Dissolved greenhouse gas emissions from agricultural groundwater irrigation in the Guanzhong Basin of China

While evidence indicates that groundwater is a potential source for greenhouse gas (GHG) emissions, information for such emissions in groundwater used for irrigation is lacking. Based on 23 wells in the mid-western Guanzhong Basin of China, we investigated the dissolved CO2, N2O, and CH4 distributions in groundwater, their relationships with water indicators, and emission fluxes during flood irrigation. We found zero methane, but CO2 and N2O were 30 and 25 times, respectively, supersaturated compared to atmospheric concentrations. Dissolved N2O in groundwater was positively correlated with NO3--N (P = 0.009), while CO2 depended mainly on low pH and high dissolved inorganic carbon. The CO2 and N2O emission fluxes detected in wellheads, especially in shallow wells, implied potential emissions. Flood irrigation experiments showed that 24.55% of dissolved CO2 and 36.81% of dissolved N2O in groundwater was degassed immediately (within 12 min of irrigation) to the atmosphere. Our study demonstrates that direct GHG emissions from groundwater used for agricultural irrigation in the Guanz-hong Basin are potentially equivalent to about 2-4% of the GHG emissions from 3 years of fertilizer use on these farmlands, so further research should focus on optimizing irrigation strategies to mitigate GHG emissions.