Potential nitrogen immobilization in grassland soils across a soil
organic matter gradient
J.E. Barrett
a,*, I.C. Burke
b,ca
Environmental Studies Program, HB, 6182 Steele Hall, Dartmouth College, Hanover, NH 03755, USA b
Department of Forest Sciences, Colorado State University, Fort Collins, CO 80523, USA c
Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO 80523, USA
Accepted 24 April 2000
Abstract
Nitrogen additions to grasslands have increased historically and are likely to continue increasing given the current and projected land use patterns, urbanization and fossil fuel use. Nitrogen retention in both grassland and forest soils is often limited by organic substrate availability, but few studies have explicitly tested the relationship between soil carbon content and nitrogen retention. We initiated a laboratory study to directly assess the in¯uence of soil organic matter content on potential nitrogen immobilization and turnover for soils collected from across a temperature gradient in the Great Plains region of the U.S. We measured soil organic carbon, total nitrogen and carbon±nitrogen ratios and estimated carbon mineralization and net nitrogen mineralization over 5- and 30-day laboratory incubations. We used the 15N pool dilution assay to estimate gross nitrogen immobilization and nitrogen turnover for 5 day laboratory incubations. Soil organic carbon concentration and soil carbon± nitrogen ratios were negatively correlated with mean annual temperature in a linear regression model that accounted for 46± 56% of the variability, respectively. Regional patterns in soil organic carbon content and small scale variability in substrate availability imposed by discontinuous plant cover together strongly in¯uenced potential nitrogen immobilization. Potential carbon mineralization and nitrogen immobilization increased with increasing soil organic matter content. Soil organic carbon content accounted for 58% of the variation in potential rates of N immobilization. A strong correlation between nitrogen immobilization and carbon mineralization further suggests that rapid stabilization of nitrogen is facilitated by an active microbial community and the availability of a readily mineralizable organic substrate.7 2000 Elsevier Science Ltd. All rights reserved.
Keywords:Nitrogen immobilization; Nitrogen retention; Soil organic matter; Semi-arid grasslands
1. Introduction
Agricultural and industrial activities have greatly enhanced global rates of nitrogen (N) cycling (Gal-loway et al., 1995; Jordan and Weller, 1996; Vitou-sek et al., 1997). In response to these increases in N availability, researchers have sought to identify natural sinks for anthropogenic N. Soils provide a
natural sink for anthropogenic N in a broad range of ecosystems and soil types, and over varying time scales depending on the mechanisms of stabilization (Burge and Broadbent, 1961; Vitousek and Matson, 1985; He et al., 1988; Hart et al., 1994; Delgado et al., 1996; Aber et al., 1998; Fenn et al., 1998; Jae-ger et al., 1999; Nadelhoer et al., 1999). For example, clay minerals and humic compounds may
sequester N over century time scales (Johnson,
1992; Stevenson, 1994), while labile pools of soil or-ganic matter (SOM) represent a short-term sink, with turnover on the order of days to years (Clark, 1977; Zak et al., 1990; Davidson et al., 1990, 1992).
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* Corresponding author. Tel.: 1689; fax: +1-603-646-1682.
Labile SOM, largely composed of microbial bio-mass, is capable of rapid N immobilization (Vitou-sek and Matson, 1984; Davidson et al., 1990; Seely and Lajtha, 1997; Jaeger et al., 1999) while recalci-trant organic matter fractions generally accumulate N more slowly (Clark, 1977; Delgado et al., 1996). Rapid stabilization of mineral N may be important to overall N retention budgets because N deposition generally occurs in mineral forms that are highly
mobile and vulnerable to losses such as NO3ÿ
leach-ing and NH3volatilization (Matthews, 1994; Jaeger et
al., 1999).
While soils may rapidly stabilize N through abio-tic mechanisms (Strickland et al., 1992), microbial immobilization generally accounts for the largest
proportion of rapidly stabilized N in 15N tracer
ex-periments (Clark, 1977; Vitousek and Matson, 1984, 1985; Schimel et al., 1989; Davidson et al., 1990). In this paper we address in¯uences over microbial immobilization of N in laboratory incubations.
