Cheever, B. M.Webster, Jackson R.Bilger, E. E.Thomas, S. A.2014-03-272014-03-272013-07B. 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. http://dx.doi.org/10.1890/12-1339.10012-9658http://hdl.handle.net/10919/46815Heterotrophic 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.en-USIn Copyrightn-15assimilationchloroform fumigationheterotrophic microbesimmobilizationnitrogen availabilitynitrogen cyclingorganic matterdecompositionstreamsforest litter decompositionsoil-nitrogenwoodland streamchloroformfumigationphosphorus dynamicsfoliar litternutrientcarbonreleaseligninArticle - RefereedEcological Society of Americahttp://www.esajournals.org/doi/pdf/10.1890/12-1339.1https://doi.org/10.1890/12-1339.1