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ABSTRACT
Soils represent the largest terrestrial sink for N, yet current understanding of nutrient cycling cannot account for some of the mechanisms and sinks that stabilize anthropogenic N. We assessed the influence of soil properties, particularly soil organic C, pH, and clay content on potential biotic and abiotic N assimilation in soils collected across a temperature gradient in the U.S. Great Plains. We pulse labeled HgCI^sub 2^-sterilized and unsterilized and unsterilized soils with ^sup 15^N to examine the relative importance of abiotic and biotic N assimilation in short-term laboratory incubations. Estimates of total N assimilation in unsterilized soils ranged from 1.21 to 2.40 mg N kg^sup -1^ soil. Soil C content accounted for 50 and 60% of the variance in estimates of biotic immobilization and total N assimilation, respectively. Estimates of abiotic N assimilation ranged from 0.089 to 0.80 mg N kg^sup -1^ soil. Abiotic N uptake represented a large proportion of total N assimilation (mean equals 20%) in short-- term laboratory incubations. In contrast to previous reports, abiotic N uptake was negatively correlated to soil clay content and pH, perhaps because of differences in mineralogy and soil organic matter composition across the gradient. These results emphasize the importance of nonbiological N uptake in semiarid soils and suggest that abiotic pools could be an important sink for N.
Abbreviations: delta, natural abundance.
SOIL ORGANIC MATTER accounts for the majority of N in terrestrial ecosystems and most of the N retained in elevated N deposition and fertilizer studies (Aber et al., 1998; Fenn et al., 1998; Nadelhoffer et al., 1999), yet it has been difficult to identify the specific mechanisms or pools responsible for N stabilization. Most biogeochemical models assume that N inputs are incorporated into detrital, microbial, and plant pools by microbes and plants with concomitant fluxes of C reflective of their respective structural requirements (i.e., their C/N ratios). However, recent studies indicate that significant amounts of added N may be stabilized without observed increases in microbial biomass, soil respiration, or plant productivity (Aber et al., 1998; Fenn et al., 1998). For example, Aber et al. (1998) reported high rates of N recovery in northeastern forest soils without observed increases in microbial biomass, soil respiration, or root biomass. Agronomic studies have long...