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DIVISION S-7-FOREST & RANGE SOILS

Biotic and Abiotic Nitrogen Retention in a Variety of Forest Soils

^a Desert Research Institute, 2215 Raggio Parkway, and Environmental and Resource Sciences, Univ. of Nevada, Reno, NV 89512 USA
^b Dep. of Environmental Studies, Univ. of California, Santa Cruz, CA 95064 USA
^c Dep. of Forest Sciences and Natural Resource Ecology Lab., Colorado State Univ., Fort Collins, CO 80523 USA

dwj{at}dri.edu

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion NOTES Summary and conclusions REFERENCES

 
Nitrogen (N) immobilization in sterilized (abiotic) and non-sterilized (biotic) O and A horizons was studied to determine the relative importance of biotic and abiotic processes in N retention in forest ecosystems. We collected samples from a variety of forest locations in Washington, Nevada, California, Tennessee, and North Carolina with differing soil types, vegetation, N status, and soil acidity. Included among these sites were adjacent stands of N₂-fixing and non-N₂-fixing species and sites of differing N status due to slope position at a given location. We treated O and A horizon samples from each site with (¹⁵NH₄)²SO₄; sterilization was achieved by adding HgCl₂, which proved to be highly effective. We found significant levels of both abiotic and biotic N immobilization in all soils. Biotic N immobilization was much greater in the N-poor sites in California and Nevada than in the other sites and was inversely related to N concentration overall. Biotic immobilization was directly related to pH and base saturation among all sites, but we hypothesize that these correlations resulted from a correlation between those parameters and N concentration. Abiotic N immobilization varied less than biotic N immobilization across sites and was unrelated to N concentration or pH. The percentage of total N immobilization as abiotic N immobilization varied considerably (from 6–90%), and was positively correlated with N concentration. These results suggest that abiotic N immobilization can be a significant process in a variety of soil types. Across soil types with increasing N saturation, biotic N immobilization decreases and abiotic N immobilization accounts for a greater proportion of total N immobilization.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion NOTES Summary and conclusions REFERENCES

 
THE FACT that most N in soils is associated with organic matter has led many forest soil scientists and ecologists to assume that N retention in forest ecosystems is controlled almost exclusively by biological processes. It is commonly assumed that competition among plant roots, heterotrophs, and nitrifiers for NH⁺₄ dominates the fate of N in ecosystems (e.g., Vitousek et al., 1979; Johnson and Edwards, 1979; Riha et al., 1986; Johnson, 1992). Relatively little attention has been given to abiotic processes controlling N retention in forest soils (i.e., NH⁺₄ retention by 2:1 clays and chemical reactions between NH⁺₄ and organic matter), even though the latter have been known to exist since the 1940s (Mattson and Koulter-Andersson, 1942, 1943; Mortland and Wolcott, 1965; Nömmik, 1965; Nömmik and Vahtras, 1982; Paul and Clark, 1989). Abiotic N retention can result from physical condensation reactions of phenols (originating from partially degraded lignin and some fungal pigments) with either amino acids or NH₃, resulting in the formation of "brown, nitrogenous humates" (Mortland and Wolcott, 1965; Nömmik, 1965; Nömmik and Vahtras, 1982; Paul and Clark, 1989). These abiotic, autocatalytic reactions are important in the production of humus (Mortland and Wolcott, 1965; Paul and Clark, 1989). Abiotic incorporation of NH₃ into humus is enhanced by high pH (because NH₃ is the reactive form of N) and high NH₃ and/or NH⁺₄ concentrations (Nömmik and Vahtras, 1982). Both of these conditions occur following urea fertilization, which is known to cause abiotic NH⁺₄ retention (Foster et al., 1985). The importance of abiotic reactions under ambient pH conditions is unclear. Nömmik (1970) found little abiotic NH⁺₄ retention in acid Norway spruce humus unless pH was raised to neutrality or higher. On the other hand, Axelsson and Berg (1988) found that abiotic NH⁺₄ retention occurred at pH 4 on decomposing Scots pine (Pinus sylvestris L.) litter. They also noted an inverse relationship between N concentration and N retention capacity; thus, litter at later stages of decay retained less N because of its higher N concentration. Schimel and Firestone (1989a), using Nömmik's (1970) methods, found that non-biological reactions accounted for 20% of the N retained in an acid (pH 4.3–4.5) forest soil in Blodgett Forest, CA. In field studies at the same site, however, the same authors attributed the immobilization of ¹⁵N from applied NH⁺₄ entirely to microbial uptake (Schimel and Firestone, 1989b). Recently, Aber et al. (1998) have acknowledged that both microbial uptake and abiotic retention could be important in ecosystem N retention, but found that neither could account for the very high rate of ¹⁵N immobilization in soils (150 kg N ha^-1 yr^-1 for over 10 yr) observed in a field study at Harvard Forest. They posed a new hypothesis whereby mycorrhizae take up N and subsequently release it as extracellular enzymes which can then be stabilized by chemical reactions with organic matter.

