Diurnal patterns of methane and nitrous oxide emissions form rice paddy under different soil moisture conditions

Paddy fields are one of the major anthropogenic sources of methane (CH4) and nitrous oxide (N2O) emissions. Soil moisture condition has been recognized as one of the most important factors that affect CH4 and N2O emission from paddy fields. Water-saving irrigation techniques for rice cultivation which have been widely used in China, lead to huge change in soil moisture conditions and soil biochemical processes, and that is expected to strongly alter trace gases emissions. Field experiments were carried out at Kunshan Experiment Station (31 degrees 15'15 '' N, 120 degrees 57'43 '' E) in East China, to investigate the effect of varied soil moisture conditions on diurnal variations of CH4 and N2O emissions from non-flooding controlled irrigation (CI) rice paddy. In traditional flooding irrigation (TI) paddy, CH4 emissions are likely got peaks in noon, and N2O emissions likely followed a similar diurnal pattern. With decrease of soil moisture in CI paddy, CH4 fluxes reduced and N2O fluxes increased. Optimal soil moisture condition for high nitrous oxide emission from CI paddy is about of 73.1% water filled pore space (WFPS). In TI paddy, the first drainage followed long-term flooding leads to even greater N2O emissions than those from CI paddy, although soil moisture in TI paddy are much higher than optimal soil moisture. Fluxes on 10:00 are always near daily average fluxes for CH4 and less than daily average fluxes for N2O emissions from TI paddy, but when it comes to CI paddy, fluxes on 10:00 h are always larger than daily average fluxes for both CH4 and N2O emissions. We concluded that non-flooding conditions in rice paddy result in both great change in emission fluxes of CH4 and N2O, and change in their diurnal patterns. Fluxes measured around 10:00 in CI paddy likely overestimate daily average emission fluxes of both CH4 and N2O.