Impacts Of Rice Cropping Technology Innovation On Rice Yield And Greenhouse Emissions Under Major Cropping Systems

Rice is a main grain crop in China and rice production is facing great challenges,such as climate change,resource limitation and environmental pollution.A sustainable increase in rice yield is necessary to ensure food security in future.Meanwhile,nearly one-third of global agricultural methane emissions are from rice fields.CH₄ is a long-lived greenhouse gas and its global warming potential is 25 times of CO₂.The increasing demand for rice has induced great concerns on the stimulation of greenhouse gas?GHG?emissions.Studies have shown that,theoretically,rice yield increase and GHG emissions reduction can be achieved through the application of effective rice cropping technologies and cropping systems.Thus,more emphasis should be paid on the new cropping technologies that can increase rice yield and reduce GHG emissions.Understanding and generalizing the new technologies will benefit the policy decisions and are significant for the food security of our country.Many studies have been conducted to study the impacts of rice cropping technologies,however,most of them were singly focused on the impacts on rice yield or GHG emissions.The integrated impacts of rice cropping technologies on rice yield and GHG emissions are still ambiguous.Therefore,we investigated impacts of rice cropping technology innovation on rice yield and GHGs emissions under major Chinese cropping systems since 1960s.The cropping technologies mainly included rice varieties evolution,rice nursery methods,and cultivation techniques innovation.All the investigations were based on three main rice cropping systems,including:annually single rice cropping system in North China?SR?,annually rice-upland rotation cropping system in Central China?RUR?and annually double rice cropping system for early rice?EDR?and late rice?LDR?in South China.Field measurements of CH₄ and N₂O fluxes were tested during rice seedling stage under flooded nursery?FN?,moist nursery?MN?and dry nursery?DN?in the major Chinese rice cropping areas.Rice yield and GHGs emissions of leading rice varieties since1960s were selected and investigated.Also,a meta analysis was conducted to quantify the impacts of rice cropping practices on the greenhouse gas emissions at the yield-scale rather than area-scale in China.Finally,a life cycle assessment?LCA?method was used to examine the impact of cropping technology innovation on the environmental impact of the rice production system.A comprehensive assessment of the carbon emissions of rice cropping systems will be helpful for the cropping technology innovation and policy selection for higher-yield with less-emission,as well as climate smart cropping.Main results of this paper are as follows:1.Chinese rice cropping technologies,such as seedling raising methods,planting technologies,irrigation and fertilization methods,have been changed greatly from 1970s to2010s.Rice seedling raising methods have been changed from traditional FN to DN and MN under all rice cropping systems.The factory seedling cultivation method will be expanded in future with the popularization of the mechanized transplanting technology.DN is the most popular raising methods in SR cropping system at present,and the factory seedling cultivation method will be expanded from 11.7%in 2010 to 30%in 2020.Trends of raising methods in RUR and DR are similar,with the highest application of MN and the less of FN.The overall trend of rice planting technology showed a decrease in manual transplanting and an increase in direct seeding,mechanized transplanting and seedling broadcasting.In 2010,the proportion of seedling broadcasting,mechanized transplanting and direct seeding have been increased to 24%,11%and 10%,respectively,while the manual transplanting was gradually decreased to 55%.And the proportion of manual transplanting will go on declining to 35%while mechanized transplanting will be increased to 30%in the next ten years.Flooded irrigation after rice transplanting accounted for 94%of irrigation methods in 1970s,and it was decreased to 11%in 2010.In contrast,intermittent irrigation became the main irrigation method in recent years.Compound fertilizer and urea are main nitrogen fertilizer types in China,followed by ammonium bicarbonate.Ammonium bicarbonate fertilizer reduced from 40%in 1998 to 20%in 2010,while the urea application rate was increased,accounting for 75%of the total nitrogen applied in 2011.The urea and compound fertilizers will continue to increase in future.2.The evolution of rice varieties has significantly increased rice yield with a declining trend in GHGs emissions.over the past fifty years.There were no significant correlation between methane emission and rice biomass and yield with the evolution of rice varieties under all cropping systems.Rice aboveground had an indirect effect on methane production,thus showing less impact on methane emission.Yield-scale GWP in DR was highest among three main rice cropping regions.Yield-scale GWP in RUR was higher than SR in 1960s and they had similar yield-scale GWP in 2000s.Leading rice varieties in 2000s had a higher yield with lower emissions than old varieties in 1960s.3.Methane emissions from FN were significantly higher than those from the MN and DN under all the rice cropping systems.Relative to FN,MN decreased total CH₄ by 74.2%,72.1%and 49.6%under the rice-upland rotation cropping system?RUR?,and the double rice cropping system for the early rice?EDR?and the late rice?LDR?,respectively.DN decreased CH₄ emission by 99.2%,92.0%,99.0%and 78.6%compared to the FN under the single rice cropping system?SR?,RUR,EDR and LDR,respectively.When comparing with FN,MN and DN increased N₂O emission by 58.1-134.1%and 28.2-332.7%,respectively. Ultimately,the net GWPs of CH₄ and N₂O were decreased by 33-68%with MN and43-86%with DN in comparison with the FD.However,the mitigating effect of MN and DN on total GWPs varied greatly across rice cropping systems,ranging from 30.78%in the LDR to 86.46%in the SR.Chinese actual emission from rice nurseries was reduced to956.66×103 t CO₂ eq from the estimate of 2242.59×103t CO₂ eq under the flooded nursery scenario·4.Higher rice yield with lower GHGs emission can be achieved through the adjustment of irrigation,fertilization methods and cropping system.Intermittent irrigation significantly reduced yield-scaled GWP by 59%,largely because of a significant reduction in CH₄ emission and a significant increment in rice yield.Nitrogen fertilization?50-300 kg N ha-1?significantly increased rice yield with slight increments in the GWP of CH₄ and N₂O emissions,resulting in significant reductions in the yield-scaled GWP.The greatest reduction occurred at the application rate of 150-200 kg N ha-1 by 37%as compared to the non-fertilization control.For organic amendments,biogas residue application had no significant effect on the yield-scaled GWP,while manure application and straw recycling significantly increased the yield-scaled GWP by 54%and 154%,respectively.Yield-scaled GWP of Chinese major rice cropping systems during rice growing season was in the order:double rice cropping system?1188.9 kg CO₂ eq Mg-1?>rice-upland crop rotation system?777.0 kg CO₂ eq Mg-1?>single rice cropping system?346.7 kg CO₂ eq Mg-1?.5.The spatial carbon footprint of Chinese rice production showed an increasing trend while the yield-scaled carbon footprint showed a decreasing trend from 1970s to 2010s. Carbon emissions of material input in rice production were increased by 115%while greenhouse gases emissions?GHGs?during rice growing stage were decreased by 28%. Thus,spatial carbon footprint of rice production were increased by 12%.Meanwhile,rice yield was increased by 37%during the past fifty years,so yield-scaled carbon footprint was decreased.The proportion of material input was gradually increased from 28.2%in 1970s to 54.1%in 2010s.In contrast,the proportion of GHGs was decreased from 69.7%in 1970s to 45.6%in 2010s.The spatial carbon footprint of Chinese rice production will be increased in 2030s.Yet,the application of new rice cropping techniques with higher yield and lower emission,such as new rice variety,seedling raising methods,irrigation and fertilization methods,can decrease yield-scaled GHGs emissions by 36.3%-57.4%in 2030.