Paddy soil drainage influences residue carbon contribution to methane emissions.

Water drainage is an important mitigation option for reducing CH ₄ (methane) emissions from residue-amended paddy soils. Several studies have indicated a long-term reduction in CH ₄ 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 CH ₄ and CO ₂ (carbon dioxide) emissions were compared to identify the balance between straw-carbon CH ₄ and CO ₂ 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 ¹³ C-enriched rice straw was applied to all treatments to identify straw-derived ¹³ C-gas emissions from soil carbon derived emissions. The highest fluxes of CH ₄ and δ ¹³ C-CH ₄ were recorded from the control treatment in the first week after straw application. The CH ₄ flux and δ ¹³ C-CH ₄ were reduced the most (0.1-0.8 μg CH ₄ 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 CH ₄ 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 CO ₂ 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 CH ₄ 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 CH ₄ emissions. Therefore, it is important to introduce the early drainage as an effective measure to mitigate CH ₄ emissions from crop residues.
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