Since N immobilization is limited by substrate avail-ability in a broad range of ecosystems and soil types (Vitousek and Matson, 1985; Schimel, 1986; Burke et al., 1989a; Hart et al., 1994; Nadelhoer et al., 1995; Janssen, 1996; Hedin et al., 1998), our objective was to assess the in¯uence of soil carbon (C) content over N immobilization in soils collected from an environmen-tal gradient in the U.S. Great Plains. Past work has shown that broad scale climatic gradients in¯uence patterns of soil organic matter storage throughout the region (Jenny, 1941; Sims and Nielsen, 1986; Parton et al., 1987; Burke et al., 1989b; Amelung et al., 1997). We hypothesized that soil organic C concentration would strongly in¯uence N immobilization. Soils with higher organic matter content and wide soil C±N ratios may immobilize more N than soils with less SOM because of a limitation of reduced C substrate to microbial metabolism. Organic substrates with wide C±N ratios often support microbial communities that are N limited and generally exhibit higher rates of N immobilization presumably because microbes require additional mineral N to metabolize material with high C content relative to N (Sollins et al., 1984; Zak et al., 1994; Janssen, 1996). This may be particularly true when the C±N ratio of the substrate is substantially greater than the C±N ratio of microbial biomass. We hypothesized that under identical climates soils with high C content and wide C±N ratios should immobi-lize more N, and exhibit lower rates of N turnover than soils with lower C content and narrow C±N
ratios. Higher N immobilization should be
ac-companied by high rates of C mineralization, since presumably C substrate facilitates nitrogen retention
2. Materials and methods
We estimated total soil C and N, and indices of mi-crobial C and N turnover in laboratory incubations conducted on soils collected from ®ve sites along a mean annual temperature gradient in the U.S. Great Plains. These sites ranged from the Panhandle of Texas through southeastern Montana (Table 1). Mean annual precipitation ranges from 320 mm in the north to 450 mm in the south but we assumed that soil water availability did not vary markedly since rates of potential evapo-transpiration are highest in the sites that receive the most precipitation (Sims et al., 1978; Webb et al., 1983). Reported estimates of net primary production are similar across this gradient (Parton et al., 1987; Sala et al., 1988; Paruelo and Lauenroth, 1995; Burke et al., 1997). Mean annual temperature decreases from 148C in the south to 78C in the north (National Climatic Data Center, 1996). The three southern sites are composed of shortgrass steppe veg-etation and the two northern sites are composed of northern mixed-grass prairie (Dodd, 1979; Epstein et al., 1996). Soil textures ranged from sandy clay loam to clay loam for the sites used in this study (Table 1).
At each site we established a 50-m transect on a level upland such that all points on the transect shared
similar topography. Two 20 cm deep5 cm diameter
soil cores were collected from ®ve randomly located points along each transect. We collected soil cores from directly under the closest living plant (under) and from the center of the closest bare soil surface (between) to the predetermined random point on the transect to account for small scale spatial variability associated with discontinuous plant cover. Past work has demonstrated a strong in¯uence of microsite dier-ences (presence or absence of aboveground vegetation) upon the spatial variability of nutrient cycling in the surface soils of semi-arid grasslands (Hook and Lauen-roth, 1991; Vinton and Burke, 1995; Burke et al., 1998). We accounted for this variability by stratifying sample collection and including microsite as an inde-pendent variable in the statistical model. All soil samples were placed into polyethylene bags and stored at 48C until analyses were completed.
