Effects And Mechanism Of Straw And Biochar Addition On Greenhouse Gas Emissions From Tobacco-planting Soil

In order to explore the effect and mechanism of straw and biochar addition on greenhouse gas emissions from tobacco planting soil,we designed four treatment modes:conventional fertilization(CK),conventional fertilization+2.25 t·hm-2 biochar-C(T1),conventional fertilization+2.25 t·hm-2 straw-C(T2),conventional fertilization+1.125 t·hm-2 biochar-C+1.125 t·hm-2 biochar-C(T1).The effects of straw and biochar on soil physical and chemical properties,soil microbial biomass,greenhouse gas emissions and relative abundance of functional genes were studied.The main results are as follows:(1)Adding biochar and straw significantly increased soil respiration rate and total CO2 emissions,T3 increased the most,while T1 increased the least.T1 promoted soil respiration in the early growth stage and inhibited it in the late growth stage.Adding biochar and straw could significantly increase the absorption of CH4.Compared with CK,T1,T2 and T3 increased the absorption of CH4 by 34.2%,9.3%and 22.1%,respectively.Compared with CK,T2 and T3 significantly increased the cumulative emissions of soil N2O by 1.2 and 1.1 folds,respectively,whileTl significantly reduced the emission of soil N2O by almost 30%.(2)The addition of biochar and straw significantly increased the relative abundance of pmoA gene,and T1 treatment had the greatest positive effect on pmoA gene at the late growth stage of tobacco.T2 treatment increased the relative abundance of mcrA gene,while T1 and T3 treatment had no significant effect on mcrA gene.The relative abundance of amoA-AOA,nirS,nirK and nosZ genes decreased with the addition of biochar,while the relative abundance of amoA-AOB increased.On the contrary,T2 treatment increased the relative abundance of amoA-AOA,nirS,nirK and nosZ genes.Compared with T2 treatment,T3 significantly increased the relative abundance of nosZ.(3)Adding biochar and straw significantly increased the contents of soil microbial biomass C,microbial biomass N and organic carbon.T3 and T1 had the greatest promotion effect on organic carbon,followed by T2,which showed T3≥T1>T2>CK,increased by 36.4%～45.1%,30.5%～42.3%and 8.4%～16.7%respectively,compared with CK.Microbial biomass C and N reached the peak at 60 days after transplanting,and the promotion effect of T2 and T3 was greater than that of T1.(4)Biochar and straw returning significantly reduced soil bulk density,in which T1 decreased the most,followed by T3.T1,T2 and T3 decreased soil bulk density by 1.16%～9.71%,2.96%～6.12%and 0.52%～9.68%respectively.The addition of straw and biochar had no significant effect on the content of available nitrogen,but significantly promoted the content of available phosphorus and available potassium,and T2 treatment had the most significant effect.(5)Adding straw and biochar significantly increased soil invertase activity,Tl and T3 increased significantly after 45 daysof transplanting,T2 and T3 increased significantly 60 days after transplanting.T1,T2 and T3 increased soil invertase activity by 16.0 mg·g-1·24h-1,14.4 mg·g-1·24h-1 and 20.7 mg·g-1·24h-1,respectively.The effect of biochar and straw application alone on urease activity was not significant.The urease activity of biochar combined with straw application was significantly increased by 12.47%,compared with CK.(6)Biochar and straw addition significantly increased the colony number of soil functional bacteria.T1 had a significant effect on nitrogen fixing bacteria,T2 had no significant effect on fungi,and T3 showed a positive interaction.(7)Correlation analysis showed that soil respiration rate had a significant exponential correlation with soil temperature.There was a significant positive correlation between cumulative CO2 emission and soil microbial biomass carbon and nitrogen,as well as the number of bacteria and actinomycetes.The structural equation model showed that the main factors affecting N2O emission were NH4+-N,NO3--N,pH,amoA-AOA and nirK genes.The main factors affecting CH4 emission were bulk density,pH,SMBC,pmoA and mcrA genes.In conclusion,adding straw and biochar can improve soil fertility to a certain extent,but straw returning can significantly increase CO2 and N2O emissions of tobacco planting soil.Compared with straw,biochar can significantly reduce greenhouse gas emissions by regulating functional microorganisms and soil physical and chemical properties.Therefore,adding biochar is an effective measure to reduce greenhouse gas emissions on the basis of improving soil fertility.