Quantification of In Situ Denitrification Rates in Groundwater Below an Arable and a Grassland System

Understanding denitrification rates in groundwater ecosystems can help predict where agricultural reactive nitrogen (N) contributes to environmental degradation. In situ groundwater denitrification rates were determined in subsoil, at the bedrock interface and in bedrock at two sites, grassland and arable, using an in situ ‘push–pull’ method with 15 N-labelled nitrate (NO 3 − –N). Measured groundwater denitrification rates ranged from 1.3 to 469.5 μg N kg −1 day −1 . Exceptionally high denitrification rates observed at the bedrock interface at grassland site (470 ± 152 μg N kg −1 day −1 ; SE, standard error) suggest that deep groundwater can serve as substantial hotspots for NO 3 − –N removal. However, denitrification rates at the other locations were low and may not substantially reduce NO 3 − –N delivery to surface waters. Denitrification rates were negatively correlated with ambient dissolved oxygen, redox potential, k s and NO 3 − (all p values, p