Soils were processed in the laboratory using a 2-mm sieve to remove litter, large roots and rocks prior to soil analyses and incubation. We weighed a sub-sample
of each soil sample and dried it at 508C and then
re-weighed the sub-sample for gravimetric soil moisture determination. We estimated ®eld capacity for each site as the amount of water held against gravity after 12 h of equilibration in sieved soils (Cassel and Niel-sen, 1986). We determined soil texture on a sub-sample of each sieved soil sample using the hydrometer method of Gee and Bauder (1986). A sub-sample was ground in a ball mill and analyzed using a LECO
CHN-1000 analyzer (St. Joseph, MI) for total C and N content. A 10 g sub-sample was extracted in 50 ml 2.0 M KCl-phenyl mercuric acetate (PMA 5 ppm) for 30 min on an orbital shaker, ®ltered through Whatman # 40 paper and analyzed using an Alpkem Flow Sol-ution 3000 Autoanalyzer (Perstorp Analytical, Silver Springs, MD) to determine initial NO3ÿ and NH4 con-centrations.
2.1. Incubations
We conducted two sets of incubations to character-ize N and C turnover dynamics using 5 day
incu-bations for potential gross mineralization and
immobilization and 30 day incubations for potential net mineralization. For the 5 day laboratory incu-bations, a 50-g sub-sample of air dry soil was
pre-incu-bated at 258C for two days in a 100-ml specimen cup
and pulse labeled with 15N and brought to ®eld
ca-pacity in a simultaneous procedure (Burke et al.,
1989a). Soils were labeled with 15N as NH4Cl (99%
enriched) at a rate of 5.0 mg 15N per gÿ1 of dry soil
with a solution of 100 mg 15N lÿ1. This amendment
was approximately 0.25 mg 15N per gÿ1 of soil N,
increasing the initial 15N pool by an average of 60%
across all sites if one assumes a natural abundance for
15N of 0.3663 at% (Knowles and Blackburn, 1993).
When necessary, we added additional de-ionized water to bring the samples to ®eld capacity. We thoroughly homogenized the soil with a glass rod to ensure
ade-quate mixing of 15N with the soil. Each specimen cup
was then sealed in a01.0 l sealed glass mason jar with a 5 ml 2 N NaOH base trap and 20 ml de-ionized water in the bottom to maintain a saturated atmos-phere to prevent the soils from drying out. Soils were incubated at 258C for 5 days. On day ®ve, 20 g of soil from the incubations were extracted in 100 ml 2 N KCl±PMA for inorganic N analysis. Base traps were
titrated with 1.0 N HCL and excess BaCl2 to
deter-mine the amount of CO2 produced during incubation,
or the potential C mineralization.
Ammonia in sub-samples of the KCl±PMA extracts was diused to a polytetra¯uoroethylene (PTFE) coated disk of Whatman # 40 paper over 7 days to
concentrate the 15N for mass spectrometry analysis.
We followed the Brooks et al. (1989) diusion pro-cedure with the technique modi®ed by Sùrensen and
Jensen (1991) and Stark and Hart (1996). Since NH4
concentrations were very low (<0.04 ppm), we treated all samples with Devarda's alloy during diusion to reduce NO3ÿ to NH4, and considered all mineral N as
a single pool for 15N analysis. Standards containing
known levels of NO3ÿ were reduced and diused along
con-tained in the acid trap against known concentrations
of NO3ÿ and NH4 in the KCl soil extracts. Recovery
values of15N from NO3ÿ standards were all over 97%.
Recovery of 15N from samples ranged from 88 to
117%. Samples with recovery rates of under 95% were eliminated from the analysis to avoid errors associated with isotope fractionation during the diusion process (Sùrensen and Jensen, 1991). Acid traps were sent for mass spectrometric analysis to the Stable Isotope
Facility at Utah State University (Logan, UT) for15N
determination.
