Effects Of Straw Returning And Combined Application Of Nitrogen,Phosphorus And Potassium On Soil Carbon Sequestration And Crop Yield In Wheat-Maize Cropping Farmland

The efficient utilization of straw resources is of great significance for ensuring national food security and mitigating climate change.However,the cycle between carbon and nutrient elements in the soil-crop system is strongly coupled.Therefore,the effects of straw returning on soil carbon sequestration and fertilization,greenhouse gas emission reduction and crop yield are largely limited by fertilization strategies.At present,it is still unclear how different fertilizers affect straw carbon turnover,farmland greenhouse gas emissions and crop nutrient utilization under straw returning conditions.Based on this,this paper takes the wheat-maize multiple cropping dry farmland in Guanzhong area as the research object in two consecutive growing seasons from 2020 to 2021,and uses the ¹³C-labeled corn straw material to set up the interaction test of straw returning to the field and different nitrogen,phosphorus and potassium fertilizers.Five treatments were designed in the wheat season:single application of straw,straw combined with nitrogen fertilizer,straw combined with nitrogen and phosphorus fertilizer,straw combined with nitrogen,phosphorus and potassium fertilizer,no straw and fertilizer as the control group,and each treatment in the corn season was applied with nitrogen 100kg·hm-2.In order to provide scientific basis and technical support for the optimization of fertilizer management under straw returning,the effects of straw returning combined with nitrogen,phosphorus and potassium fertilizers on the spatial and temporal dynamics of soil available nutrients,the composition and size of soil carbon pool,straw carbon turnover,greenhouse gas emission characteristics,and crop yield formation were explored.The main results are as follows:（1）Compared with the control without straw and chemical fertilizer,straw returning reduced the average content of nitrate nitrogen and ammonium nitrogen in soil during crop growth period,increased the content of total nitrogen and available potassium in soil,and significantly increased the content of available phosphorus in soil during wheat and maize harvest period by 82.1%and 27.8%.Compared with single straw returning,straw combined with chemical fertilizer reduced soil total nitrogen content,but significantly increased soil available phosphorus content.Among them,straw combined with nitrogen and phosphorus fertilizer increased the most,wheat and corn harvest period increased by 53.4%and 38.4%,respectively.During the wheat-maize multiple cropping period,soil nutrients decreased with time.（2）Straw returning significantly increased the average content of SOC in the wheat harvest period,but had no significant effect on the soil organic carbon content in the maize harvest period.The content of soil organic carbon decreased with the increase of nitrogen,phosphorus and potassium application types at wheat harvest stage,but increased with the increase of nitrogen,phosphorus and potassium application types at maize harvest stage.Compared with the single application of straw,straw combined with nitrogen,nitrogen and phosphorus,nitrogen,phosphorus and potassium fertilizers decreased the soil organic carbon content by 2.1%,2.9%,8.3%and 9.7%,3.7%,0.5%,respectively.With the extension of straw returning time,the proportion of straw carbon in soil organic carbon showed an increasing trend.Whether in wheat or maize season,with the application of phosphorus and potassium fertilizer,the proportion of straw carbon in soil gradually decreased.The correlation analysis showed that during the wheat-maize multiple cropping period,the contents of soil organic carbon and particulate organic carbon decreased gradually with the extension of the test time,and the content of mineral-bound organic carbon increased gradually.（3）Compared with no straw returning and fertilization,straw returning increased the cumulative emission of nitrous oxide by 20.3%,the cumulative emission of carbon dioxide by 21.2%,and the absorption of methane by 15.0%.Compared with the single application of straw,the cumulative emission of nitrous oxide increased significantly by 114.26%,89.4%and 80.96%,the cumulative emission of carbon dioxide increased by 10.5%,4.5%and 2.1%,and the absorption of methane decreased by 14.2%,16.8%and 19.5%,respectively,indicating that both straw returning and fertilization could promote the emission of greenhouse gases in farmland.Compared with straw combined with nitrogen fertilizer,straw combined with nitrogen,phosphorus or nitrogen,phosphorus and potassium fertilizer could slow down the emission of nitrous oxide and carbon dioxide,and reduce the absorption of methane in farmland.（4）Crop productivity,aboveground dry matter and NPK accumulation increased with the increase of nitrogen,phosphorus and potassium fertilizer.Compared with no straw and chemical fertilizer,single straw treatment had a decreasing trend on the grain yield of wheat in the current season,but it could increase the grain yield of maize in the next crop.Compared with single straw application,straw combined with nitrogen,nitrogen and phosphorus,nitrogen,phosphorus and potassium fertilizer significantly increased the yield of wheat and maize by 114.1%,128.8%,131.6%and 25.4%,24.4%,23.7%,respectively.Balanced fertilization promoted nitrogen use efficiency,nitrogen partial factor productivity and nitrogen agronomic efficiency of wheat grain,but had no significant effect on the following maize.In summary,although straw combined with nitrogen and phosphorus fertilizer increased the comprehensive warming potential,it also improved soil nutrients and carbon sequestration capacity,and had a positive impact on crop productivity.Therefore,in order to increase grain yield and improve farmland carbon sequestration capacity in Guanzhong area,it is suggested that wheat and maize straw should be returned to the field for a long time in the wheat-maize multiple cropping system,and reasonable nitrogen and phosphorus fertilizers should be applied to reduce the frequency of potassium fertilizer use.