The purpose of this paper is to report the results of a laboratory study on the biotic and abiotic incorporation of ¹⁵NH⁺₄ into O and A horizon soils from a variety of forest sites with differing vegetation, soils, climate, and N status. On the basis of the literature reviewed above, we hypothesized that (i) both biotic and abiotic N retention would be inversely correlated with N concentration, (ii) both biotic and abiotic N retention would be directly correlated with lignin concentration, and (iii) abiotic N retention would be positively correlated with Ph.

	Materials and methods

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion NOTES Summary and conclusions REFERENCES

 
Sites
We samples soils from a set of sites located in Washington, California, Nevada, Tennessee, and North Carolina, representing a wide range of conditions in terms of vegetation, soils, climate, and N status (Table 1) . Each site has a good background data set on N cycling from previous studies (Johnson and Lindberg, 1992; Johnson and Todd, 1990; Johnson et al., 1991, 1997; Van Miegroet and Cole, 1984). Nitrogen deposition ranged from 0.3 to 27.1 kg ha^-1 yr^-1, and inputs via N fixation at the red alder (Alnus rubra Bong.) (RA) site at Thompson, WA, are estimated to have ranged from 100 to 300 kg ha^-1 yr^-1 over the life of the stand (Van Miegroet and Cole, 1984). Nitrogen leaching ranged from 0.04 to 38.9 kg ha^-1 yr^-1, indicating widely varying degrees of "N-saturation" (Aber et al., 1989). Forest floor N contents ranged from 0.1 to 2.2 mg ha^-1, and soil N contents ranged from 1.2 to 9.1 mg ha^-1.

Methods
After considering and testing several methods for soil sterilization (including autoclaving, radiation, and other biocides), a 5% (w/v) HgCl₂ solution was selected because (i) it would remain in the samples and maintain its effectiveness throughout the experiments (whereas radiation may not) and (ii) its use would produce little change in the properties of organic matter, as autoclaving might (Wolf and Skipper, 1994). We tested the effectiveness of HgCl₂ as a biocide by measuring CO₂ evolution in laboratory incubations of selected soils and found the treatment to be completely effective in precluding soil respiration for periods much longer than needed for the completion of the ¹⁵N experiments. In the biocide tests, we treated 1 g soil with 1 mL of either distilled water or 5% HgCl₂ and incubated the sample in Corning 15 mL centrifuge tubes (Corning Incorporated, Big Flats, NY) fitted with a septum. Each day for 6 d, CO₂ in the headspace was measured by extracting 100 µL of headspace gas with a Hamilton gas syringe and analyzing for CO₂ with a LI-COR 6250 CO₂ Analyzer (LI-COR, Inc., Lincoln, NE). We tested two Sierran soils which we considered to be potentially problematic because of their inherent hydrophobicity (Fig. 1) ; HgCl₂ was completely effective in preventing soil respiration.

