58 1.11 0.83 0.66 1.56 1.08Nitrogen Saturation and Soil Texture
10 2.0 45
A B C
MAOM-N (g kg silt + clay -1)
POM-N (g kg total soil -1)
8 40
1.5
POM C/N Ratio
6 35
1.0
4 30
0.5
2 25
0 0.0 20
400 500 600 700 800 900 1000 0 2 4 6 8 10 0 2 4 6 8 10
Sand (g kg soil-1) MAOM-N (g kg silt + clay -1) MAOM-N (g kg silt + clay -1)
Figure 2. Black triangles indicate A horizon samples and gray circles indicate B horizon samples. Solid lines indicate the
model fit. A Sand concentration and mineral-associated organic matter nitrogen concentration in the silt + clay soil
fraction (MAOM-N) were positively correlated within soil horizons. For the A horizon y = 0.02x - 7.28; R = 0.78,
P < 0.0001 and for the B horizon y = 0.01x + -1.8, R = 0.68, P = 0.0034). B MAOM-N concentration in the silt + clay
soil fraction was positively correlated with particulate organic matter N (POM-N) in the total soil (Log(y) = 1.34logx +
-1.13, R = 0.81, P < 0.0001). Data were log10 transformed prior to analysis to correct for heteroscedasticity, thus, no
regression line is displayed. C MAOM-N concentration in the silt + clay soil fraction was negatively correlated with POM
C/N ratio within horizons. For the A horizon y = 23.33 + 20.65e(-0.38x); R = 0.72; P = 0.0005. For the B horizon
y = 18.57 + 33.49e(-0.79x); R = 0.68; P = 0.0165.
and net ammonification and nitrification rates The absolute mass of NH4-N transferred to
(Online Appendix). Gross NH4 consumption was insoluble MAOM calculated per mass of MAOM-N
positively correlated with total soil N, POM-N (g NH4-N transferred to MAOM kg-1 MAOM-N),
and MAOM-N. However, gross NO3 consumption was negatively correlated with total N, POM-N, and
was not correlated with soil properties (Online MAOM-N (Online Appendix). These relationships
Appendix). were similar and best fit by an exponential decay
The fraction of NH4-N that was transferred to equation. The mass of NH4-N transferred to POM
insoluble MAOM during the 3-day in situ incuba- and the mass of NH4-N that was not recovered in
tion was negatively correlated with sand, total N, insoluble OM (kg-1 MAOM-N) were relatively
MAOM-N, and POM-N (Figure 4A; Online poorly correlated with soil properties (Online
Appendix). These relationships were all similar and Appendix).
best fit by exponential decay equations. Of these Gross and net ammonification and nitrification
relationships, the fraction of NH4-N transferred to rates were negatively correlated with the fraction of
MAOM was most closely correlated with MAOM-N NH4-N transferred to MAOM but positively corre-
concentrations in silt + clay (Figure 4A). In con- lated with the fraction and absolute mass of NH4-N
trast, the fraction of NH4-N that was retained in that was not recovered in POM or MAOM (mg NH4-
insoluble POM after the 3-day in situ incubation N unrecovered in POM or MAOM kg-1 soil;
was positively related to POM-N in the total soil Figure 5; Online Appendix). Gross and net ammo-
and this relationship was best fit by an exponential nification and nitrification rates were positively
rise to maximum equation (Figure 4B). The frac- correlated with the absolute mass of NH4-N trans-
tion of NH4-N that was not recovered in insoluble ferred to POM (mg NH4-N transferred to POM kg-1
OM (POM or MAOM) was positively correlated soil; Online Appendix). Gross and net ammonifica-
with sand, total N, POM-N, and MAOM-N. These tion and nitrification rates were negatively corre-
relationships were all best fit by an exponential rise lated with the absolute mass of NH4-N that was
to maximum equation. Of these relationships, the transferred to MAOM (mg NH4-N in MAOM kg-1
fraction of NH4-N that was not recovered in MAOM-N). These relationships were best fit by
insoluble OM was most closely correlated with exponential decay equations (Online Appendix).
