The relative importance of exogenous and substrate-derived nitrogen for microbial growth during leaf decomposition.
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 microg 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.
Department of Biological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA. bethcheever@trentu.ca
B M
BM
Cheever
J R
JR
Webster
E E
EE
Bilger
S A
SA
Thomas
D031002:Acer / Q000737:chemistry / Q000382:microbiology; D001419:Bacteria / Q000254:growth & development*; D018533:Biomass; D004784:Environmental Monitoring; D005658:Fungi / Q000254:growth & development*; D009584:Nitrogen / Q000737:chemistry* / Q000378:metabolism*; D058458:Nitrogen Cycle; D018515:Plant Leaves / Q000737:chemistry* / Q000382:microbiology*; D045483:Rivers; D013997:Time Factors
D009584:Nitrogen