View larger version (28K): [in this window] [in a new window]   Fig. 1 Carbon dioxide concentration in the headspace of incubated soils with and without HgCl2

Total and abiotic N immobilization were calculated by subtracting natural abundance ¹⁵N from measured ¹⁵N in treated subsamples (termed "excess ¹⁵N"), and dividing this value by 0.736, the fraction of the N in the added (¹⁵NH₄)₂SO₄ solution that was ¹⁵N. Biotic N immobilization was calculated as the difference in N immobilization between samples treated with (¹⁵NH₄)₂SO₄ only and those treated with (¹⁵NH₄)₂SO₄ plus HgCl₂. We recognize that the separation of biotic and abiotic N immobilization in this study is somewhat artificial and arbitrary; such is invariably the case for soil fractionation schemes. The protocols we adopted could have been biased toward abiotic N immobilization, for example, if microbial competition for NH⁺₄ in nonsterilized soils reduces abiotic immobilization. We see no way to resolve this issue with present techniques, however, and will proceed to utilize the scheme described above for separating biotic and abiotic N immobilization with the caveat that it, like many other soil analyses, should be regarded as in index to be used for comparisons among sites and treatments at a given site rather than as an absolute value.

Statistical analyses were performed by Microsoft Excel software. For comparisons across sites, regression analyses were used, accepting the significance level of P < 0.10. For comparisons between adjacent sites (N₂-fixers vs. non- N₂-fixers, ridgetop and cove sites), Student's t-tests were used, accepting the significance level of P < 0.10.

	Results and discussion

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion NOTES Summary and conclusions REFERENCES

 
Patterns in Nitrogen Concentration, Carbon:Nitrogen Ratios, Lignin, pH, Base Saturation and Natural Nitrogen-15 Abundance Among Sites
The N concentration and C:N ratio data from the study sites varied by more than twofold, reflecting the many factors (species, soils, atmospheric deposition, N fixation) affecting the N status of each site (Fig. 2 and 3) . The Sierran sites (SHF, SHP, ULVC, ULVP, and MET) fell on the low end of the scale, reflecting low N deposition rates and frequent N losses due to fire. The Smokies sites (SB and ST) fell on the high end of the scale, reflecting high N deposition rates and absence of fire. The positive effects of N₂-fixers was evident in the higher N concentrations and lower C:N ratios in all horizons of the red alder (RA) site as compared to the adjacent Douglas-fir (DF) site at Thompson, WA. The effects of N₂ fixers was less apparent at the adjacent Ceanothus (ULVC) and Jeffrey pine (ULVP) sites at Upper Little Valley, NV, however. Oe horizon and A horizon N concentrations were greater in the Ceanothus than in the adjacent pine site ( , Student's t-test, respectively), but there were no significant differences in N concentration in the Oi horizon or in C:N ratio in any horizon (Fig. 2 and 3). The effect of slope position was reflected in N concentrations at Walker Branch, TN, where the cove (WB98) site had significantly higher N concentrations than in the ridgetop site (WB42) for in the Oi and Oe horizons horizons ( , respectively, Student's t-test). There were no significant differences in N concentration in the Oa and A horizons at the two Walker Branch sites (Fig. 2 and 3). Lignin concentrations (not shown) varied little among sites.

View larger version (26K): [in this window] [in a new window]   Fig. 2 Nitrogen concentration in samples from the study sites (see Table 1 for code). (Note that Oa horizons were either not present or too thin to be sampled at the ULVC and SB sites.)

View larger version (28K): [in this window] [in a new window]   Fig. 3 C:N ratio in samples from the study sites (see Table 1 for code)

View larger version (25K): [in this window] [in a new window]   Fig. 4 Oa and A horizon pH and A horizon base saturation for samples taken from the study sites (see Table 1 for code)

View larger version (24K): [in this window] [in a new window]   Fig. 5 15N natural abundance in samples from the study sites. (see Table 1 for code.)

Patterns in Nitrogen Immobilization Among Sites
Biotic Immobilization
Biotic N immobilization in the O horizons varied by a factor of 10 or more among the sites, and was generally greater in the N-poor sites (especially SHF, and SHP, and ULVP) sites than in the others (Fig. 6) . Simple linear regressions revealed statistically significant, negative correlations between biotic N immobilization and N concentration in the Oe and A horizons (Table 2) . Simple linear regression gave a reasonable fit to the data in the A horizons. Linear regressions did not provide a good fit for biotic N immobilization versus N concentration in the O horizons, however; there appeared to be a threshold effect in which biotic N immobilization increased substantially as N concentration dropped below 10 g g^-1 (Fig. 7) . Power functions [of the form y = a(x^b)] improved the correlation coefficients for the O horizon regressions somewhat; however, the power function curve fit was poor and did not adequately describe the observed threshold effect (Fig. 7 and Table 2).

