Spatial heterogeneity of nitrogen fixation and denitrification

Stream ecosystems exhibit high degrees of spatial heterogeneity in environmental conditions, communities of organisms, and ecosystem processes at nested scales from landscapes to microhabitats. This heterogeneity may facilitate the co-occurrence of biogeochemical processes that are favored under incompatible environmental conditions, like dinitrogen (N 2 gas) fixation and denitrification. We hypothesized that environmental variation at the patch scale (1-10’s m) would facilitate the co-occurrence of N 2 fixation and denitrification through the formation of ecosystem control points or patches that show high reaction rates relative to the surrounding area. We measured rates of N 2 fixation and denitrification and relative abundances of the genes nif H and nir S (genes that encode for the enzymes nitrogenase and nitrite reductase respectively) in patches determined by channel geomorphic units and substrate type in seven streams encompassing a gradient of N and P concentrations. We found ecosystem control points, where rates of N2 fixation and denitrification were 1 to 4 times higher than reach-average rates (0.2 – 1400 μg m -2 h -1 and 350 – 60000 μg m -2 h -1 , respectively), occurred in all study streams. Most N 2 fixation control points were in patches with rock substrates, while denitrification rates and relative abundances of nif H and nir S were higher in fine sediment patches. Yet, in two of the streams, rates in the top 25% of all patches for both denitrification and N 2 fixation occurred in the same patches, suggesting that variation in conditions at the sub-patch scale can also facilitate co-occurrence of these processes. Across all streams and patches, organic matter and dissolved oxygen concentrations were important predictors of rates of N 2 fixation, denitrification, and nif H relative abundance, while P concentration was important to N 2 fixation and denitrification. Our results demonstrate that understanding the spatial ecology of microbially-driven nutrient cycling is required to characterize nutrient fluxes more completely in stream ecosystems.