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 NO
3-N concentrations from about 5 to 900 μg NO
3-N/L. Ambient NO
3- 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.
Keywords
15N,
assimilation,
chloroform fumigation,
heterotrophic microbes,
immobilization,
nitrogen availability,
nitrogen cycling,
organic matter decomposition,
streams
Citation
Cheever, B.M.; Webster, J. R.; Bilger, E. E.; Thomas, S. A. 2013. The relative importance of exogenous and substrate-derived nitrogen for microbial growth during leaf decomposition. Ecology 94(7):1614–1625. https://doi.org/10.1890/12-1339.1
