The relative importance of exogenous and substrate-derived nitrogen for microbial growth during leaf decomposition

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Ecological Society of America

Heterotrophic microbes colonizing detritus obtain nitrogen (N) for growth by assimilating N from their substrate or immobilizing exogenous inorganic N. Microbial use of these two pools has different implications for N cycling and organic matter decomposition in the face of the global increase in biologically available N. We used sugar maple leaves labeled with N-15 to differentiate between microbial N that had been assimilated from the leaf substrate (enriched with N-15) or immobilized from the water (natural abundance N-15:N-14) in five Appalachian streams ranging in ambient NO3-N concentrations from about 5 to 900 g NO3-N/L. Ambient NO3- concentration increased sugar maple decomposition rate but did not influence the proportion of microbial N derived from substrate or exogenous pools. Instead, these proportions were strongly influenced by the percentage of detrital ash-free dry mass (AFDM) remaining. Substrate-derived N made up a large proportion of the microbial N after the first 24 h in all streams. Detrital and microbial isotopic N-15 signatures approached that of the water as decomposition progressed in all streams, suggesting that exogenous N may be the predominant source of N for meeting microbial requirements even when exogenous N concentrations are low. Our results support predictions of more rapid decomposition of organic matter in response to increased N availability and highlight the tight coupling of processes driving microbial N cycling and organic matter decomposition.

n-15, assimilation, chloroform fumigation, heterotrophic microbes, immobilization, nitrogen availability, nitrogen cycling, organic matter, decomposition, streams, forest litter decomposition, soil-nitrogen, woodland stream, chloroform, fumigation, phosphorus dynamics, foliar litter, nutrient, carbon, release, lignin
B. M. Cheever, J. R. Webster, E. E. Bilger, and S. A. Thomas 2013. The relative importance of exogenous and substrate-derived nitrogen for microbial growth during leaf decomposition. Ecology 94:1614-1625.