View larger version (29K): [in this window] [in a new window]   Fig. 6 Biotic N immobilization samples from the study sites (see Table 1 for code). In the A horizons, values are plotted on the basis of soil weight (100 x mg N kg soil-1) and on the basis of organic matter weight in soil (mg N kg OM-1). Values on the soil weight basis are multiplied by 100 for scale

View this table: [in this window] [in a new window]   Table 2 Sign of slope (+ or -) and correlation coefficients (r2) for statistically significant regressions (linear) of various soil properties against N immobilization.

View larger version (27K): [in this window] [in a new window]   Fig. 7 Plots of biotic and Abiotic N immobilization against N concentration. Correlation coefficients (r2) are shown for power function regressions of the form y = a(xb) of biotic N immobilization and N concentration in the O horizons

The effects of N fixers and slope position on biotic immobilization at any one location were mixed. At Upper Little Valley, biotic immobilization was significantly lower in the Ceanothus (ULVC) than in the adjacent jeffrey pine (ULVP) sites in the O horizons (P = 0.03 and 0.06, respectively for the Oi and Oe horizons; Student's t-test) (Fig. 6). (Note that the Oa horizon was too thin and indistinct to sample in the ULVC site.) In the A horizons, however, the differences reversed: biotic immobilization was significantly greater in the Ceanothus than in the pine soil (P = 0.02). Biotic immobilization was lower in the Oi horizon of red alder (RA) than in the Oi horizon of the adjacent Douglas-fir (DF) site at the Thompson, WA, site (P = 0.09); however, the differences in the other horizons were not significant (Fig. 6). Biotic immobilization was significantly lower in the Oi (P = 0.01) and Oe (P = 0.06) horizons of the N-rich cove site (WB98) than in the N-poor ridgetop site (WB42) at Walker Branch, TN (Fig. 6). Differences in the other horizons at the Walker Branch site were not significant.

There were no consistent patterns of biotic N immobilization with stage of litter decomposition. At some sites (ULVP, WB42, ST), biotic N immobilization decreased from Oi to Oe horizons, whereas in other sites the reverse was true (SHP, SB), and in others there was no significant difference (Fig. 6). Similarly, biotic immobilization was lower in Oa than in Oe horizons in some sites (SHF, SHP, ULVP, MET) but not significantly different in others (DF, RA, WB42, WB98, ST). Biotic N immobilization in the A horizons was generally equal to or lower than that in the Oa horizons when expressed on the basis of soil weight. However, when, biotic N immobilization in the A horizons was expressed on the basis of weight of organic matter, it far exceeded that in the O horizons in all sites, and soils with low organic matter (i.e., ULVP and MET) showed the highest values (Fig. 6). Thus, while comparisons among the O horizons suggest that biotic N immobilization decreases during later stages of litter decomposition at some sites (SHF, SHP, ULVP, WB42), the opposite seems to hold true as litter transforms into soil organic matter.

Abiotic Immobilization
We found significant levels of abiotic immobilization in nearly all samples. Abiotic N immobilization in the O horizons varied by a factor of two to three among the sites, but showed only a weak and non-significant downward trend with increasing site N status (Fig. 8) . Abiotic N immobilization in the Oi and Oe horizons was greatest in the Sagehen red fir (SHF), but there was no particular trend among the other sites. In the Oa horizons, the SHF and ULVP sites had greater values but there were no particular trends among the other sites. There were no statistically significant correlations between abiotic N immobilization and N concentration, C:N ratio, lignin concentration, lignin:N ratio, pH, or base saturation within any horizon (Table 2). There were no statistically significant differences in abiotic immobilization between the N₂-fixing and non-N₂-fixing sites at Upper Little Valley, NV, (ULVC and ULVP), or Thompson, WA, (RA and DF). Abiotic N immobilization was lower in the Oi horizon of the cove site (WB98) than in the ridgetop site (WB42) at Walker Branch, TN (P = 0.01, Student's t-test), but differences were not significant in other horizons.