MAOM-N concentrations in the silt + clay fraction
(Figure 4C). In comparison to the fraction of NH4-N
DISCUSSION
transferred to OM, relationships between the
absolute mass of NH4-N that was transferred to, or Data from this site suggest that concentrations of
not recovered in OM were similar and significant MAOM on silt and clay particles affect N minerali-
but not as strong (Online Appendix). zation and retention. Transfer of NH4-N to insolubleM. J. Castellano and others
4 4
A C
Potential Net Ammonification
3 3
Gross Ammonification
(mg N kg soil d )
-1 -1
(mg N kg soil d )
-1 -1
2 2
1 1
0 0
B D
Potential Net Nitrification
3 3
(mg N kg soil d )
(mg N kg soil d )
Gross Nitrification
-1 -1
-1 -1
2 2
1 1
0 0
0 2 4 6 8 10 0 2 4 6 8 10
MAOM-N (g N kg silt + clay -1) MAOM-N (g N kg silt + clay -1)
Figure 3. Across both soil horizons, gross and net ammonification and nitrification rates were positively correlated with
mineral-associated organic matter nitrogen (MAOM-N) concentrations in the silt + clay fraction: A Log(y) = 0.89-
log(x) + -0.48; R = 0.63, P < 0.0001. B Log(y) = 0.62 log(x) + -0.90; R = 0.70, P < 0.0001. C Log(y) = 0.92-
log(x) + 0.84; R = 0.64, P < 0.0001. D Log(y) = 1.09log(x) + 0.74; R = 0.76, P < 0.0001. Data were log10 transformed
prior to analysis to meet analytical assumptions. Original data are displayed; thus, models are not shown. Black triangles
indicate A horizon samples and gray circles indicate B horizon samples.
MAOM was negatively correlated with MAOM 2007). Using a particle size gradient and soil hori-
concentrations in silt + clay (Figure 4; Online zons, we maximized variation in OM stabilization
Appendix) whereas gross and net ammonification capacity and inputs to provide an initial test of our
and nitrification were positively correlated with model (Table 1; Figure 1), and several key predic-
MAOM concentrations in silt + clay (Figure 3). tions were supported. Concentrations of MAOM in
Moreover, the transfer of NH4-N to MAOM was the silt + clay particle fraction and concentrations
negatively correlated with gross and net ammonifi- of POM in the total soil were positively correlated
cation and nitrification (Figure 5; Online Appen- (Figure 2B). This pattern is consistent with C sat-
dix). Across the texture and SOM gradients at our uration principles that suggest increasing MAOM-C
site, POM, MAOM, and total N were consistently concentrations in fine particles will lead to an
better correlated with N dynamics than texture or accumulation of POM-C and an increase in C
C/N ratios (Figures 3, 4, 5; Online Appendix). mineralization (Stewart and others 2007; Gulde
and others 2007). Our data extend this relationship
to N by demonstrating that increasing MAOM
Conceptual Model Evaluation
concentrations are also coincident with an increase
Our conceptual model for N retention and miner- in POM-N and a decrease in POM C/N ratios
alization is based on the capacity of fine soil parti- (Figure 2C). Based on C saturation theory and
cles to physico-chemically stabilize C and N inputs. these data, increasing MAOM concentrations
The capacity of fine soil particles to stabilize OM is should also increase N mineralization. Indeed, N
affected by particle size and the cumulative amount mineralization was positively correlated with
of OM inputs (Hassink 1997; Stewart and others MAOM (Figure 3).Nitrogen Saturation and Soil Texture
Fraction of NH 4 -N
Fraction of NH4 -N retained in MAOM Fraction of NH4 -N retained in POM Unrecovered in POM or MAOM
1.0
A B C
0.8
0.6
0.4
0.2
0.0
0 2 4 6 8 0.0 0.5 1.0 1.5 0 2 4 6 8 10
MAOM-N (g Nitrogen kg silt + clay -1 ) POM-N (g N kg soil-1 ) MAOM-N (g Nitrogen kg silt + clay -1 )
Figure 4. Across both horizons, the fraction of the initial NH4-N pool that was transferred to insoluble organic matter or
not recovered in insoluble organic matter during a 3-day in situ incubation: A The fraction of NH4-N transferred to
insoluble silt + clay mineral associated organic matter (MAOM) was most closely correlated with MAOM-nitrogen
(MAOM-N) concentrations in silt + clay (y = 0.09 + 0.42e(-0.51x); R = 0.82; P < 0.0001). B The fraction of NH4-N
transferred to insoluble POM was most closely correlated with POM nitrogen (POM-N) concentrations in the total soil
y = 0.05(1 - e(-15.15x)); R = 0.50, P = 0.0010). C The fraction of NH4-N unrecovered in insoluble organic matter (POM or
MAOM) was most closely correlated with MAOM-N concentrations in silt + clay y = 0.81(1 - e(-1.26x)); R = 0.65,
P < 0.0001). Solid lines indicate model fits. Fractions displayed across the three panels sum to 1.0. Black triangles indicate A
horizon samples and gray circles indicate B horizon samples.
