Study On Carbon And Nitrogen Sequestration And Greenhouse Gas Emissions Reduction Effects Of Nitrogen Fertilizer And Straw Return In Wheat-Maize Cropping System

Greenhouse gas emissions exacerbate global warming and seriously disrupt the balance of ecosystems.As an important guarantee of food security in China,the intensive production mode of wheat-yew cropping is an important source of greenhouse gas emissions.However,the current research on the effect of nitrogen fertilizer and straw return on carbon sequestration and emission reduction in farmland under wheat-maize rotation system is relatively lagging,especially the annual research.Both N(nitrogen)fertilizer and straw return as conventional on-farm management measures can affect crop yield and soil carbon sequestration and emission reduction capacity if they are not applied properly.In order to investigate the effects of nitrogen application and straw return on carbon and nitrogen fixation and greenhouse gas emissions in the wheat and jade cropping areas,this study was conducted in the wheat-maize cropping system in Guanzhong,Shaanxi Province.Four N fertilizer levels(N₀:no fertilizer in the maize season;N₁₅₀:150 kg/hm² in the maize season;N₂₂₅:225 kg/hm² in the maize season;N₃₀₀:300kg/hm²in the maize season)and two straw management practices(S₀:no straw return;S₁:straw return)were conducted in this study,with a total of eight treatments.The experiment systematically investigated the effects of nitrogen fertilizer and straw return on the crop-soil carbon and nitrogen pools and greenhouse gas emissions,and analyzed the mechanisms of the effects of nitrogen fertilizer and straw return on soil biological properties,and comprehensively evaluated the effects of different nitrogen application rates with straw return on crop-soil-environment benefits using factor analysis to provide a theoretical and practical basis for realizing green agricultural production and developing low-carbon agriculture in the region.The main findings and results are as follows.(1)Both nitrogen application and straw return significantly increased maize and wheat yields and carbon sequestration,with total crop yield and carbon sequestration increasing and then flattening out with increasing nitrogen application under the same straw management.14.0%,12.8%and 12.8%for maize,and 8.7%,8.3%and 8.3%for wheat,respectivelySoil organic carbon storage,carbon sequestration and sequestration rate increased in each N application treatment after three continuous crop growth cycles compared with those before the experiment,and the trend was increasing with the increase of N application.The soil organic carbon content and carbon sequestration rate increased on average by 18.6%and 142.3%,respectively,for each treatment of straw return compared with without straw return.Soil readily oxidizable organic carbon content showed a trend of increasing and then slightly decreasing with increasing N application,ranging from 4.59 to 6.60 mg/kg,with an average increase of0.91 mg/kg at the same N application rate for straw returned compared to the corresponding non-returned field,with the largest difference between the two at N₂₂₅.Both N application and straw return significantly increased microbial carbon content,while microbial quantity carbon was also influenced by season,mainly showing that soil microbial carbon was higher in wheat harvest than in maize harvest.Combining the two agronomic measures,the effect of nitrogen fertilizer management on crop carbon sequestration was significantly greater than that of straw management,while for soil carbon pool,straw management was the most important factor affecting soil carbon sequestration(2)Nitrogen fertilizer and straw return significantly increased nitrogen uptake in both wheat and maize during different years,and there was a significant interaction between nitrogen and straw management.The nitrogen utilization data from six crops over three years showed that nitrogen application significantly reduced nitrogen fertilizer productivity,nitrogen fertilizer uptake efficiency and apparent recovery rate in the maize season and increased nitrogen fertilizer productivity and uptake efficiency in the wheat season.Relative to without straw returned,straw return significantly increased nitrogen utilization in maize and wheat,with an average increase of 5.58%to 13.84%,16.94%to 20.30%and 15.44%to 52.71%in nitrogen fertilizer productivity,nitrogen fertilizer uptake efficiency and nitrogen fertilizer apparent recovery in maize,respectively,and an increase of 8.23%in nitrogen fertilizer productivity and nitrogen uptake efficiency in wheat and 6.98%.After three management cycles,the soil total N content in the 0-20 cm soil layer of N₀-S₀and N₁₅₀-S₀ decreased compared with that before the experiment,while the soil total N content and storage in the N₂₂₅-S₀ and N₃₀₀-S₀ treatments increased significantly.The straw return treatments with the same N application significantly increased the soil total N content and N sequestration rate compared to the no straw return.The increasing effect of straw return on nitrate-N storage was mainly concentrated above the 0-100 cm soil layer,with 7.9%,7.2%,32.6%and 42.1%lower nitrate-N accumulation in the 100-200 cm soil layer in the N₀,N₁₅₀,N₂₂₅ and N₃₀₀ treatments,respectively,compared with no straw return.Nitrogen application significantly increased microbial N content,with N₂₂₅?N₃₀₀>N₁₅₀>N₀.At the same N application rate,straw return increased soil microbial N content compared with no return,and N_150-S₁ and N₂₂₅-S₁ treatments significantly increased microbial N.