Yield-scaled greenhouse gas emissions from flood irrigated rice under long-term conventional tillage and no-till systems in a Humid Subtropical climate

•NT reduced soil CH4 emissions by 21%, but had no effect on soil N2O emissions.•CT leads to high DOC and low Eh in deeper soil layers, thus increasing CH4 emission.•CH4 accounted for 96.5% of partial GWP in flooded subtropical rice systems.•Rice grain yields (7.8Mgha−1) were not affected by tillage systems.•NT had 23% lower yield-scaled pGWP due to lower CH4 emission and no yield reduction. Soils under flooded rice (Oryza sativa L.) production are one of the major anthropogenic source of CH4 emissions, an important greenhouse gas (GHG) with a 25-times larger global warming potential (GWP) than CO2. No-till systems (NT) systems may be a viable alternative to mitigate GHG emissions in comparison to conventional tillage (CT). The objectives of this study were to evaluate on a field scale the long-term effects of CT and NT systems on soil CH4 and N2O emissions, rice yields and yield-scaled emissions during five growing seasons (GS) in Southern Brazil. In addition, a short-term greenhouse experiment was conducted to isolate the effect of winter crop [ryegrass (Lolium multiflorum L.)] biomass incorporation on soil CH4 efflux. Averaged across years, the NT system resulted in 21% lower seasonal CH4 emissions than the CT system, at 408 and 517kg CH4ha−1 GS−1, respectively. No significant effect of tillage system on N2O emissions was observed. Methane emission was responsible for 96.5% of partial GWP (pGWP=CH4×25+N2O×298), stressing the importance of this GHG for developing low GHGs rice systems. No significant effect of tillage system on rice grain yields (average of 7.8Mgha−1GS−1) was detected. Consequently, the NT system resulted in 23% lower yield-scaled pGWP, at 1.35 and 1.76kg CO2eqkg−1 grain for NT and CT treatments, respectively. According to the greenhouse study, the incorporation of ryegrass biomass into the soil resulted in 2.8 times larger cumulative CH4 emission than the surface application of biomass, at 347.4 and 125.5g CH4m−2, respectively, due to higher dissolved organic carbon (DOC) concentration and reduced soil environment in subsurface soil layers. Our results indicate that biomass incorporation is the main cause of higher CH4 emissions from conventionally tilled soil and that NT system is a viable alternative to reduce yield-scaled GHG emissions from flooded rice fields.