Cattle biogas effluent application with multiple drainage mitigates methane and nitrous oxide emissions from a lowland rice paddy in the Mekong Delta, Vietnam

Household biogas production from livestock manure is popular in Vietnam. Effluent from the biogas production can be effectively used as organic fertilizer for rice cultivation. However, there is concern about enhanced methane (CH4) emission from the flooded soil under reductive conditions because the effluent contains labile organic carbon, the substrate for microbial CH4 production. We therefore combined multiple drainage with “cattle biogas effluent” (BE) application to mitigate CH4 emission by ameliorating the reductive soil conditions while obtaining grain yields comparable to the conventional practice, a combination of synthetic fertilizers (SY) application and continuous flooding (CF). We performed a 1-year experiment on a lowland (0 m a.s.l.) paddy field in the Mekong Delta, Vietnam, under a triple cropping system. We examined the effects of fertilizer type (SY and BE) and water management (CF, alternate wetting and drying [AWD], and midseason drainage followed by intermittent irrigation [MiDi]) on rice growth and yield and on CH4 and nitrous oxide (N2O) emissions. For AWD, a surface water level of −15 cm was used as the threshold for re-irrigation after natural drainage, whereas MiDi used a fixed irrigation schedule with one-time forced drainage during the midseason drainage period. Surface water level did not drop sufficiently in either AWD or MiDi due to the lowland conditions, and both practices caused similar seasonal changes in water level except for the forced drainage in MiDi. A significant (P = 0.05) interaction between fertilizer type and water management on grain yield was found only in dry season (winter–spring) with the highest yield level. Despite the poorly drained conditions, CH4 emission was significantly reduced by AWD and MiDi (by 27–30%), and both practices offset the significant CH4 emission enhancement by BE application. N2O emission was significantly reduced by BE application, whereas it was not affected by water management because flooded conditions were maintained during every nitrogen fertilizer application. Our results indicate that the combination of BE application and multiple drainage can mitigate CH4 and N2O emissions from a lowland rice paddy in the Mekong Delta without yield loss.