Methane emissions from double rice agriculture under long-term fertilizing systems in Hunan, China

We measured methane emissions from double-crop rice fields with long-term fertilizing systems in Taoyuan county in Hunan province, China. These long-term experiments have been ongoing since 1990, with six different fertilizing systems including balanced mineral fertilizers or combined organic/mineral fertilizers as following: nutrient K deficit (NP), nutrient P deficit (NK), balanced mineral nutrients (NPK), combined organic/mineral fertilizers at full rate (FOM) or at reduced rate (ROM) and no-fertilizer as a control (CK). Results showed that the average CH4 fluxes ranged between 11.8 and 19.4 mg m−2 h−1 for the early rice season and between 23.3 and 43.8 mg m−2 h−1 for the late rice season. Out of the total emissions, 66–71% from the double-cropping system occurred during the late rice season. Cumulative emissions of CH4 from two rice crops ranged from 65.9 to 121 g m−2, ranking as following: FOM > ROM > NPK > NK > NP > CK. Results indicated that high CH4 emission rates from rice fields in Hunan province were dominated comprehensively by rice straw incorporation and extended flooded periods. Therefore, it is recommendable to drain the flooded field intermittently for double rice cropping systems. In particular, rice straw should not be incorporated into soil before transplanting while milk vetch as green manure could be encouraged with regards to crop productivity and greenhouse gas mitigations.