Responses to atrazine of respiration, nitrification, and denitrification in stream sediments measured with oxygen and nitrate microelectrodes
Atrazine from agricultural runoff commonly pollutes streams in the Midwestern United States. Potential effects of atrazine on nitrification and denitrification were studied in sediment cores collected from a stream receiving relatively low inputs of atrazine. Nitrate and oxygen microprofiles were measured in these sediment cores using microelectrodes. After initial light and dark profile measurements, atrazine was added to overlying water resulting in concentrations of 0, 10, 50, and 100 μg l−1 active ingredient. Microprofiles were measured again 1 and 6 days following the addition of atrazine. Rates of oxygen respiration, nitrification, and denitrification were determined from the microprofiles using a reaction-diffusion model. Oxygen respiration increased relative to its initial value in cores exposed to all atrazine concentrations. This resulted in a shallow oxic layer in sediment cores exposed to atrazine. Nitrification was inhibited in sediments exposed to 50 and 100 μg l−1 atrazine and was significantly higher than control in cores exposed to 10 μg l−1. In the light, denitrification increased relative to control in sediment exposed to 10 μg l−1 atrazine. Denitrification was inhibited by 100 μg l−1 atrazine in the dark. Total daily carbon mineralization resulting from aerobic and nitrate respiration was significantly higher in cores exposed to 10 μg l−1 atrazine than in control cores. These results suggest that atrazine can significantly alter nitrogen and carbon cycling in streams. Atrazine could affect stream recovery from nitrate pollution and could reduce total organic matter retention in streams.