Indirect NO emissions from groundwater under high nitrogen-load farmland in eastern China.

Current estimates of global indirect NO emissions are based on a relatively small dataset and remain a major source of uncertainly in the global NO budget. Nitrogen (N)-enriched groundwater from agricultural fields may act as an important source of indirect NO emissions as it discharges to adjacent watershed areas. During 2015-2017, dissolved NO concentrations in groundwater were measured and indirect NO emission factors (EF) calculated under three typical high-N land-use types (vineyard, vegetable field and paddy field) in eastern China. The average dissolved NO concentrations in groundwater were 58.1 ± 40.4, 18.5 ± 11.5 and 0.72 ± 0.27  μg N L for vineyard, vegetable field and paddy field, respectively. The dissolved NO was over-saturated and was therefore a net source of NO to the atmosphere. The indirect NO emission factors (EF) 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 NO emissions from the underlying groundwater. In contrast, the EF of the paddy field (0.0019) was slightly lower than the default EF proposed by IPCC (2006) and contributed minimal indirect NO emissions to the atmosphere. However, the current IPCC method may have overestimated the contribution of groundwater NO to the global N cycle because it took residual but not initial groundwater NO-N concentration into account when calculating EF. Therefore, we proposed the adoption of an improved method for calculating the EF 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 EF.