The positive correlations between gross and net tent with research that demonstrates ecosystems
ammonification and nitrification and MAOM with high N retention capacity typically have low
(Figure 3) suggest that retention of N inputs in nitrification whereas ecosystems with low N
MAOM decreases with increasing MAOM concen- retention capacity typically have high nitrification
trations in the silt + clay fraction. We explicitly (Emmett and others 1998). Our data advance large
examined this process by measuring 15NH4-N scale across-ecosystem research by demonstrating:
transfer to insoluble MAOM during a 3-day in situ (1) how N mineralization–immobilization processes
incubation (Figures 4, 5; Online Appendix). Nega- can be affected by biophysical soil properties and
tive correlations between 15NH4-N transfer to (2) significant variation in these processes can oc-
MAOM (by fraction and mass) and concentrations cur at a relatively small scale within vegetation
of MAOM in silt + clay implicate a direct effect of communities and N deposition amounts.
MAOM concentrations on N retention (Figure 4; In contrast to our conceptual model predictions
Online Appendix). In fact, the fraction of 15NH4-N (Figure 1), increases in POM concentrations and
transfer to MAOM and the absolute mass of 15NH4- gross and net ammonification and nitrification
N transfer to MAOM per kg of soil were also neg- rates were linearly, rather than exponentially,
atively correlated with concentrations of MAOM in correlated with concentrations of MAOM in
the total soil (Online Appendix). Similar, but more silt + clay (Figures 2B, 3). These data are inconsis-
variable, negative exponential relationships be- tent with soil carbon saturation theory that sug-
tween NH4-N transfer to SOM and total N have gests POM-C concentrations increase exponentially
been observed (Johnson and others 2000). with increasing MAOM concentrations in silt +
The ability of a soil to rapidly immobilize mineral clay (Stewart and others 2007). However, small
N inputs into insoluble OM appears to be an portions of exponential relationships can appear
important control on gross and net ammonification linear. It is possible that the range of MAOM con-
and nitrification rates. We observed gross and net centrations in our experiment was not sufficient to
nitrification to be negatively correlated with 15NH4- observe the broader relationships between these
N transfer to MAOM and positively correlated with variables. Figure 2A indicates that silt + clay parti-
15
NH4-N that was not transferred to insoluble OM cles at our site were not saturated with MAOM
(Figure 5; Online Appendix). Our data are consis- (C-saturated) because the increase in MAOM withM. J. Castellano and others
Figure 5. The fraction of
1.4 A C NH4-N that was retained
in insoluble mineral
(mg N kg-1 soil day -1)
1.2
Gross Nitrification
associated organic matter
(MAOM) during a 3-day
1.0
in situ incubation was
negatively correlated with
0.8
gross nitrification (A)
y = 1.11e(-7.95x);
0.6
R = 0.69; P < 0.0001 and
potential net nitrification
0.4
(B) y = 2.16e(-5.55x);
R = 0.78; P < 0.0001.
0.2
The fraction of NH4-N
that was not recovered in
0.0
B D insoluble POM or MAOM
1.4
was positively correlated
with gross nitrification
Potential Net Nitrification
1.2
(mg N kg-1 soil day -1)
(C) y = 0.01e(7.80x);
1.0 R = 0.69; P < 0.0001 and
potential net nitrification
0.8 (D) y = 0.01e(5.41x);
R = 0.74; P < 0.0001.
0.6 Solid lines indicate model
fit, Black triangles indicate
0.4 A horizon samples, and
gray circles indicate B
0.2 horizon samples.
0.0
0.0 0.1 0.2 0.3 0.4 0.5 0.5 0.6 0.7 0.8 0.9 1.0
Fraction of NH 4-N transferred to MAOM Fraction of NH 4-N Unrecovered in OM
sand was linear rather than asymptotic at a maxi- NH4-N to DON and subsequent sorption to silt and
mum mass of MAOM-N kg-1 silt + clay (Stewart clay particles was likely the dominant mechanism
and others 2007). Nevertheless, the fraction of affecting NH4-N transfer to OM at our site. In fact,
NH4-N transferred to MAOM-N appears to reach a recent chemical characterization of SOM indicates
minimum asymptote at around 10% (Figure 4A). stable MAOM pools may be disproportionately
These data may suggest MAOM-N concentrations comprised of microbial-derived N compounds
in the silt + clay fraction are approaching satura- (Grandy and Neff 2008).
