Amino acid abundance and proteolytic potential in North American soils
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Studies of nitrogen (N) cycling have traditionally focused on N mineralization as the primary process limiting plant assimilation of N. Recent evidence has shown that plants may assimilate amino acids (AAs) directly, circumventing the mineralization pathway. However, the general abundance of soil AAs and their relative importance in plant N uptake remains unclear in most ecosystems. We compared the concentrations and potential production rates of AAs and NH4 +, as well as the edaphic factors that influence AA dynamics, in 84 soils across the United States. Across all sites, NH4 + and AA-N comprised similar proportions of the total bioavailable N pool (~20%), with NO3 − being the dominant form of extractable N everywhere but in tundra and boreal forest soils. Potential rates of AA production were at least comparable to those of NH4 + production in all ecosystems, particularly in semi-arid grasslands, where AA production rates were six times greater than for NH4 + (P < 0.01). Potential rates of proteolytic enzyme activity were greatest in bacteria-dominated soils with low NH4 + concentrations, including many grassland soils. Based on research performed under standardized laboratory conditions, our continental-scale analyses suggest that soil AA and NH4 + concentrations are similar in most soils and that AAs may contribute to plant and microbial N demand in most ecosystems, particularly in ecosystems with N-poor soils.
KeywordsSoil N cycle Proteolysis Ammonium Amino acid Organic N Nitrification Nitrogen mineralization Protein mineralization
We thank Rebecca McCulley for comments that greatly improved the manuscript, and Sean Berthrong and Isabel Heine for lab assistance. We are also grateful to the many people who generously collected field samples for this study. Support for this work was provided by a NSF Postdoctoral Fellowship to NF and by grants from NIGEC/NICCR (through the office of Biological and Environmental Research at the Department of Energy) and the National Science Foundation (DEB 02-35425 and 07-17191) to RBJ.
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