We calculated gross N mineralization and
immobil-ization using the 15N pool dilution assay and the
fol-lowing equations developed by Kirkham and
Bartholomew (1954).
where m = gross N mineralization, i = gross N
im-mobilization, H015N g soil-1 at time t0, M0 =
total N 14N + 15Ng soilÿ1) at time t0, H15N g soilÿ1at timett,M = total N 14N+15Ng soilÿ1)
at time tt,t5 days. This technique estimates
gross N ¯uxes based upon the dilution of an 15N
labeled mineral pool by unlabeled N mineralized from organic matter. These equations assume that the inor-ganic N pools are small relative to N turnover and that the rates of N mineralization and N immobiliz-ation are constant throughout the incubimmobiliz-ation (Kirk-ham and Bartholomew, 1954). Additionally, these equations assume that no immobilized N is re-minera-lized through the course of the incubation. Although these soils had relatively low estimates of gross N min-eralization compared to other semi-arid ecosystems (Schimel, 1988; Burke et al., 1989a), the total N
miner-alized was 4±20the initial concentration of mineral
N in these soils. The assumptions concerning constant rates of mineralization and no re-mineralization can only be approximated over short durations so we con-®ned the incubations to 5 days (Kirkham and Bartho-lomew, 1954).
Net N mineralization over the 5 day incubation was calculated as the dierence between gross mineraliz-ation and gross immobilizmineraliz-ation. N turnover was used as an indication of total ¯ux relative to standing pools of N for comparison of sites with dierent levels of SOM. We estimated N turnover as the proportion of total N mineralized over the 5 day incubation (gross N
mineralized total Nÿ1 100% sensu Schimel, 1986).
Microbial growth eciency (YC) was estimated to
index microbial growth per unit respiration. We used
YC as an indication of both microbial eciency and
substrate quality. We used the following approxi-mation developed by Schimel (1988):
YC iNC:N
iNC:N C mineralization 3
where iN is the rate of gross N immobilization and
C:N is the C±N ratio of microbial biomass. We used a C±N ratio of seven for microbial biomass based upon published estimates for this region (Schimel, 1988; Metherell et al., 1993).
For the 30 day laboratory incubations, 25 g dry soil were brought to ®eld capacity and prepared as above in 1.0 l mason jars with 10 ml 2.0 N NaOH base traps and 20 ml de-ionized water in the bottom to maintain a saturated atmosphere. We incubated the soils at 258C for 30 days and then extracted 20 g of soil in 50 ml 2 M KCI±PMA. We determined ammonium and nitrate concentrations as described above. Potential net N mineralization was calculated as the dierence between initial NH4 + NO3ÿ and ®nal NH4 + NO3ÿ: Base traps were titrated with 1.0 N HCl and excess
BaCI to determine the amount of CO2 produced
during incubation, or the potential C mineralization.
2.2. Statistical analysis
We used the multiple regression procedure in SPSS version 6.1 to test the following hypotheses: (1) poten-tial N immobilization and C mineralization increase with increasing soil C content, and (2) nitrogen turn-over decreases with increasing C content. We charac-terized trends in soil organic matter content across the temperature gradient by regressing soil organic C and total soil N against mean annual temperature. We regressed potential N immobilization, mineralization, N turnover and microbial activity against soil C con-tent. Microsite position (between or under plants) was entered as a categorical independent variable into the statistical models to account for the small scale spatial variability imposed by discontinuous plant cover. We used an a level of <0.100 as criteria for entry into the model. Signi®cance for all statistical analyses was accepted ataR0:05:
3. Results
3.1. Soil organic matter
con-tent than soils from between plants, but microsite pos-ition was not a signi®cant term in the regression model. Soil N content ranged from 0.05 to 0.19% by weight across the temperature gradient and was not signi®cantly related to mean annual temperature or soil micro-site position. Soil C±N ratios were highest in the coldest sites (Fig. 1c). Mean annual temperature
accounted for 56% of the variation in soil C±N ratios
p0:012). Plant position had no signi®cant in¯uence over soil C±N ratios (Fig. 1c). In general there was a good linear ®t between mean annual temperature and soil organic C (Fig. 1a) and soil C±N (Fig. 1c) with the exception of soils collected from the southernmost sites which had greater SOM content and wider C±N ratios than predicted by our model. It is likely that dierences in site history, litter chemistry and seasonal-ity among the sites contributed to variation in SOM
Fig. 2. Plots of (a) 5 day potential gross N immobilization, (b) 5 day potential gross mineralization, and (c) 5 day N turnover vs. soil or-ganic C for a series of soils collected from a temperature gradient in the U.S. Great Plains. Triangles represent soils collected from under plants. Circles represent soils collected from the bare soils between plants. Regression lines represent the linear regression between the dependent variable (averaged over both microsite positions) and total soil C. Error bars are 1 standard deviation.
