Effects Of Long-term Straw Returning And Water And Nitrogen Management On Rice And Wheat Yields And Greenhouse Gas Emissions

The increasing global population and greenhouse effect accompanied by decreases in the availability of resources put lots of pressure on modern agriculture.The modern agriculture should be high yielding,good quality,high efficiency,and low in greenhouse gas emission.Wheat-rice rotation is the main cropping system in eastern China,with an annual planting area of 26 million hectares,playing a crucial role in ensuring food security in both China and the world.Returning of rice and wheat straws to the field is an important practice to rationally utilize straw resources and increase soil fertility,but it could increase greenhouse gas emissions such as methane.Irrigation and nitrogen fertilization are two major cultivation techniques in crop production,which play important regulatory roles in crop yield,grain quality,and greenhouse gas emissions.However,it remains unclear how to improve water and fertilizer management measures to achieve high crop yield,efficient utilization of water and fertilizer,and reduction in greenhouse gases emissions under the condition of straw returning.This study was conducted in a wheat-rice rotation system,and two rice varieties Huaidao 5(HD5,conventional japonica rice)and Yongyou 2640(Y2640,indica japonica hybrid super rice)and two wheat varieties Yangmai 20(M20),and Yangmai 16(M16)were used as materials.The positioning experiment of straw returning to the field began in 2015,with four straw returning treatments including no straw returning(CK),previous wheat straw returning before rice transplanting(WR),previous rice straw returning before wheat sowing(RR),and both rice and wheat straws returned to the field(DR).Starting from 2021,two irrigation regimes,conventional irrigation(CI)and alternate wetting and moderate drying(AWMD),were added to the straw returning experiment,and two types of nitrogen fertilizer treatments,conventional urea(NU)and slow-release urea(CU),were added to the straw returning experiment.The effects of long-term straw returning and water and nitrogen management on rice and wheat yields and greenhouse gas emissions from the field were investigated and their principles were explored from aspects such as crop population quality and soil properties.The following main results were obtained.1.Effects of straw returning and irrigation regime on rice yield and crop populationqualityThe rice yield,root morphology and activity,as well as aboveground population quality indicators were determined in the designated four straw returning treatments(CK,WR,RR,and DR)during the rice growth season from 2019 to 2021.The results showed that,compared with CK,WR,RR,and DR treatments significantly increased the total spikelet number and increased the yield by 7.22%-14.3%.In comparison with CI,AWMD increased the grain number per panicle and 1000-grain weight,and grain yield by 3.18%-9.73%.the increase in grain yield under straw returning and AWMD conditions was mainly attributed to the improvement in rice population quality,including increases in root oxidation activity(ROA),leaf area index(LAI),leaf area duration(LAD),leaf net photosynthetic rate(Pn),leaf nitrate reductase(NR)and glutamate synthase(GOGAT)activities,plant nitrogen uptake rate(NUR)and plant nitrogen accumulation(NUA).All the straw returning treatments increased nitrogen partial factor productivity(PFP).The AWMD significantly reduced irrigation water and significantly improved water use efficiency(WUE).2.Effects of straw returning and irrigation regimes on soil properties in the paddy fieldThe soil properties in the paddy field were determined in the designated four straw returning treatments(CK,WR,RR,and DR)during the rice growth season from 2019 to 2021.The results showed that,in comparison with CK,WR,RR and DR treatments significantly increased contents of soil organic carbon(SOC),soil total nitrogen(STN),soil microbial biomass carbon(MBC),soil microbial biomass nitrogen(MBN),and dissolved organic carbon(DOC),enhanced soil urease(SU),sucrose reductase(SRR),and catalase(CAT)activities after the panicle initiation(PI)stage,and increased soil bacterial population abundance and diversity,especially the relative abundance of Proteobacteria,Bacteroidetes and Firmicutes at the PI stage,which contributed to the significant increase in soil nitrate(NO3-)and ammonium(NH4+)content at the same stage.Under the straw returning treatments and relative to CI,AWMD increased activities of the above soil enzymes and MBN content,increased the relative abundance of Proteobacteria,Actinobacteria and Firmicutes,and significantly increased soil NO3-content.The results indicate that straw returning can promote the carbon and nitrogen cycling in the paddy soil,and significantly improve the soil fertility from the PI to maturity,and AWMD can enhance the effect of straw returning on improving soil fertility.3.Effects of straw returning and irrigation regimes on greenhouse gas emissions from the paddy fieldThe greenhouse gas emissions from the paddy field were determined in the designated four straw returning treatments(CK,WR,RR,and DR)during the rice growth season from 2019 to 2021.The results showed that,compared with CK,WR,RR,and DR treatments significantly increased the cumulative emissions of methane(CH4),carbon dioxide(CO2)and nitrous oxide(N2O),global warming potential(GWP,excluding CO2)and GWPa(including CO2),and greenhouse gas emission intensity(GHGI,excluding CO2)and GHGIa(including CO2).The increase in greenhouse gas emissions in the RR treatment was smaller than those