Soil Greenhouse Gas Emission And Influence Factors In Different Depths Of Straw Returning

As a large agricultural production country,there are huge amounts of crop straw produced every year in China.Straw returning as an important soil improvement method,which can not only solve the problem of agricultural waste treatment and disposal,but also improve soil structure,increase soil organic matter and soil productivity.However,after straw returning to different soil depths,due to the different mineralization of organic matter,the number and activity of soil microorganisms also differ,so that the use of carbon and nitrogen in straw also changes with the depth of burial,which affects soil greenhouse gas emissions.Therefore,it is of great significance to study the influence factors of greenhouse gas emissions in different straw returning depths to mitigate global climate change and to define a reasonable depth of returning.Four returning treatments were set up,which were 0-10 cm(T1),10-20 cm(T2),20-30 cm(T3)and 30-40 cm(T4),meanwhile,non straw returning as a control(CK).The characteristics of CO2,CH4 and N2 O emissions from soil during the whole growth stages of maize were measured by static box-gas chromatography,and measured the changes of soil properties in different soil layers at different growth stages of maize at the same time,to explore the characteristics of greenhouse gas emissions and their influence factors at different returning depths of straw.The main research conclusions are as follows:(1)During the whole maize growing season,both CO2 and N2 O showed emission,but CH4 showed absorption.The cumulative emission of CO2 was the highest under T3 treatment,which increased by 28.58% significantly compared with CK.The increase rate of cumulative emission of CO2 under T4 treatment was the least,which was significantly increased by 17.10% compared with CK(P < 0.05),but the difference between T1 and T4 treatment was not significant;the cumulative emission of N2 O was the highest under T2 treatment.Compared with CK,the cumulative amount of N2 O increased significantly by 111.26%,the increase rate under T4 treatment was the least,and the CK increased significantly by 12.80%(P<0.05).However,CH4 showed absorption,and the absorption capacity of CH4 in farmland soil was reduced after straw returning;the absorption capacity was CK>T4>T1>T3>T2,and there were significant differences between treatments and CK(P<0.05).From the perspective of global warming potential(GWP)and greenhouse gas emission intensity(GHGI),returning to 10-20 cm contributes most to the global warming potential of farmland and returning to 30-40 cm can reduce the intensity of greenhouse gas emissions on a 100-year time scale.(2)Straw returning treatments significantly increased the content of soil ammonium nitrogen and reduced the content of soil nitrate nitrogen,which were higher in 20-30 cm soil layer.Straw returning can increase soil urease,catalase,and sucrase activities significantly.Urease activity showed a trend of first decrease and then increase with growth time,and the activity is the lowest during the large bell period;catalase and sucrase activities decreased as the soil depth increase.Straw returning significantly increased the microbial biomass carbon and nitrogen content.As the depth of the soil layer increased,the microbial biomass nitrogen content increased first and then gradually decreased.The content was highest in the 20-30 cm soil layer.(3)Through redundant analysis,soil Eh,catalase,soil temperature,and MBC are the main environmental factors affecting soil greenhouse gas emissions.There was a positive correlation among soil CO2 emission flux with soil Eh,catalase activity,soil temperature,and MBC,and soil N2 O emission was related to soil Eh,catalase,soil temperature,MBN,NO3--N and NH4+-N have a positive correlation,while CO2 and N2 O have a negative correlation with soil p H,moisture,and urease.(4)The total root length,total root volume,total root surface area and average root diameter of each treatment were increased,among them,the effect of straw returning to 30-40 cm was the most obvious.The total root volume,total root surface area and average root diameter were positively correlated with CO2 emission,which indicated that the root promoted soil CO2 emission.The distribution of root system with different diameter in total root length,total root surface area and total root volume was also different under different straw returning depths.The root system with diameter of 0-1 mm was the main root system in each treatment,while the root system with diameter of more than 4 mm accounted for more than 50% of the total root volume and total root surface area.