Paddy soil drainage influences residue carbon contribution to methane emissions.

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