(3)Compared with no straw return,straw return significantly improved the stability of soil aggregation structure and enzyme activities.Under the same straw management,sucrase,urease and alkaline phosphatase enzyme activities tended to increase and then decrease with increasing N application,reaching a maximum at 225 kg/hm² of N application.The OTU,Chao1 and Shannon index of soil microorganisms were significantly affected by N application,and all three indexes showed a parabolic trend of increasing and then decreasing with N application.Regardless of whether the straw was returned or not,the dominant bacterial groups in different nitrogen application treatments were aspergillus,acidobacter,actinobacteria and green bacteria.According to LEf Se analysis,the high nitrogen treatment(N₃₀₀)produced significantly more differential flora than the lower nitrogen application treatments(N₀,N₁₅₀and N₂₂₅),and the differential flora were mainly amoebacteria and?-Amoebacteria(p?Proteobacteria).Soil total N,SOC and NO₃^--N concentrations were the main environmental factors affecting the soil microbial community under the treatment of no straw return,while soil temperature,urease activity and NO₃^--N concentration were the main influencing factors under the straw return.(4)The average emission flux and cumulative emissions of N₂O showed a pattern of high N fertilization and high release.Compared with no straw return,straw return intensified the average emission flux and cumulative emission of N₂O,increasing on average by 48.1%,65.1%and 34.4%,32.9%during the growing period of maize and wheat.Soil CO2 emissions had a significant seasonal effect,showing a high summer and low winter.Under the same straw management practice,CO₂ emissions tended to increase with increasing N application;under the same N application,straw return significantly increased CO₂ emission fluxes and cumulative emissions compared with the corresponding no straw treatment,with larger increases in the N₂₂₅ and N₃₀₀ treatments.In northern drylands,CH₄ emission fluxes were usually negative,indicating that drylands are a sink for CH₄.From a year round perspective,both N application and straw return had a tendency to reduce soil uptake of CH₄,and the N₃₀₀-S₁ treatment had the lowest cumulative CH₄ uptake.Nitrogen fertilizer and straw return increased GHG warming potential,and when considering the amount of soil carbon sequestration,N application significantly reduced the net GHG warming potential,with an overall performance of N₂₂₅>N₁₅₀>N₃₀₀>N₀,straw return increased the absolute value of greenhouse gas emissions,but the increase in soil organic carbon sequestration and crop yield significantly reduced the net greenhouse gas emission intensity,with the lowest net emission intensity in the N₂₂₅-S₁treatment.(5)By using factor analysis,the seven indicators of net benefit,crop carbon sequestration,SOC storage,mean weight diameter of agglomerates,OTU,cumulative N₂O emissions and NO₃^--N storage in 100-200 cm soil layer were grouped into two main factors,and the cumulative variance that could be explained was 94.04%,and the overall ranking order of each treatment was N₂₂₅-S₁>N₁₅₀-S₁>N₃₀₀-S₁>N₀-S₁>N₁₅₀-S₀>N_225-S₀>N₃₀₀-S₀>N₀-S₀,i.e.,the total benefit of straw return with N fertilizer was better than that of straw not returned to the field,with N₂₂₅-S₁ being the best.In conclusion,straw return and nitrogen fertilizer application can significantly improve soil agglomeration structure,activate soil biological properties,increase soil organic carbon and total nitrogen storage as well as carbon and nitrogen fraction content,while improving crop carbon sequestration and nitrogen absorption capacity,reducing the net greenhouse effect of farmland,and contributing to the improvement of farmland carbon sequestration and emission reduction capacity.The N₂₂₅-S₁ treatment(225 kg N/hm² in maize season and 195 kg N/hm²in wheat season with straw return)is the best combination of crop,soil and environmental benefits,which can effectively improve field productivity,enhance crop and soil carbon and nitrogen fixation capacity,and have significant effects on enriching soil microbial communities and reducing greenhouse gas emissions as well as NO₃^--N accumulation in the soil layer below100 cm.