content. These dierences, particularly in seasonality, may account for the lack of ®t for the southern sites, which typically experience a longer growing season and greater more late summer precipitation (Paruelo et al., 1995; Paruelo and Lauenroth, 1998).
3.2. Five day incubations
Gross N immobilization was highest in high C soils and under plants (Fig. 2a). Soil C content and micro-site together explained 76% of the variation in esti-mates of N immobilization p<0:0001 (Table 2). Gross N mineralization also increased with increasing soil C content, though the slope was less than that for gross N immobilization (Fig. 2b). Soil C content accounted for 32% of the variation in estimates of
gross N mineralization p0:028, Table 2). Though
estimates of gross N mineralization were generally higher for soils collected from under plants compared to soils collected from the adjacent bare ground between plants, this trend was not statistically signi®-cant p>0:1). Soil C content and microsite position had no signi®cant in¯uence upon N turnover (gross N
mineralizedtotal Nÿ1100%) (Fig. 2, Table 2). Soil
C content and microsite position together accounted for 73% of the variation in estimates of 5 day C min-eralization (Fig. 3a p0:0001). Soil C content alone accounted for 29% of the variation in indices of mi-crobial growth eciency (Fig. 3b, Table 2).
3.3. Thirty day incubations
Potential net N mineralization increased signi®cantly with increasing soil C content over 30 day incubations
but there was no signi®cant eect of microsite position upon 30 day estimates of net N mineralization (Fig. 4a) (Table 2). Soil C content accounted for 61% of the
variation in estimates of net N mineralization p
0:0001 (Table 2). C mineralization increased with
increasing soil C content p0:002)(Fig. 4b). Soil C content and microsite explained 55% of the variation in 30 day estimates of C mineralization (Table 2).
4. Discussion
Regional trends in N cycling are strongly in¯uenced by broad scale environmental gradients as well as by microscale soil heterogeneity (Charley and West, 1977; Burke et al., 1989b, 1997; Hook and Lauenroth, 1991; Vitousek et al., 1995; Vinton and Burke, 1995, 1996; Schlesinger et al., 1996). We found that regional trends in soil organic C content and local scale spatial varia-bility imposed by discontinuous plant cover together controlled N immobilization and microbial activity in laboratory incubations. We found the highest rates of N immobilization in high C soils and in soils collected from under plants, corresponding to the highest rates of C mineralization and microbial growth eciency (Figs. 2(a), 3(a) and (b)). These results contribute to a growing body of literature demonstrating that organic matter availability limits N retention within the soil (e.g. Vitousek and Matson, 1985; Hart et al., 1994; Nadelhoer et al., 1995; Delgado et al., 1996; Downs et al., 1996; Fenn et al., 1998; Zink and Allen, 1998). The data presented here, while limited in scope by the circumstantial nature of laboratory soil incubations, corroborate the ®ndings of these previous studies over
Table 2
ANOVA values from multiple regression analysis of N and C ¯uxes in soils collected from across a soil organic carbon (SOC) gradient in the U.S. Great Plains. The cut-o for entry into the model wasp0:100:ModelR20
s are in bold face type
Dependant variable Source DF F p Slope R2
5 day gross N immobilization Model 2 31.457 0.0001 ± 0.76
SOC 1 5.602 0.0001 0.934 0.58
Microsite 1 4.493 0.0001 0.782 0.18
5 day gross N mineralization Model 1 5.103 0.028 ± 0.32
SOC 1 2.259 0.028 0.595 0.32
5 day N turnover Model 1 0.860 0.464 ± ±
5 day Net N mineralization Model 2 0.0664 0.797 ± ±
5 day C mineralization Model 2 26.096 0.0001 ± 0.73
SOC 1 4.079 0.0002 10.406 0.54
Microsite 1 5.101 0.0001 13.585 0.20
Microbial growth eciency (YC) Model 1 4.179 0.0468 ± 0.29
SOC 1 2.044 0.0468 4.64 0.29
30 day net N mineralization Model 1 28.101 0.0001 ± 0.61
SOC 1 5.301 0.0001 0.287 0.61
30 day C mineralization Model 2 10.043 0.0002 ± 0.55
SOC 1 3.169 0.0027 6.287 0.30
a continuous range of soil carbon content and without the confounding in¯uences of co-varying factors such as disturbance, arti®cial C amendments or large dier-ences in clay content (i.e. >20%). Soil C content and microsite position alone explained almost 80% of the
variability in estimates of N immobilization despite a broad geographical range of soils and their unique site histories.
