Continuous, automated nitrous oxide measurements from paddy soils converted to upland crops

Since rotation of irrigated rice (Oryza sativa L.) and upland crop cultivations is a major land use management widely conducted in Japan, dynamics of greenhouse gas emission from drained paddy fields are required to be clarified. In the present study, the seasonal courses of N2O flux from a drained paddy field were measured continuously over one and a half years with an automated monitoring system at an interval of six times per day. The experimental paddy field with Gray lowland soil was drained and cultivated with two upland cropping systems: single cropping of upland rice and double cropping of soybean [Glycine mar (L.) Merrill] and wheat (Triticum aestivum L.). Unlike many of the previous studies, the increase in N2O flux after fertilizer application was not distinctive, which suggests that the contribution of nitrification to the N2O flux is relatively small in this field. Temporal peaks of N2O flux were often observed after heavy rainfalls and harvest of the crops, which lasted for a few to approximately 15 d. In all the crop cultivations, high peaks of N2O flux were observed when the crops were in the flowering to ripening stages, which suggests significant influence of crop development on N2O flux. N2O production in the rhizosphere or possible pathway for transport through the plant body is possible loss mechanisms. In particular, the amounts of N2O flux during the flowering and ripening stages of the summer crops were significant and were most responsible for the cumulative N2O emissions. Cumulative N2O emissions increased 4.0 to 5.3 times by the land use change from paddy rice cultivation to upland crop cultivations.