Denitrification and nitrate consumption in an herbaceous riparian area and perennial ryegrass seed cropping system

Riparian ecosystems have the capacity to lower NO3- concentrations in groundwater entering from nonpoint agricultural Sources. The processes responsible for decreases in riparian groundwater NO3- concentrations in the Willamette Valley of Oregon are not well understood. Our objective was to determine if denitrification and/or dissimilatory NO3- reduction to NH4+ (DNRA) could explain decreases in groundwater NO3- moving from a perennial ryegrass cropping system into a mixed-herbaceous riparian area. In situ denitrification rates (DN) were not different: between the riparian area (near-stream or near-cropping system) and cropping system the first year. In the second year, during the transition to a clover planting, DN was highest just inside of the riparian/cropping system border. Median denitrification enzyme activity (DEA) rates ranged from 29.5 to 44.6 mg N2O-N kg(-1) d(-1) for surface soils (0-15 cm) and 0.7 to 1.7 mu g N2O-N kg(-1) d(-1) in the subsoil (135-150 cm). Denitrification enzyme activity rates were not different among the zones and were most often correlated to soil moisture and NH4+ Nitrate additions to Surface soils increased DEA rates, indicating a potential to denitrify additional NO3-. Based on groundwater velocity estimates, NO3- (3.8 mg NO3--N L-1) entering the riparian surface soil could have been consumed in 0.2 to 7 m by denitrification and 0.03 to 1.0 m by DNRA. Denitrification rates measured ill the subsoil could not explain the spatial decrease in NO3-. However, with the potentially, slow movement of water in the Subsoil, denitrification and DNRA (0 to 264 mu g N kg(-1) d(-1)) together Could have completely consumed NO3- within 0.5 m of entering the riparian zone.