tion because the rate of N transfer to MAOM Nevertheless, abiotic mechanisms of NH4-N
should decrease at an increasingly slower pace as transfer to insoluble OM are included in our mea-
MAOM accumulates (Figure 1; Stewart and others surements. It is uncertain if abiotic N retention will
2007). The rate of exponential decline in N transfer follow our conceptual model. Some evidence sug-
to MAOM in our conceptual model may be too gests abiotic N retention may not be affected by soil
shallow (Figure 1). N status (Johnson and others 2000). However, re-
The accumulation of MAOM-N primarily occurs ports of greater total inorganic N immobilization in
through sorption of DON compounds to silt and paired sterilized versus live soil laboratory incuba-
clay particles (Sollins and others 1996). Although tions suggest microbes effectively compete with
the transfer of NH4-N to insoluble OM could differ abiotic N sinks (Fitzhugh and others 2003; Fricks
from sorption of DON, laboratory and field research and others 2009; Lewis 2011). Therefore, some
indicate that biotic processes dominate NH4-N relationship between soil N status (microbial N
transfer to insoluble OM through microbial trans- demand) and abiotic N retention could be ex-
formation of NH4-N to DON (Norton and Firestone pected. Mineralogy is also an important soil prop-
1996; Barrett and Burke 2000; Fitzhugh and others erty affecting DON sorption (Kaiser and Zech 2000)
2003). Accordingly, microbial transformation of although we did not explore these relationships.Nitrogen Saturation and Soil Texture
Soil Texture and Nitrogen Saturation high regardless of texture differences (Figures 1, 4;
Theory Stewart and others 2007). In these situations when
N stabilization is close to 100% efficient, properties
Current N retention conceptual models do not that are positively associated with N cycling and
explicitly include the capacity for silt + clay parti- silt + clay (for example, water content) might be
cles to stabilize mineral N inputs. However, more important factors affecting N cycling than
empirical evidence demonstrates that ecosystems texture.
differ in absolute capacity for N retention and the
rate of N saturation (Aber and others 1998; Emmett The Role of Subsoils
and others 1998; Magill and others 2000). For
example, a 90-year-old forest on sandy soils within Conventional scalars of N mineralization and
the Michigan Gradient study exhibited N saturation retention, including C/N ratios and total organic C
symptoms immediately after N additions (30 kg (Lovett and others 2002; Booth and others 2005),
NO3-N ha-1 y-1; Pregitzer and others 2004). In could not characterize the wide range of gross and
contrast 50- and 70-year-old forests at the Harvard net ammonification and nitrification rates across
Forest exhibited relatively few symptoms of N sat- surface and subsoils at our site (Online Appendix).
uration after 9 years of N additions (50 kg In contrast, the full range of gross and net ammo-
NH4NO3-N ha-1 y-1; Magill and others 2000). nification and nitrification rates observed at our site
Initial MAOM concentrations in the silt + clay soil was scaled across both horizons by individual OM
fraction could partially explain the differences in fractions (Figure 3). Total organic C is often posi-
the absolute capacity for N retention and the rate at tively correlated with N retention in A horizons
which different ecosystems experience N satura- due to high microbial activity and biomass (Na-
tion, especially after N additions (Figure 1). delhoffer and others 1999; Barrett and others 2002;
Our conceptual model and results complement Booth and others 2005). However, at our site, B
another recent conceptual model of N saturation horizons with low total organic C retained a similar
that distinguishes between ‘‘capacity’’ N saturation, absolute mass of 15NH4-N as C-rich A horizon soils,
which occurs when N sinks are zero, and ‘‘kinetic’’ and a larger fraction of 15NH4-N in MAOM (Ta-
N saturation, which occurs when N input rates ble 1; Figure 4). These results build on a growing
exceed sink strength (Lovett and Goodale 2011). body of research that demonstrates subsoils can
Relative C saturation (MAOM/MAOMMaximum) provide a significant mineral N sink (for example,
should be considered as a potential factor affecting Idol and others 2003; Dittman and others 2007). As
N sink strength and this sink could be particularly OM and mineral N inputs are greatest in surface
strong in the subsoil. Indeed, concentrations of soils, our data suggest that subsoils will become
MAOM can affect capacity and kinetic N saturation progressively more important N sinks as surface soil
processes. Soil texture can affect capacity N satu- silt + clay fractions accumulate N. Consistent with
ration whereas MAOM concentrations in silt + clay pedogenic development, the capacity of soils to
can affect kinetic N saturation. Soils with high stabilize OM should initially saturate in surface
silt + clay contents should have high N retention soils; afterward, saturation would be expected to
capacity; soils with low MAOM concentrations in progress downward through the soil profile.
