Mitigation of methane emissions from paddy fieldsunder controlled irrigation

Abstract: In order to explore the influence of soil moisture regulation on methane (CH4) emissions under water-saving irrigation and look for an irrigation mode for rice that would save water while simultaneously limiting CH4 emissions, the regularity of CH4 emissions from paddy fields under controlled irrigation was analyzed based on 5 years' of field experiment data. Two different irrigation managements were conducted as follows with the same rice variety, cultivation practices and plant protection measures: controlled irrigation (CI) and irrigated intermittently flooded single aeration irrigation (FI). Rice was cultivated in a 150 m2 (20 m×7.5 m) plot between 2006 and 2007, each plot was separated by brick concrete with a height of 30 cm and width of 30 cm. Plastic anti-seepage films were inserted into soil plough layer to a depth of 50cm to isolate water exchange between the plots. For exploring the influence of rainfall on CH4 emissions from irrigated paddy fields and controlling soil moisture accurately, experiments were conducted in lysimeter with mobile canopy between 2009 and 2011, and each plot area was 5 m2(2.5 m×2 m). Gas samples were collected using manual static chamber and were analyzed by a gas chromatograph (ShimadzuGC-14B) with a flame ionization detector (FID) for CH4 concentration. The results showed that the total CH4 emissions from CI paddy fields were 1.07±0.17g/m2, which was significantly reduced by 83.5% compared with FI paddy fields (6.49±0.17 g/m2). Total seasonal and annual CH4 emissions from CI and FI paddy fields located in Southeast China were lower than the reported values. Meanwhile, the total annual CH4 emissions from controlled irrigation paddy fields was lower than that of paddy filed in most parts of the World. When compared to irrigated intermittently flooded multiple aeration, irrigated intermittently flooded single aeration and irrigated continuously flooded paddy fields, seasonal CH4 emissions from CI paddy fields decreased 94.7%, 97.1% and 99.0% respectively. Annual total CH4 emissions from CI paddy fields was reduced by 87.4%, 84.0% and 33.8% than that of irrigated continuously flooded, irrigated intermittently flooded single aeration and irrigated intermittently flooded multiple aeration paddy fields in most parts of the World. The controlled irrigation mode significantly changed CH4 emissions law from paddy fields and also reduced the emission peak. The CH4 emissions from paddy fields under controlled irrigation were high between turning-green stage and middle tillering stage (DAP≤18), then reduced and maintained low level beginning from late tillering stage (DAP=21). The peak flux of methane emission from paddy fields under controlled irrigation was 3.69 mg/(m2?h), which was 69.0% lower than that of irrigated intermittently flooded single aeration irrigation. The CH4 emissions from paddy fields under controlled irrigation and irrigated intermittently flooded single aeration irrigation were began to reduce due to persistent rainfall. The increase of water depth due to the persistent rainfall, and decrease of CH4 emissions was observed in the rainfall. Meanwhile, the low soil temperature maintained in the progress of persistent rainfall also impeded CH4 production. The soil moisture regulation under controlled irrigation resulted in a series of dewatering in paddy field and changed the oxygen supply in soil rooting profile, finally reduced the total CH4 emissions from paddy fields. The controlled irrigation mode could significantly reduce CH4 emissions from paddy fields.