Mitigating methane emission via annual biochar amendment pyrolyzed with rice straw from the same paddy field

To develop an economic and sustainable biochar application strategy for mitigating methane emission from paddy fields, a four-year field experiment was conducted to compare two biochar amendment methods. The annual low (AL) rate pyrolyzed biochar returning method used the same amount of biochar as was harvested from rice straw in the field, 2.8 t ha^-1 yr^-1. The high single (HS) biochar returning method consisted of a single application of 22.5 t ha^-1 biochar only in the first year, 2015. Our results showed that the AL biochar returning strategy prevailed over the HS strategy in mitigating methane emission from paddy fields. On average, AL and HS could reduce methane emissions by 41% and 38.25% in four years, respectively. Methane accumulation per unit rice production was 45.8% and 43.1% in AL and HS, respectively. AL showed a stable effect on mitigating methane emission over four successive years, which resulted from the continuously increasing methanotrophs due to annual fresh biochar application. Aged biochar weakened the promotion of methanotrophs, leading to lower methane reduction rates in HS than in AL in the 4 years. Our results indicate that AL is a highly sustainable strategy for methane mitigation in paddy fields due to its high efficiency, practical operation, and economical acceptance.