Can midseason drainage mitigate methane emissions from a paddy field with milk vetch (Astragalus sinicus L.) incorporation?

Midseason drainage (MSD) has been proven as an effective strategy for mitigating methane (CH4) emissions in paddy fields. However, its efficacy under milk vetch (Astragalus sinicus L., MV) incorporation, a paddy green manure widely cultivated in East Asian countries, remains uncertain. A three-year field trial was conducted, including five treatments: common level of chemical fertilizer with continuous flooding (CF-F), MV incorporation and reduced chemical fertilizer with continuous flooding (MVCF-F) or with 5 (MVCF-D5), 10 (MVCF-D10), and 15 (MVCF-D15) days of MSD. The results showed that the MVCF-F treatment enhanced rice productivity and soil fertility but promoted CH4 emissions relative to the CF-F treatment. However, compared with the MVCF-F treatment, the MVCF-D5, MVCF-D10, and MVCF-D15 treatments significantly decreased average CH4 emissions by 20%, 31%, and 35%, respectively. Furthermore, the MVCF-D5 and MVCF-D10 treatments led to a significant reduction in CH4 production potentials (PPs) and an increase in CH4 oxidation potentials (OPs). Compared to the MVCF-F treatment, the two treatments significantly decreased mcrA gene copy numbers from 23.44 x 107 copies g-1 to 12.62 x 107 copies g-1 and 11.26 x 107 copies g-1, while increasing pmoA gene copy numbers from 11.32 x 106 copies g-1 to 18.94 x 106 copies g-1 and 20.56 x 106 copies g-1, respectively. The results from the partial least squares-path model revealed that the reduction in CH4 emissions under MSD was mainly due to a decrease in CH4 PPs rather than an increase in CH4 OPs. The mcrA copy numbers, negatively regulated by soil Eh, was essential in regulating CH4 PPs. These findings provide evidence that MSD with a rational duration, even under exogenous carbon input through MV incorporation, can still effectively mitigate CH4 emissions in paddy fields. Considering that MSD with 15 days sacrificed the yield-increasing benefits of MV incorporation, MSD with 10 days is advisable in the study area.