Mustard catch crop enhances denitrification in shallow groundwater beneath a spring barley field

Abstract Over-winter green cover crops have been reported to increase dissolved organic carbon (DOC) concentrations in groundwater, which can be used as an energy source for denitrifiers. This study investigates the impact of a mustard catch crop on in situ denitrification and nitrous oxide (N 2 O) emissions from an aquifer overlain by arable land. Denitrification rates and N 2 O-N/(N 2 O-N + N 2 -N) mole fractions were measured in situ with a push–pull method in shallow groundwater under a spring barley system in experimental plots with and without a mustard cover crop. The results suggest that a mustard cover crop could substantially enhance reduction of groundwater nitrate NO 3 - -N via denitrification without significantly increasing N 2 O emissions. Mean total denitrification (TDN) rates below mustard cover crop and no cover crop were 7.61 and 0.002 μg kg −1  d −1 , respectively. Estimated N 2 O-N/(N 2 O-N + N 2 -N) ratios, being 0.001 and 1.0 below mustard cover crop and no cover crop respectively, indicate that denitrification below mustard cover crop reduces N 2 O to N 2 , unlike the plot with no cover crop. The observed enhanced denitrification under the mustard cover crop may result from the higher groundwater DOC under mustard cover crop (1.53 mg L −1 ) than no cover crop (0.90 mg L −1 ) being added by the root exudates and root masses of mustard. This study gives insights into the missing piece in agricultural nitrogen (N) balance and groundwater derived N 2 O emissions under arable land and thus helps minimise the uncertainty in agricultural N and N 2 O-N balances.