Paddy soil drainage influences residue carbon contribution to methane emissions

Water drainage is an important mitigation option for reducing CH₄ (methane) emissions from residue-amended paddy soils. Several studies have indicated a long-term reduction in CH₄ emissions, even after re-flooding, suggesting that the mechanism goes beyond creating temporary oxidized conditions in the soil. In this pot trial, the effects of different drainage patterns on straw-derived CH₄ and CO₂ (carbon dioxide) emissions were compared to identify the balance between straw-carbon CH₄ and CO₂ emissions influenced by soil aeration over different periods, including effects of drainage on emissions during re-flooding. The water treatments included were: continuous flooding [C] as the control and five drainage patterns (pre-planting drainage [P], early-season drainage [E], midseason drainage [M], pre-planting plus midseason drainage [PM], early-season-plus-midseason drainage [EM]). An equal amount of ¹³C-enriched rice straw was applied to all treatments to identify straw-derived ¹³C-gas emissions from soil carbon derived emissions. The highest fluxes of CH₄ and δ¹³C-CH₄ were recorded from the control treatment in the first week after straw application. The CH₄ flux and δ¹³C-CH₄ were reduced the most (0.1-0.8 μg CH₄ g^-1 soil day^-1 and -13 to -34‰) in the pre-planting and pre-planting plus midseason drainage treatments at day one after transplanting. Total and straw-derived CH₄ emissions were reduced by 69% and 78% in pre-planting drainage and 77% and 87% in pre-planting plus midseason drainage respectively, compared to control. The early-season, midseason, pre-planting plus midseason and early-season-plus-midseason drainage treatments resulted in higher total and straw-derived CO₂ emissions compared to the control and pre-planting drainage treatments. The pre-planting and pre-planting plus midseason drainage treatments lowered the global warming potential by 47-53%, and early-season and early-season-plus-midseason drainage treatments reduced it by 24-31% compared to control. By using labelled crop residues, this experiment demonstrates a direct link between early drainage and reduced CH₄ emissions from incorporated crop residues, eventually leading to a reduction in total global warming potential. It is suggested that accelerated decomposition of the residues during early season drainage prolonged the reduction in CH₄ emissions. Therefore, it is important to introduce the early drainage as an effective measure to mitigate CH₄ emissions from crop residues.