in the WR or the DR treatment.Straw returning treatments increased soil organic carbon sequestrations(SOCs).Compared with CI,AWMD reduced CH4 emissions,GWP and GHGI;and increased CO2 emissions.The variety Y2640 showed a lower GHGI than the variety HD5,due mainly to a higher grain yield for Y2640.4.Effects of straw returning and controlled release nitrogen fertilizer on wheat yield and crop population qualityThe wheat yield,root morphology,and crop population quality indicators were determined in the designated four straw returning treatments(CK,WR,RR,and DR)and two types of nitrogen fertilizer treatments(NU and CU)during the wheat growth season from 2019 to 2022.The results showed that,in comparison with CK,WR,RR and DR treatments significantly increased the total grain number and grain yield by 3.60%-10.4%.CU could increase the number of grains per spike and 1000-grain weight and increased the yield by 0.84%-4.33%when compared with NU under the same straw returning treatment.The increase in grain yield under straw returning and CU treatments was accounted for the improvement of crop population quality including increases in root oxidation activity(ROA),leaf area index(LAI)and leaf area duration(LAD),leaf net photosynthetic rate(Pn),leaf nitrate reductase(NR)and glutamine synthetase(GS)activities,plant nitrogen uptake rate(NUR)and plant nitrogen accumulation(NUA).The straw returning treatments increased nitrogen partial factor productivity(PFP).Application of CU could enhance the efficiency of improvement in grain yield under the straw returning treatments.5.Effects of straw returning and controlled release nitrogen fertilizer on soil properties in the wheat fieldSoil properties in the wheat field were determined in the designated four straw returning treatments(CK,WR,RR,and DR)and two types of nitrogen fertilizer treatments(NU and CU)during the wheat growth season from 2019 to 2022.The results showed that,compared with CK,WR,RR and DR treatments significantly increased soil organic carbon(SOC)and total nitrogen(STN),soil microbial biomass carbon(MBC),nitrogen(MBN)and dissolved organic carbon(DOC)contents,soil urease(SU),sucrose reductase(SRR),and catalase(CAT)activities from the booting stage,and increased the relative abundance of Ascomy.cota,Basidiomycota and Mortierella,soil nitrate(NO3-)and ammonium(NH4+)contents at the booting stage.application of CU increased activities of above soil enzymes and MBN content,increased the relative abundance of Ascomycota,Basidiomycota and Mortierella,and significantly increased soil NO3-and NH4+ contents under the same straw returning treatment.The results suggest that straw returning can facilitate the carbon and nitrogen cycling in the wheat field soil,and significantly improve the soil fertility from the booting stage to maturity,and application CU can enhance the effect of straw returning on improving soil fertility.6.Effects of straw returning and controlled release nitrogen fertilizer on greenhouse gas emissions from the wheat fieldGreenhouse gas(GHG)emissions from the wheat field were determined in the designated four straw returning treatments(CK,WR,RR,and DR)and two types of nitrogen fertilizer treatments(NU and CU)during the wheat growth season from 2019 to 2022.The results showed that,in comparison with CK,the treatments of WR,RR,and DR significantly increased the cumulative emissions of methane(CH4),carbon dioxide(CO2)and nitrous oxide(N2O),global warming potential(GWP,excluding CO2)and GWPa(including CO2),greenhouse gas emission intensity(GHGI,excluding CO2)and GHGIa(including CO2),and increased soil organic carbon sequestrations(SOCs).A higher grain yield in the WR treatment made a lower GHGI than that of RR and DR treatments.Compared with NU,CU increased SOCs and reduced the total GHG(CH4,CO2 and N2O)emissions,and consequently,decreased GWP and GHGI.7.Effects of long-term straw returning on the annual rice and wheat yields and greenhouse gas emissions from the fields and their differences between yearsIn the field with designated four straw returning treatments(CK,WR,RR,and DR),grain yield of both rice and wheat,soil properties and GHG emissions from the paddy and wheat field were determined at the first anniversary(2015-2016),the fourth anniversary(2018-2019)and the seventh anniversary(2021-2022)of straw returning.The results showed that,compared with CK,continuous seven-year straw returning significantly increased annual grain yield of rice and wheat,soil organic carbon(SOC)and sequestration(SOCs),optimized soil physical and chemical properties(improvement in soil bulk density and soil redox potential),and significantly increased GHG,including methane(CH4),carbon dioxide(CO2)and nitrous oxide(N2O)from the field.The annual increments of SOCs and grain yield were the largest in the fourth anniversary,followed by the seventh anniversary and the smallest in the first anniversary of straw returning.The increase of soil CH4 emission in the seventh anniversary of SR was significantly lower than that in the fourth anniversary,whereas the increase of soil CO2 emissions was opposite,which made the global warming potential(GWP,excluding CO2),GWPa(including CO2),greenhouse gas emission intensity(GHGI,excluding CO2)in the seventh anniversary lower than those in the fourth anniversary,especially in the RR treatment.Compared with NU-CI,CU-AWMD increased grain yield with lower environmental loss under straw returning treatments.The results demonstrate that long-term straw returning can reduce the environmental loss by reducing the CH4 emissions,and CU-AWMD has a beneficial effect on it in the wheat-rice rotation system.