Denitrification in Shallow Groundwater Below Different Arable Land Systems in a High Nitrogen‐Loading Region

To evaluate the risk of nitrate (NO3--N) in groundwater, it is necessary to know the denitrification capacity. In this study, observations were carried out for 2years to investigate the groundwater denitrification capacity below three arable land systems in eastern China. Denitrification capacity was assessed by measuring the concentration and distribution patterns of nitrous oxide (N2O) and dinitrogen (N-2) in groundwater. The results revealed that groundwater denitrification activity was high and consumed 76%, 83%, and 65% of the NO3--N in the vineyard (VY), vegetable field (VF), and paddy field (PF), respectively. The high denitrification activity might be attributed to the strong reducing conditions, with high dissolved oxygen concentrations in groundwater, which promotes denitrification. During the sampling period, we observed high dinitrogen (excess N-2) accumulation in groundwater, which exceeded the total reactive nitrogen (N) in the deep layer. The large amount of excess N-2 observed in VY and VF indicated that considerable N was stored as gaseous N-2 in groundwater and should not be ignored when balancing N budgets in aquifers where denitrification is high.