Denitrification and Nitrate Consumption in an Herbaceous Riparian Area and Perennial Ryegrass Seed Cropping System

Abstract

Riparian ecosystems have the capacity to lower NO 3 − concentrations in groundwater entering from nonpoint agricultural sources. The processes responsible for decreases in riparian groundwater NO 3 − concentrations in the Willamette Valley of Oregon are not well understood. Our objective was to determine if denitrification and/or dissimilatory NO 3 − reduction to NH 4 + (DNRA) could explain decreases in groundwater NO 3 − 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 N 2 O‐N kg −1 d −1 for surface soils (0–15 cm) and 0.7 to 1.7 μg N 2 O‐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 NH 4 + Nitrate additions to surface soils increased DEA rates, indicating a potential to denitrify additional NO 3 − Based on groundwater velocity estimates, NO 3 − (3.8 mg NO 3 − ‐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 NO 3 − 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 NO 3 − within 0.5 m of entering the riparian zone.