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 μg 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 in 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 μg N kg⁻¹ d⁻¹) together could have completely consumed NO₃⁻ within 0.5 m of entering the riparian zone.