The effects of land use on the relationships among denitrification, NO3−-N, dissolved organic C (DOC), and other environmental parameters were examined in 9 headwater streams (3 each in forested, agricultural, and urban-dominated subwatersheds) in the Kalamazoo River Watershed, Michigan. Sediment denitrification rates were determined using the chloramphenicol-amended acetylene inhibition technique. Agricultural streams had high concentrations of NO3−-N, DOC, soluble reactive P, and NH4 -N, whereas forested streams had the lowest concentrations of these nutrients, and urban streams generally exhibited intermediate concentrations. Sediment denitrification rates were highest in agricultural streams and lowest in forested streams throughout the study period. Availability of NO3−-N was the dominant environmental predictor of sediment denitrification rates, limiting denitrification when NO3−-N concentrations were below a calculated threshold of 0.4 mg NO3−-N/L. Other potential controlling variables (e.g., DOC, dissolved O2, water temperature, and sediment organic matter content) influenced denitrification rates secondarily. Despite higher denitrification rates in agricultural and urban streams compared to forested streams, sediment denitrification removed a smaller proportion of the stream NO3−-N load in agricultural and urban streams relative to forested streams.
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Vol. 24 • No. 2