Characteristics of dissolved N2O and indirect N2O emission factor in the groundwater of high nitrate leaching areas in northwest China

Numerous studies have demonstrated high concentrations of dissolved N2O and indirect N2O emission factors in groundwater affected by agriculture. However, the characteristics of seasonal and vertical dimensional difference in groundwater in high nitrate leaching areas are relatively lacking. We monitored the concentrations of dissolved and wellhead N2O of 23 groundwater wells over a one year period to understand the seasonal characteristics of dissolved and wellhead N2O concentrations and indirect N2O emission factors (EF5) of the shallow and deep groundwater in a high nitrogen leaching area and analyze the reasons for their differences. The mean dissolved N2O concentration in groundwater was 9.71 (9.03) mu g/L, which was 1.5-fold higher during the wet season relative to the dry season. Further-more, the leaching of soil N2O caused by rainfall and irrigation could be a pivotal factor affecting seasonal variation in the dissolved N2O. Shallow wells were found to have higher dissolved and wellhead N2O concentrations compared with deep wells in all seasons. The low wellhead N2O concentrations during the dry season were attributed to the sea-sonal decrease of the groundwater table and dissolved N2O concentrations. We concluded that indirect N2O emission factors did not vary in the vertical dimension but were higher during the wet season than that during the dry season. In addition, the mean indirect N2O emission factor in the groundwater was 0.025%, which was one order of magnitude below the current IPCC value (0.25 %). Thus, we proposed that such a low indirect N2O emissions factor could imply a low indirect N2O emission potential in groundwater with high dissolved oxygen and nitrogen loads. Our study further indicated that seasonal differences in dissolved N2O concentrations and indirect N2O emission factors should be considered when estimating the potential emissions of dissolved N2O in groundwater.