View larger version (32K): [in this window] [in a new window]   Fig. 8 Abiotic N immobilization samples from the study sites (see Table 1 for code). In the A horizons, values are plotted on the basis of soil weight (100 x mg N kg soil-1) and on the basis of organic matter weight in soil (mg N kg OM-1). Values on the soil weight basis are multiplied by for scale

Contrary to what was hypothesized and what has been found previously in the literature, we found no relationship between either N status or pH and abiotic immobilization among these samples. The lack of correlation between pH and abiotic immobilization does not imply that changing pH on an individual sample would have no effect on abiotic N immobilization; indeed, the opposite has been shown in detailed laboratory studies in which pH was manipulated (Nömmik, 1970; Nömmik and Vahtras, 1982). Experiments involving pH manipulations of pH and abiotic immobilization on these samples are underway at this time.

The percentage of total N immobilization accounted for abiotic immobilization varied considerably. Among the Oi horizons, this percentage ranged from a low of 9% in SHP to a high of 78% in RA (Fig. 9) . Among the Oe horizons, it ranged from 9 (SHP) to 90% (ULVC); in the Oa horizons it ranged from 26 (SHF) to 63% (ULVP); and in the A horizons it ranged from 18 (ULVC) to 76% (ST). We note that our values for percentage abiotic N immobilization in jeffrey pine O horizons in this laboratory study (6–63%) well encompass the value of 20% reported by Schimel and Firestone (1989a) in a field study of N immobilization in the O horizon of a ponderosa pine forest.

View larger version (59K): [in this window] [in a new window]   Fig. 9 Percentage abiotic and biotic N immobilization in O and A horizons from the study sites (see Table 1 for code)

	Summary and conclusions

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion NOTES Summary and conclusions REFERENCES

 
Our study demonstrates that abiotic N immobilization occurs in a wide variety of soil types, and that the levels could be significant for ecosystem processes. Our results indicated that biotic N immobilization in the laboratory was greatly reduced in sites with greater N availability (because of N fixation, slope position, or atmospheric deposition). Abiotic N immobilization, on the other hand, tended to remain more constant and was not significantly related to N status. In contrast to previous laboratory studies, we found little evidence that either pH or lignin were important factors affecting either biotic or abiotic N immobilization among these sites. This does not necessarily imply that pH is unimportant for abiotic N immobilization; it merely implies that no overall correlation between pH and abiotic N immobilization across the sites we studied was found. We hypothesize that manipulating pH of individual samples from this study would result in changes in abiotic N immobilization, as has been found in the past.

Laboratory studies of this nature cannot provide a complete picture of soil N cycling processes because laboratory conditions differ substantially from field conditions (especially with respect to the presence of plants). Laboratory studies of this kind can, however, provide comparisons of selected processes among a number of sites that would be prohibitively expensive in a field setting. The results of this study suggest the hypothesis that abiotic N immobilization is much less affected by N status than biotic immobilization is, and that abiotic immobilization may therefore become relatively more important in sites that are "N saturated" due to atmospheric deposition, fertilization, or natural processes. Field studies are needed to test this hypothesis.Effland 1977

	ACKNOWLEDGMENTS

 
Research supported by the National Science Foundation and the Nevada Agricultural Experiment Station, University of Nevada, Reno. Field assistance by Don Todd and Robb Harrison is gratefully acknowledged. Lignin analysis by Dr. Mark Davis and technical assistance by Valerie Yturiaga and Donn Geisinger are greatly appreciated.

	NOTES

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion NOTES Summary and conclusions REFERENCES

 
¹ The reader should recall that Oa horizons were either absent or too thin to be sampled in the ULVC and SB sites; thus, these sites are not included in any statistical analyses of patterns in Oa horizons.

	REFERENCES

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion NOTES Summary and conclusions REFERENCES

 

• Aber J.D., McDowell W., Nadelhoffer K., Magill A., Bernston G., Kamakea M., McNulty S., Currie W., Rustad L., Fernandez I. Nitrogen saturation in temperate forest ecosystems. Bioscience 1998;48:921-934.[ISI]
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