Contribution of winter cover crop amendments on global warming potential in rice paddy soil during cultivation

Winter cover crop cultivation during the fallow season has been strongly recommended in mono-rice paddy soil to improve soil quality, but its impact in increasing the greenhouse gases (GHGs) emissions during rice cultivation when applied as green manure has not been extensively studied. In order to recommend a preferable cover crop which can increase soil productivity and suppress GHG emission impact in paddy soil, the effect of winter cover crop addition on rice yield and total global warming potential (GWP) was studied during rice cultivation. Two cover crops (Chinese milk vetch, Astragalus sinicus L., hereafter vetch, and rye, Secale cerealis) having different carbon/nitrogen (C/N) ratios were cultivated during the rice fallow season. The fresh above-ground biomasses of vetch [25 Mg fresh weight (FW) ha(-1), moisture content (MC) 86.9 %, C/N ratio 14.8] and rye (29 Mg rye FW ha(-1), MC 78.0 %, C/N ratio 64.3) were incorporated as green manure 1 week before rice transplanting (NPK + vetch, and NPK + rye). The NPK treatment was installed for comparison as the control. During the rice cultivation, methane (CH4) and nitrous oxide (N2O) gases were collected simultaneously once a week using the closed-chamber method, and carbon dioxide (CO2) flux was estimated using the soil C balance analysis. Total GWP impact was calculated as CO2 equivalents by multiplying the seasonal CH4, CO2, and N2O fluxes by 25, 1, and 298, respectively. Methane mainly covered 79-81 % of the total GWP, followed by CO2 (14-17 %), but the N2O contribution was very small (2-5 %) regardless of the treatment. Seasonal CH4 fluxes significantly increased to 61 and 122 % by vetch and rye additions, respectively, compared to that of the NPK treatment. Similarly, the estimated seasonal CO2 fluxes increased at about 197 and 266 % in the vetch and rye treatments, respectively, compared with the NPK control plots. Based on these results, the total GWP increased to 163 and 221 % with vetch and rye applications, respectively, over the control treatment. Rice productivity was significantly increased with the application of green manure due to nutrient supply; however, vetch was more effective. Total GWP per grain yield was similar with the vetch (low C/N ratio) and NPK treatments, but significantly increased with the rye (high C/N ratio) application, mainly due to its higher CH4 emission characteristic and lower rice productivity increase. A low C/N ratio cover crop, such as vetch, may be a more desirable green manure to reduce total GWP per grain yield and to improve rice productivity.