Upward diffusion of nitrous oxide produced by denitrification near shallow groundwater table in the summer: a lysimeter experiment

Movement of nitrous oxide (N2O) produced in subsoil and shallow groundwater is important in determining the direct and indirect N2O emissions from agricultural soils. From the results of our previous study in a lysimeter-contained Gray lowland soil in the summer, we hypothesized that if a large amount of N2O is produced near shallow groundwater table in the summer, it will diffuse upward to the atmosphere. To examine this hypothesis, we conducted a one-year experiment in the same lysimeters for the cultivation of soybean wheat double cropping (SW) or upland rice (UR). Dissolved N2O concentration in the drainage water in the UR plots exceeded 0.4 mg N-L-1 in the summer, whereas that in the SW plots remained <0.1 mg NL-1.Analyses of the concentrations of nitrate and dissolved N2O in the drainage water and their nitrogen and oxygen isotopic compositions (delta N-15 and delta O-18) during the summer revealed that denitrification was the main process for the N2O production near the groundwater table. There was a significant positive correlation between the dissolved N2O concentration and soil-surface N2O flux in the summer. Calculated upward diffusive N2O fluxes at three soil depths by Fick's law also supported our hypothesis. The delta N-15 values of N2O in the soil-surface flux were similar to those in the shallow groundwater in the UR plots during the summer. Such similarity was not found in the SW plots. We conclude that our hypothesis was confirmed by the above results. Comparison of the monitored data with other seasons indicates that low soil water content was a driving force for the upward N2O diffusion as well as the high dissolved N2O concentration.