Optimum application level of winter cover crop biomass as green manure under considering methane emission and rice productivity in paddy soil

The combined seeding and cropping of non-leguminous and leguminous cover crops during the cold fallow season is recommended as an important agronomic practice to improve total biomass productivity and soil fertility in mono-rice (Oryza sativa L.) cultivation system. However, application of plant residues as green manure can increase methane (CH4) emission during rice cultivation and affect rice quality and productivity, but its effects are not well examined. In this field study, the mixture of barley (Hordeum vulgare R.) and hairy vetch (Vicia villosa R., hereafter, vetch) seeds with 75 % recommended dose (RD 140 kg ha(-1)) and 25 % RD (90 kg ha(-1)), respectively, were seeded after rice harvesting in late November, 2010, and harvested before rice transplanting in early June 2011. Total aboveground biomass was 36 Mg ha(-1) (fresh weight basis with 68 % moisture content), which was composed with 12 Mg ha(-1) of barley and 24 Mg ha(-1) of vetch. In order to determine the optimum recycling ratio of biomass application that can minimize CH4 emission without affecting rice productivity, different recycling ratios of 0, 25, 50, 75, and 100 % of the total harvested biomass were incorporated as green manure 1 week before rice transplanting in a typical temperate paddy soil. The same rates of chemical fertilizers (N-P2O5-K2O = 90-45-58 kg ha(-1)) were applied in all treatments. Daily mean CH4 emission rates and total CH4 fluxes were significantly (p < 0.05) increased with increasing application rates of cover crop biomass. Rice productivity also significantly (p < 0.05) increased with biomass application, but the highest grain yield (53 % increase over the control) was observed for 25 % recycling. However, grain quality significantly (p < 0.05) decreased with increasing cover crop application rates above 25 % recycling ratio, mainly due to extended vegetative growth periods of rice plants. Total CH4 flux per unit grain yield, an indicator used to simultaneously compare CH4 emission impact with rice production, was not statistically different between 25 % biomass recycling ratio and the control but significantly increased with increasing application rates. Conclusively, the biomass recycling ratio at 25 % of combined barley and vetch cover crops as green manure might be suitable to sustain rice productivity without increasing CH4 emission impact in mono-rice cultivation system.