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

Authors

B. M. Cheever, Virginia Polytechnic Institute and State University, Blacksburg, VirginiaFollow
J. R. Webster, Virginia Polytechnic Institute and State University, Blacksburg, Virginia
E. E. Bilger, Virginia Polytechnic Institute and State University, Blacksburg, Virginia
S. A. Thomas, University of Nebraska-LincolnFollow

Date of this Version

2013

Citation

Ecology, 94(7), 2013, pp. 1614–1625

Comments

Copyright 2013 by the Ecological Society of America

Abstract

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 ¹⁵N to differentiate between microbial N that had been assimilated from the leaf substrate (enriched with ¹⁵N) or immobilized from the water (natural abundance ¹⁵N:¹⁴N) in five Appalachian streams ranging in ambient NO₃-N concentrations from about 5 to 900 lg 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 ¹⁵N 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.