Early mid-season drainage can mitigate greenhouse gas emission from organic rice farming with green manure application

Organic rice farming is acknowledged as a safe and environmentally friendly rice production method. However, the application of fresh organic matter as fertilizer can increase methane (CH4) emissions during the rice-growing period because it is a carbon (C) source for CH4 under anaerobic conditions. In this study, we evaluated the CH4 emissions and net greenhouse gas emissions (NGHGE) from rice paddy fields managed by long-term organic farming. We also assessed the effect of early incorporation of green manure and mid-season drainage (to reduce CH4 emission and NGHGE) on the rice grain yield. In the first year, we measured CH4 and nitrous oxide (N2O) emissions, C budget, and yield in conventionally managed (CF) and organically managed (GM) rice paddy fields, in which white clover was grown in the fallow period. In the second year, we set up four treatments with dried hairy vetch (Vicia villosa) as green manure, incorporated in the conventional season (G), one week (GE1), and three weeks (GE3) earlier, and early mid-season drainage (GED), in which green manure was incorporated in the same way as in G. In the second year, we measured the same factors as in the first year. In the first year, C was accumulated in GM due to the application of C from green manure, despite C being lost in CF. On the other hand, the large amount of CH4 emission induced in GM contributed to global warming due to high NGHGE compared to CF. Early incorporation of green manure in rice paddy fields did not increase CO2 or decrease CH4 emissions before rice transplanting in the second year. A significant reduction in grain yield in GE3 suggested that three weeks earlier incorporation of green manure is not appropriate in terms of stable agricultural management. Meanwhile, early mid-season drainage reduced CH4 emissions, which contributed to both soil C sequestration and NGHGE reduction. These results indicate that mid-season drainage a week earlier than conventional practice could be a workable way of maintaining the brown rice yield and soil C sequestration and mitigating global warming in rice paddy fields receiving green manure.

Automated summary

Organic rice farming is considered safe and environmentally friendly, but the use of fresh organic matter as fertilizer can lead to increased methane emissions. This study evaluated methane and net greenhouse gas emissions in long-term organic rice paddy fields, and looked at the effects of green manure and mid-season drainage on rice grain yield. Early incorporation of green manure did not increase CO2 or decrease CH4 emissions, while early mid-season drainage reduced CH4 emissions and contributed to soil carbon sequestration and greenhouse gas reduction.