A seven-year study on the effects of fall soil tillage on yield-scaled greenhouse gas emission from flood irrigated rice in a humid subtropical climate

Abstract

In southern Brazil, flood irrigated rice is grown during the summer, while a self-reseeding ryegrass is grown during the winter months without irrigation. Soil tillage operations that incorporate the rice and ryegrass residues into the soil are performed only in the spring season, which may increase methanogenesis due to higher substrate availability in reduced subsurface soil layers. It was hypothesized that anticipating the soil tillage from spring to the fall season reduces yield-scaled greenhouse gas emissions during the summer rice season due to lower availability of C compounds to methanogenic bacteria in subsurface soil layers. A seven-year study was conducted to determine soil methane (CH4) and nitrous oxide (N2O) emissions, partial global warming potential [pGWP=(CH4*25)+(N2O*298)], rice yields, and yield-scaled pGWP emissions (yield-scaled pGWP=pGWP/yield) from flood irrigated rice under spring and fall tillage treatments. No significant effect of tillage treatments on soil N2O emissions and rice yields was detected. When averaged across treatments and growing seasons (GSs), rice yield was 7.9 Mgha−1 GS−1, whereas cumulative N2O emissions were 3.65kg N2Oha−1 GS−1. Soil CH4 emissions were responsible for 91.5% of pGWP. The spring tillage treatment resulted in an earlier and larger first peak of CH4 efflux, likely due to higher labile C availability originated from the rice and ryegrass biomass decomposition in subsurface soil layers. In contrast, in the fall tillage treatment the easily decomposable compounds of the rice residue was utilized during the winter months, which combined with the ryegrass biomass kept on the soil surface resulted in lower labile C availability in subsurface soil layers. The fall tillage treatment significantly reduced cumulative CH4, pGWP and yield-scaled pGWP emissions by 24, 21, and 25%, respectively. Averaged across GSs, CH4, pGWP and yield-scaled pGWP emissions for the fall and spring tillage treatments were 316 and 417kg CH4ha−1 GS−1, 8.6 and 10.9Mg CO2eqha−1 GS−1, and 1.06 and 1.41kg CO2eqkg−1 grain, respectively. Our results indicate that shifting soil tillage operations from spring to fall can successfully mitigate yield-scaled pGWP emissions from regional flooded rice fields.