Microbial and metazoan effects on nutrient dynamics during leaf decomposition in streams

I investigated the drivers of nutrient cycling by heterotrophic microbes during leaf decomposition in streams. My research addressed two overarching questions: 1) how do exogenous and endogenous factors interact to drive microbial nitrogen (N) cycling during organic matter decomposition in stream ecosystems, and 2) what affect will the global increase in biologically active N have on these factors and resulting fluxes? I conducted studies in natural streams and laboratory mesocosms to address these questions and used general stoichiometric theory to conceptualize diverse microbial assemblages as a single functional unit within stream ecosystems. First, I described spatial and temporal patterns of N and phosphorus uptake and mineralization by leaf-associated microbial assemblages in five southern Appalachian streams which spanned a gradient of nitrate availability. I found wide variations in nutrient fluxes across time and space, perhaps due to macroinvertebrate-induced changes in microbial assemblage composition. Secondly, I explored the roles of endogenous and exogenous N in meeting microbial requirements. I isolated microbial biomass from leaves that had been labeled with 15 N and incubated in the same five Appalachian streams. The importance of exogenous N increased as decomposition progressed and was particularly important in streams with high N availability. Finally, I tested potential interactions between two exogenous drivers of microbial nutrient cycling: N availability and animal activity. I used mesocosms to test the effects of consumer nutrient recycling (CNR) and grazing by two shredders on microbial uptake under different N regimes. Animals only influenced microbial uptake under low N conditions. Shredder CNR generally stimulated uptake while grazing had a negative effect. My research provides a robust model describing N cycling by detritus-associated microbes over the course of decomposition. According to this model, microbes assimilate endogenous N during the initial stages of decomposition and immobilization of