Land use influences the spatiotemporal controls on nitrification and denitrification in headwater streams

N and C cycles in headwater streams are coupled, and land use can modify these cycles by increasing N availability and removing riparian vegetation. To increase our understanding of how land use modifies the controls on N cycling, we quantified rates of 2 microbial N transformations in a total of 18 agricultural and urban streams (with and without riparian buffers) for 3 y to examine how riparian vegetation and land use influence sediment nitrification and denitrification. Nitrification rates were highest in agricultural streams in late spring. Nitrification was not related to streamwater NH4+ concentrations but was positively related to sediment C content (linear regression, r(2) = 0.72, p < 0.001). This result suggests that benthic decomposition provided NH4+ (via mineralization) to increase sediment nitrification. Denitrification rates did not differ among landuse types but were positively related to sediment C content and streamwater NO3- concentration (multiple linear regression, R-2 = 0.78, p < 0.001). Sediment C content, the primary predictor of denitrification rates, did not differ among land uses, but strearnwater NO3- concentration, the secondary predictor of denitrification rates, was highest in winter and in agricultural streams, indicating that land use and season were more important determinants of denitrification than coupled nitrification. Substrate availability (N and Q for N transformations generally did not differ between buffered and unbuffered streams within a similar landuse type, probably because of the confounding influence of tile drainage systems, which effectively decouple stream channels from their riparian zones. Land use influenced the delivery of the necessary substrates for N transformations but decreased the role of riparian zones in stream N cycling by simplifying the drainage network of headwater streams.