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

Abstract

Movement of nitrous oxide (N 2O) produced in subsoil and shallow groundwater is important in determining the direct and indirect N 2O 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 N 2O 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 N 2O concentration in the drainage water in the UR plots exceeded 0.4 mgNL -1 in the summer, whereas that in the SW plots remained <0.1mgNL -1. Analyses of the concentrations of nitrate and dissolved N 2O in the drainage water and their nitrogen and oxygen isotopic compositions (δ 15N and δ 18O) during the summer revealed that denitrification was the main process for the N 2O production near the groundwater table. There was a significant positive correlation between the dissolved N 2O concentration and soil-surface N 2O flux in the summer. Calculated upward diffusive N 2O fluxes at three soil depths by Fick's law also supported our hypothesis. The δ 15N values of N 2O 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 N 2O diffusion as well as the high dissolved N 2O concentration. © 2011 Japanese Society of Soil Science and Plant Nutrition.