Estimates of gross mineralization and immobiliz-ation were slightly lower than previous estimates of gross N ¯uxes reported for semi-arid ecosystems (Schi-mel, 1986; Burke et al., 1989a). For example, Schimel (1986) estimated rates of gross mineralization in the
range of 5.5±6.7 mg N kg soilÿ1 dayÿ1 for native
grassland soils from southwestern North Dakota, while Burke et al. (1989a) reported gross mineraliz-ation rates for a Wyoming sagebrush steppe in the range of 2.0±4.0 mg N kg soilÿ1dayÿ1. We report esti-mates of gross N mineralization in the range of 1.2±
3.5 mg N kg soilÿ1 dayÿ1 for soils of generally lower
organic matter content than soils used in Schimel (1986) and Burke et al. (1989a).
Estimates of gross N immobilization increased with soil organic C content. These results are supported by Fig. 3. Plots of (a) 5 day potential C mineralization vs. soil organic
C content, (b) microbial growth eciency vs. soil organic C, and (b) 5 day estimates of potential gross N immobilization vs. C mineraliz-ation for soils collected from across a temperature gradient in the U.S. Great Plains. The equation for the regression of potential gross N immobilization and C mineralization is y0:880:032 X,
r20:46 p0:001). Triangles represent soils collected from under plants. Circles represent soils collected from the bare soils between plants. Error bars are 1 standard deviation of the mean.
a broad survey of ecosystems and soil types in which N retention has been found to be linked to C avail-ability (Burge and Broadbent, 1961; Monreal et al., 1981; Schimel, 1986; Strickland et al., 1992; Hart et al., 1994; Nadelhoer et al., 1995; Fenn et al., 1998; Downs et al., 1996; Zink and Allen, 1998). In forested ecosystems, availability of labile C can limit N immo-bilization, which is particularly important in reducing losses of mobile N from disturbed forests (Vitousek and Matson, 1984, 1985; Nadelhoer et al., 1995; Downs et al., 1996). Schimel et al. (1986) and Delgado et al. (1996) found that N retention increased with increasing clay and SOM content across a catena in a semi-arid grassland over 1 to 2 years and after 10 years.
Estimates of N immobilization were greater in soils collected from under plants than in soils collected from the bare ground between plants because of greater availability of labile C and higher rates of mi-crobial activity under plant canopies than in the bare soil positions between plants. Small scale spatial het-erogeneity associated with discontinuous plant cover has a strong in¯uence over substrate availability and microbial dynamics in the surface soils of arid and semi-arid ecosystems (Charley and West, 1977; Schle-singer et al., 1996; Hook and Lauenroth, 1991; Vinton and Burke, 1995). These results show that regional patterns in soil C and small scale heterogeneity in sub-strate availability imposed by discontinuous plant cover together control N immobilization.
