Nitrous oxide emission from a rice paddy field in Japan

The flux of nitrous oxide (N2O) from a rice paddy field to the atmosphere was measured at Ryuhgasaki experiment station in Ibaraki Prefecture of Japan by closed chamber method, from the summer of 1992 to the summer of 1993. During the rice-cultivated and flooding periods when methane (CH4) was emitted, no emission or uptake of N2O was measured because the flux values were below the detection limits. After the final water drainage for harvest in August or September, N2O began to emit from the soil surface while the emission of CH4 was stopped, and N2O was emitted continually until the re-flooding day in the following spring. In the first few months after the final water drainage, the N2O flux was in the range of 10–20 µgN/m2/hour, then in the latter several months during the cold season, the N2O flux was less than 10 µgN/m2/hour. The vertical profiles of N2O, CO2 and CH4 concentrations in the plowed layer of the soil down to a depth of 20 cm, were also measured six times in the fallow season. The maximum concentrations of N2O and CO2 were found in the plowed layer in the early period, and which demonstrates that most of the N2O was produced in the plowed layer through nitrification, due to the decomposition of organic matter accumulated in the plowed layer during the rice-growing and water-flooding period. On the contrary, the vertical profiles in the cold season showed a gradual increase in the concentrations of N2O and CO2 in the plowed layer. It clearly indicates that a small amount of N2O was emitted to the atmosphere by diffusion through the plowed layer from the sub-soil layer where a large source of N2O was expected to exist.