Manipulation of the Dissolved Organic Carbon Pool in an Agricultural Stream: Responses in Microbial Community Structure, Denitrification, and Assimilatory Nitrogen Uptake

Carbon (C) and nitrogen (N) are strongly coupled across ecosystems due to stoichiometrically balanced assimilatory demand as well as dissimilatory processes such as denitrification. Microorganisms mediate these biogeochemical cycles, but how microbial communities respond to environmental changes, such as dissolved organic carbon (DOC) availability, and how those responses impact coupled biogeochemical cycles in streams is not clear. We enriched a stream in central Indiana with labile DOC for 5 days to investigate coupled C and N cycling. Before, and on day 5 of the enrichment, we examined assimilatory uptake and denitrification using whole-stream N-15-nitrate tracer additions and short-term nitrate releases. Concurrently, we measured bacterial and denitrifier abundance and community structure. We predicted N assimilation and denitrification would be stimulated by the addition of labile C and would be mediated by increases in bacterial activity, abundance, and a shift in community structure. In response to the twofold increase in DOC concentrations in the water column, N assimilation increased throughout the enrichment. Community respiration doubled during the enrichment and was associated with a change in bacterial community structure (based on terminal restriction fragment length polymorphisms of the 16S rRNA gene). In contrast, there was little response in denitrification or denitrifier community structure, likely because labile C was assimilated by heterotrophic communities on the stream bed prior to reaching denitrifiers within the sediments. Our results suggest that coupling between C and N in streams involves potentially complex interactions with sediment texture and organic matter, microbial community structure, and possibly indirect biogeochemical pathways.