Estimates of N immobilization were associated with active microbial populations as indexed by C mineral-ization (Fig. 3a). Estimates of both 5 and 30 day C mineralization shared similar patterns with N immobil-ization and increased with increasing C content across the temperature gradient (Figs. 3a and 4b). In fact, 5 day estimates of potential C mineralization and N im-mobilization shared 46% of their variation, supporting our hypothesis that high N immobilization would accompany high rates of C mineralization and that the greatest immobilization potentials would occur in soils with the most active microbial populations (Fig. 3c)
r20:46,p0:001). Schimel (1986) found a similar
relationship between potential C mineralization and N immobilization in soils collected from grasslands and crop lands in southeastern North Dakota.
Estimates of N turnover did not support our hy-potheses. There was no relationship evident in po-tential N turnover and SOM despite increases in N immobilization with increasing soil C content (Figs. 2c and 3a). Schimel (1986) and Burke et al. (1989a) reported that high N turnover occurred in soils that tended to be C rather than N limited, and that N turnover was inversely related to SOM content. We propose that this relationship may be complicated by dierences in SOM quality across environmental
gradients. For example, although we could discern no signi®cant trends in N turnover across the SOM gradient, we found a strong relationship between
microbial growth eciency (potential microbial
growth per unit respiration) and soil organic car-bon, suggesting that in addition to changes in SOM quantity, there may exist a gradient in SOM quality as well. Murphy et al. (1996) reported a positive correlation between lignin content of aboveground biomass and mean annual temperature, and Ame-lung et al. (1997, 1999) reported a decrease in soil polysaccharide content and an increase in soil alkyl-C content with increasing mean annual temperature across a similar range of temperatures and soils for this region. Additional information about the com-position of SOM across this gradient, particularly at the southern end of the range, may help elucidate the relationship between mean annual temperature and organic matter turnover.
Estimates of potential 30 day net N mineralization and C mineralization further suggest that dierences in SOM composition across the gradient in¯uence poten-tial microbial activity in laboratory incubations. Esti-mates of 30 day potential mineralization in laboratory
incubations represent labile, readily mineralizable
pools of C and N. For soils collected over large regions, or across environmental gradients, incubations (standardized with respect to temperature and water content) re¯ect dierences in the relative limitations of microbial substrate and climate over C and N turnover in the ®eld (Zak et al., 1994; Vinton and Burke, 1996). For example, Zak et al. (1994) reported large values of potential N mineralization for soils collected from semi-arid sites, comparable to rates of mineralization for soils collected from mesic and even humid grass-lands.
narrow C±N ratios, higher decomposition rates and high N turnover.
5. Conclusions
Estimates of potential N immobilization in grass-lands soils suggests that controls over N immobiliz-ation may operate at both local and regional scales. At a local scale, patterns in substrate availability imposed by discontinuous plant cover in¯uence the spatial dynamics of N retention. At regional scales, soil C content alone accounts for almost 60% of the vari-ation in potential N immobilizvari-ation. The strong corre-lation between N immobilization and C mineralization shows that rapid stabilization of N is facilitated by an active microbial community and the availability of a readily mineralizable C substrate. Regional patterns in N turnover could not be accounted for by regional trends in SOM content and may be complicated by in-teractions between soil C content, SOM quality and microbial dynamics. More information about the com-position of SOM across this gradient could help explain patterns in N turnover.
Acknowledgements
We thank Marcos Robles and Rick Gill for assist-ance in the ®eld, and Becky Riggle, Judy Hendricks and Dan Reuss for assistance in the laboratory. Howie Epstein, Rick Gill, Ken Murphy, Peter Adler and Bill Lauenroth contributed greatly to the ideas expressed in this paper and we thank David Coleman and two anonymous reviewers for thoughtful reviews of this manuscript. This research was supported by the U.S. National Science Foundation and the U.S. Presidential Faculty Fellowship Program.
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