Effects Of Straw Returning To Field On Degradation Of Polycyclic Aromatic Hydrocarbons In Contaminated Soil

Polycyclic aromatic hydrocarbons（PAHs）are one of toxic persistent organic pollutants with carcinogenicity,teratogenicity and mutagenic and can be widely found in environmental media such as atmosphere,water,and soil.PAHs are mainly derived from human activities,including traffic,industrial emissions,combusition of coal and fossil fuel,etc.Soil is an important sink of PAHs,of which in various environmental media are eventually integrated into the soil through wet and dry sedimentation.PAHs in soils not only are poisonous to plants and micobes but also enter into the human body through skin contact,breathing and dietary ingestion,thereby threatening human health.Therefore,it is of great significance to reduce concentration of PAHs in soil and uptake and accumulation of PAHs in agricultural products.In this study,PAHs contaminated soil was selected as the research object to investigate the effects of returning of corn straw and stubble on PAHs degradation,the effects of adding straw,glucose,straw and glucose,sodium azide on PAHs degradation,speciation conversion,bioavailability,microbial community and functional genes in soil were studied and the effect of different proportions of corn straw（1%,2.5%,and 5.5%,w/w）on the uptake and accumulation of PAHs in winter wheat.The main conclusions are as following:The amendment of corn straw or stubble significantly（P<0.05）increased the emission rate of CO₂ in soil and the cumulative emission of CO₂ during 70 days.The emission rate of CO₂ increased with the increase of corn straw or stubble added into the soil.The amendment of corn straw and stubble can increase concentration of dissolved organic carbon（DOC）and microbial biomass carbon（MBC）in soil.Under5%（w/w）treatment,corn straw returning was more conducive to the increase of DOC and MBC than stubble returning.The amendment of corn straw and stubble apparently improved the solubility and mobility of PAHs in soil and promoted degradation of PAHs.Under 5%（w/w）treatment,corn straw returning was more conducive to the degradation of PAHs than stubble returning.After 70 days incubation,the addition of glucose and straw significantly（P<0.05）increased the degradation rate of PAHs in soil.Compared with the control soil,the degradation rate of PAHs in soil under the addition of glucose and straw and the combination of straw and glucose was increased by 13.01%、20.62%and 29.81%,respectively.At the same time,the concentration of available PAHs in soil was significantly decreased,and the concentration of available PAHs under glucose(17.12μg kg^-1),straw(16.87μg kg^-1)and the combination of straw and glucose(15.27μg kg^-1)treatments was significantly（P<0.05）lower than that in control(46.17μg kg^-1).The addition of glucose and straw and the combination of straw and glucose significantly increased the content of DOC and MBC.The accumulation of soluble organic matter increased the solubility of PAHs in soil,which lead to the adsorbed PAHs transform into bound PAHs,and then be utilized by soil microorganisms.The addition of glucose and straw and the combination of straw and glucose changed the soil microbial community,increased the relative abundance of PAH-degrading bacteria and the proportion of PAH-degrading genes,and promoted the degradation of PAHs in soils.The content of PAHs in the rhizosphere soil and non-rhizosphere soil was significantly（P<0.05）decreased by adding straw.The order of the degradation rate of PAHs in rhizosphere soil and non-rhizosphere soil was 5%>2.5%>1%.Because of the decrease of PAHs residue in soil,PAHs concentration in wheat shoots and grains decreased with the decrease of PAHs uptake by wheat roots.Under 5%（w/w）straw treatment,concentration of Ba P in wheat grain was lower than the limit of 5.0μg kg^-1in the national food safety standard.In conclusion,the degradation rate of PAHs was higher in the soil with glucose,straw and the combination of straw and glucose,and theconcentration of available PAHs（water-soluble PAHs and acid-soluble PAHs）was significantly（P<0.05）reduced,and the increase of soluble organic carbon promoted the conversion of adsorbed PAHs to bound PAHs in the soil.When the addition ration was 5%（w/w）,corn straw returning was more conducive to the degradation of PAHs than stubble returning.