Seasonal variation of carbon dioxide and methane fluxes at single cropping paddy fields in central and western Japan

Based on the results of long-term flux measurement at two paddy flux sites, Mase (MSE) in central Japan and Hachihama (HCH) in western Japan, we present seasonal variation of carbon dioxide (CO₂) and methane (CH ₄) exchanges between single rice cropping paddy fields and the atmosphere in 2003. CO₂ flux was measured by the eddy covariance method at the two sites, while CH₄ flux was measured at MSE site by the modified aerodynamic method. Net ecosystem CO₂ exchange (NEE) in the 2003 growing period showed a distinct seasonal variation with rice growth, and reached the maximum daily CO₂ uptake of 9.2-9.5 g C m ^-2 d^-1 in the middle growing period. The total NEE in the growing period at HCH site was more negative than that at MSE site by 84 g C m^-2, 60% of which was caused by larger gross primary production (GPP) and the remainder by smaller ecosystem respiration (RE) at HCH site. The inter-site difference in GPP was principally caused by larger amount of incident photosynthetically active radiation at HCH site, which resulted from longer cultivation period at HCH site and shorter sunshine duration at MSE site in the mid-growing period. The inter-site difference in RE was attributed to that the rice growth at HCH site was out of phase with the seasonal variation of temperature. By including NEE in the non-growing period, we estimated the annual NEE at MSE site at a range between -192 and -284 g C m^-2. CH ₄ emission flux at MSE site increased with days after flooding, and showed a flush of CH₄ after the pre-harvest drainage. The amount of CH₄ emission during the 2003 growing period was 9.3 g C m ^-2, which was negligible in the carbon budget of the paddy field when compared with the annual NEE, but it had significant influence on the greenhouse gas budget because of the large global warming potential of CH ₄.