the silt + clay fraction should have the ability to
rapidly immobilize large amounts of N. However,
the maximum rate of N immobilization should
Conclusions
decrease exponentially as MAOM accumulates By demonstrating that the capacity for silt + clay
(Figures 1, 4A). soil fractions to stabilize mineral NH4-N inputs is
Based on these N retention processes and sinks, similar to the capacity of silt + clay fractions to
our conceptual model (Figure 1) can reconcile stabilize organic C inputs (Figure 4), we extend C
inconsistent relationships between soil texture and saturation theory to mineral N and link it with N
N mineralization. Positive relationships between retention and saturation theories. As the global N
sand and N mineralization may be due to MAOM cycle accelerates, the capacity for ecosystems to
saturation in sandy soils. In contrast, negative retain N inputs will become an increasingly
relationships between sand and N mineralization important ecosystem service. Across a wide array of
may be limited to soils with low MAOM concen- sites, research shows that soils are the dominant
trations in silt + clay (Figure 1). In such soils with a sink for N inputs. However, there is substantial
large saturation deficit, the proportion of OM in- variation among sites. Our research leads to a new
puts that are stabilized in MAOM would be very interpretation of that variation—soil texture,M. J. Castellano and others
through its influence on the capacity to stabilize OM, stand age and soil texture in the boreal mixedwood. Soil Biol
can explain a large fraction of variation in soil N Biochem 32:1079–90.
retention. A mechanistic understanding of soil tex- Dittman JA, Driscoll CT, Groffman PM, Fahey TJ. 2007.
Dynamics of nitrogen and dissolved organic carbon at the
tural controls on N saturation complements prior Hubbard Brook experimental forest. Ecology 88:1153–66.
models emphasizing N input levels, vegetation type, Emmett BA, Boxman D, Bredemeier M, Gundersen P, Kjonaas
and microbial turnover (Aber and others 1998; OJ, Moldan F, Schleppi P, Tietema A, Wright RF. 1998. Pre-
Lovett and others 2004; Lovett and Goodale 2011). dicting the effects of atmospheric nitrogen deposition in
The relative importance of soil texture and MAOM conifer stands: evidence from the NITREX ecosystem-scale
controls on N retention will be maximized when experiments. Ecosystems 1:352–60.
other ecosystem properties that affect N dynamics, Fitzhugh RD, Lovett GM, Venterea RT. 2003. Biotic and abiotic
immobilization of ammonium, nitrite, and nitrate in soils
such as vegetation and N inputs, are similar (Lovett developed under different tree species in the Catskill Moun-
and others 2004). Nevertheless, the fundamental tains, New York, USA. Glob Change Biol 9:1591–601.
relationship between MAOM concentrations in the Fricks B, Kaye J, Seidel R. 2009. Abiotic nitrate retention in
silt + clay fraction and N retention that operated agroecosystems and a forest soil. Soil Sci Soc Am J 73:
across our within-site texture gradient could operate 1137–41.
under the same mechanisms to explain across site Giardina CP, Ryan MG, Hubbard RM, Binkley D. 2001. Tree
variation. Nitrogen addition studies can be exploited species and soil textural controls on carbon and nitrogen
mineralization rates. Soil Sci Soc Am J 65:1272–9.
to test the broad applicability of our results; sites with
Grandy AS, Neff JC. 2008. Molecular C dynamics downstream:
little or no capacity for OM stabilization in the the biochemical decomposition sequence and its impact on
silt + clay soil fraction would be expected to exhibit soil organic matter structure and function. Sci Total Environ
N saturation symptoms. 404:197–307.
Gulde S, Chung H, Amelung W, Change C, Six J. 2007. Soil
ACKNOWLEDGMENTS carbon saturation controls labile and stable carbon pool
dynamics. Soil Sci Soc Am J 72:605–12.
Discussions with David Lewis improved the design Hart SC, Stark JM, Davidson EA, Firestone MK. 1994. Nitrogen
of this experiment. This project was funded by NSF mineralization, immobilization, and nitrification. In: Methods of
(DEB Dissertation Improvement Grant-090999) Soil Analysis, Part 2—microbiological and biochemical proper-
and NOAA (National Estuarine Research Reserve) ties. Madison (WI): SSSA Book Series, no. 5. pp 985–1018.
to MJC. JPK was funded by NSF DEB 0816668. Hassink J. 1997. The capacity of soils to preserve organic C and N
by their association with clay and silt particles. Plant Soil
MJC was funded by USDA (National Needs 2005- 191:77–87.
38420-15774) to HL. Idol TW, Pope PE, Ponder F. 2003. N mineralization and N up-
take across a 100-year old chronosequence of upland hard-
wood forests. Forest Ecol Manag 176:509–18.
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