Annual emissions of dissolved CO2, CH4, and N2O in the subsurface drainage from three cropping systems

Indirect emission of nitrous oxide (N2O), associated with nitrogen (N) leaching and runoff from agricultural lands is a major source of atmospheric N2O. Recent studies have shown that carbon dioxide (CO2) and methane (CH4) are also emitted via these pathways. We measured the concentrations of three dissolved greenhouse gases (GHGs) in the subsurface drainage from field lysimeter that had a shallow groundwater table. Aboveground fluxes of CH4 and N2O were monitored using an automated closed-chamber system. The annual total emissions of dissolved and aboveground GHGs were compared among three cropping systems; paddy rice, soybean and wheat, and upland rice. The annual drainage in the paddy rice, the soybean and wheat, and the upland rice plots was 1435, 782, and 1010 mm yr-1, respectively. Dissolved CO2 emissions were highest in the paddy rice plots, and were equivalent to 1.05-1.16% of the carbon storage in the topsoil. Dissolved CH4 emissions were also higher in the paddy rice plots, but were only 0.03-0.05% of the aboveground emissions. Dissolved N2O emissions were highest in the upland rice plots, where leached N was greatest due to small crop biomass. In the soybean and wheat plots, large crop biomass, due to double cropping, decreased the drainage volume, and thus decreased dissolved GHG emissions. Dissolved N2O emissions from both the soybean and wheat plots and the upland rice plots were equivalent to 50.3-67.3% of the aboveground emissions. The results indicate that crop type and rotation are important factors in determining dissolved GHG emissions in the drainage from a crop field.