Effects Of Long-term Straw Return On N₂O And NO Emissions From Wheat-maize Rotation System In Purplish Soil

Crop straw is an important biological resource in agricultural production,which contains a lot of nutrients.China is rich in straw resources,but the utilization rate is only about 40%.In order to rationally and effectively make use of crop straw,most scholars advocate returning straw to the soil.However,straw return will change the original C and N cycles of soil,and then affects the nitrous oxide（N₂O）and nitric oxide（NO）production and emission.Therefore,this study was based on a long-term purplish soil experimental platform（established in 2005）,and explored the effects of long-term straw return on the crop yield,N₂O and NO emissions of the wheat-corn rotation system in purple soil.There are 4 treatments in this study,namely:no fertilizer as a control（CK）,conventional fertilization（NPK）,conventional fertilization+direct straw return（NPK+SR）and conventional fertilization+burning ash amendment in situ（NPK+SB）.The experiment was based on the entire wheat-maize rotation system（from November 2019 to October 2020）measurements of N₂O and NO fluxes,using a static opaque chamber-gas chromatography and a static opaque chamber-chemiluminescent analysis,and the content of environmental factors was measured.The main findings are as follows:（1）First of all,in terms of yield,the grain and straw yields of fertilization treatments（NPK,NPK+SR and NPK+SB）in the wheat season were 2.97～3.47 t ha^-1and 4.53～5.19 t ha^-1,respectively.The corresponding grain and straw yields for maize season were 4.96～5.55 t ha^-1and 5.50～6.16 t ha^-1,respectively.The crop grain yield and straw yield were:1.47～9.02 t ha^-1 and 3.68～11.31 t ha^-1 during the annual rotation cycle,respectively.Compared with NPK,the grain yield and straw yield of the NPK+SR treatment increased by 13.7%and 12.8%,respectively,while the NPK+SB treatment only significantly increased the crop grain yield,but had no significant effect on the straw yield.In general,direct straw return had the effect of increasing crop yield.（2）The range of annual N₂O fluxes for the fertilized treatments（NPK,NPK+SR and NPK+SB）was 2.29～2.77 kg N ha^-1 yr^-1.Compared with NPK,NPK+SR and NPK+SB increased the annual N₂O fluxes by 10.8%and 20.8%,respectively.Based on the background N₂O emissions of no fertilization treatment(0.39 kg N ha^-1 yr^-1),the direct emission factor of N₂O for the fertilized treatments（NPK,NPK+SR and NPK+SB）was calculated to be 0.68～0.85%for the annual rotation cycle.Compared with NPK,NPK+SR treatment significantly increased the direct emission factor of N₂O by 13.1%;compared with NPK+SR,NPK+SB significantly increased the direct emission factor of N₂O by 10.6%.The yield-scaled N₂O emission of the fertilization treatments was calculated to be 282.3～331.6 g N t^-1.Straw return did not significantly affect the yield-scaled N₂O emission,but NPK+SR significantly reduced the N₂O emission intensity by 14.8%compared with NPK+SB.（3）On the anniversary of wheat-maize rotation,annual NO flux for the fertilized treatments was 0.24～0.33 kg N ha^-1 yr^-1,the direct emission factor of NO for the fertilized treatments was 0.08～0.11%,and the range of yield-scaled NO emissiones was 27.4～41.7 g N t^-1.Compared with NPK,the annual NO flux,direct emission factor of NO and yield-scaled NO emission of NPK+SR and NPK+SB were significantly reduced by 25.4%and 26.4%、27.2%and 28.2%、34.4%and 30.2%,respectively,but there was no significant difference between NPK+SR and NPK+SB.（4）On one-year scale,soil moisture,ammonium nitrogen（NH₄⁺-N）,nitrate nitrogen（NO₃^--N）and dissolved organic carbon（DOC）were main environmental factors that affect N₂O and NO fluxes.Among them,the content of NH₄⁺-N and DOC were,the main factors affecting N₂O emission,which were in a positive correlation;and DOC and inorganic nitrogen content（NH₄⁺-N and NO₃^--N）were the main factors affecting NO emission,which were also in a positive correlation.In summary,compared with NPK,both NPK+SR and NPK+SB had the effects of increasing production and reducing NO emissions.For N₂O,compared with NPK+SB,NPK+SR significantly reduced N₂O emission intensity.In addition,the straw of NPK+SB will cause air pollution and harm human health in the combustion process.Therefore,from the perspective of economic（crop yield）and comprehensive environmental benefits,direct return straw combined with inorganic fertilizer application may be a climate and environment friendly management measure for winter wheat-summer maize rotation system in alkaline purplish soil in Southwest China.