Methane mitigation in transplanting and direct-wet seeding rice fields treated with fertilizers under condition of alternately flooding and soil aerating

Rice is main staple crop of the world. Growing rice in flooded water entails methane (CH4) emission. CH4 is one of greenhouse gases contributing to global warming. The experiment aimed to clarify the influence of fertilizer and water management on total methane emission (TME), methane mitigation and rice yields (RY). The experimental design was a split - split plot with 3 replications taking 2 cultivation in main plots, transplanting (TP) rice and direct-wet seeding (DWS) rice fields; 2 basal fertilizers, 16-16-8, 20 kg/rai and chicken manure pallet (CMP), 105 kg/rai in sub plots; and 3 top dressing fertilizers 1) none, 2) urea (46% N), 15 kg/rai and 3) ammonium sulfate (AS, 21% N), 30 kg/rai in sub-sub plots. It also examined relationship between quantity of paddy-soil water, TME and RY of both cultivations. Methane emission rate (MER) occurred during the whole growth period and was characterized by 2 large peaks: one from after transplanting or broadcasting to maximum tillering stage and the other from flowering to yellow ripening stage. Rapid declines of MER were dictated by soil aeration recognized as 3-5 days cracks. In TP rice plot based with CMP, 105 kg/rai, topped with AS, 30 kg/rai, TME decreased to 73.0% and RY increased to 14.7% over that of untreated plots with top dressing fertilizer, while in that topped with urea, 15 kg/rai, TME decreased to 68.9% and RY increased to 16.9%. In all of DWS rice plots which were topped with AS or urea, declines of TME ranged from 27.3 to 56.4% and increase of RY ranged from 31.3 to 47.9% over those without top dressing. In both TP and DWS plots, TMEs were closely correlated with the quantity of paddy-soil water (r = 0.83 and 0.86, respectively) and with submergence days (r = 0.94 and 0.89, respectively). Hence, saturated condition in paddy soil is a primary factor for methanogenesis. Moreover, for TP rice, the relationship between TME and RY was weakly positive (r = 0.16), whereas that for DWS rice was obviously negative (r = - 0.65). DWS rice cultivation could provide high RY and mitigated methane emission. In addition to this, the quantity of paddy-soil water was negatively related with RY (r = - 0.077), indicating that DWS cultivation was able to mitigate methane emission, produce high RY and reduce water use