Shallow flooding and optimal N applications could lead to 34.3% reduction of GHG emissions (CH4, N2O, and CO2) on a national scale.
China emits approximately 7.41 Tg CH4 per year, one of the primary non-CO2 greenhouse gases (GHG), to the atmosphere due to rice planting alone. In this study, we use the DeNitrification-DeComposition (DNDC) model to explore the potential of irrigation and fertilization managemetns for GHG mitigation from rice rotated cropping systems in China. For water schemes, switching continuous flooding to one or several drainages in paddy fields could reduce total GHG emissions from rice planting because the overall radiative forcing of CH4 reduction suppresses that of N2O stimulation. For fertilizer management practices, activities of N fertilization have found to strongly influence N2O emission through denitrification. Then we set varied scenarios of water use in more than 1600 counties, and derived optimal rates of N application for each county in accordance to water use scenarios. Our results suggest that 0.88 ± 0.33 Tg per year (mean ± standard deviation) of synthetic N could be reduced without reducing rice yields, which accounts for 15.7 ± 5.9% of current N application in China. Field managements with shallow flooding and optimal N applications could lead to 34.3% reduction of GHG emissions (CH4, N2O, and CO2), 2.8% reduction of overall N loss (NH3 volatilization, denitrification and N leaching) and 1.7% increase of rice yields, as compared to current management conditions.
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At a nationwide scale, changing water regimes from CF to MD and SF could lead to 32.3% and 53.6% reduction of GHG emissions, respectively. In comparisons, changing observed N rates to optimal rates could reduce national GHG emissions by 2.2 ± 1.7%, mostly due to reductions in N2O emission rather than CO2 and CH4 emissions. Meanwhile, yield-scaled GHG emissions are reduced from 1.37 ± 0.51 to 1.32 ± 0.52 kg CO2-equivalents kg-1-grain yield yr-1 (or 3% reduction in equivalent). Among the nine scenarios, yield-scaled GHG emissions are minimized under SF water regime with optimal N rates. Compared to the reference scenario (mid-season drainage with actual N rates), changing water management practices from MD to SF contributes to 31.5 % reduction of yield-scaled GHG emissions and further 4.2% decreases are achieved through reducing current N inputs to optimal rates. Among provinces with major rice production, Jiangsu, Yunnan, Guizhou, and Hubei could achieve more than 40% reduction of GHG emissions under appropriate water managements. Our modeling efforts suggest that China is likely to benefit from reforming water and fertilization managements for rice rotated cropping system in terms of sustainable crop yields, GHG emission mitigation and N loss reduction, and the reformation should be prioritized in the above-mentioned provinces.