Response of sediment denitrification rates to environmental variables in streams heavily impacted by agriculture

Environmental controls of sediment denitrification were examined at four agricultural, channelized streams in Eastcentral Illinois between June 2003 and February 2005. Differences in sediment properties, water temperature, and nitrate-N (NO₃-N) supply varied temporally and spatially among the four streams. Sediment denitrification rates ranged from 0 to 0.3 μg N g dry mass (DM)⁻¹ h⁻¹ at stream sites characterized by low benthic organic matter content and coarse-textured sediments, 0 to 1.5 μg N (g DM)⁻¹ h⁻¹ at stream sites characterized by moderate benthic organic matter content and fine-textured sediments, and 0.2 to 3.3 μg N (g DM)⁻¹ h⁻¹ in emergent grass beds characterized by high benthic organic matter content and fine-textured sediments. Seasonally, sediment denitrification rates were greatest in the summer and least in the winter, with water temperature explaining 42 to 65% of the variation in sediment denitrification rate in three of the four streams. Benthic organic matter concentration explained 21 to 67% of the variation of sediment denitrification in all streams. Denitrification assays conducted on sediments amended with increasing concentrations of NO₃-N indicated that sediment denitrification was limited by N availability in fine-textured sediments but not in coarse-textured sediments when stream NO₃-N concentrations were <1 mg L⁻¹, which was typical in late summer.