Indirect N2O emissions from groundwater under high nitrogen-load farmland in eastern China.
Current estimates of global indirect N2O emissions are based on a relatively small dataset and remain a major source of uncertainly in the global N2O budget. Nitrogen (N)-enriched groundwater from agricultural fields may act as an important source of indirect N2O emissions as it discharges to adjacent watershed areas. During 2015-2017, dissolved N2O concentrations in groundwater were measured and indirect N2O emission factors (EF5g) calculated under three typical high-N land-use types (vineyard, vegetable field and paddy field) in eastern China. The average dissolved N2O concentrations in groundwater were 58.1 ± 40.4, 18.5 ± 11.5 and 0.72 ± 0.27  μg N L-1 for vineyard, vegetable field and paddy field, respectively. The dissolved N2O was over-saturated and was therefore a net source of N2O to the atmosphere. The indirect N2O emission factors (EF5g) of vineyard (0.0091) and vegetable (0.0092) fields were much higher than the current Intergovernmental Panel on Climate Change (IPCC) default value of 0.0025 which indicated that these land-uses may have led to indirect N2O emissions from the underlying groundwater. In contrast, the EF5g of the paddy field (0.0019) was slightly lower than the default EF5g proposed by IPCC (2006) and contributed minimal indirect N2O emissions to the atmosphere. However, the current IPCC method may have overestimated the contribution of groundwater N2O to the global N cycle because it took residual but not initial groundwater NO3--N concentration into account when calculating EF5g. Therefore, we proposed the adoption of an improved method for calculating the EF5g and compared it to the current IPCC (2006) method using data from the present study and other published data. The results of the comparison showed that the improved method was more scientifically appropriate measurement for calculating EF5g.