Effect Of Biochar On Atrazine Dissipation In Soil And Biochemical Mechanisms

As one of the most common herbicides,atrazine(AT)has the characteristics of high toxicity,long persistence and high mobility threating the ecological environment.Biochar(BC)is a carbon-rich solid substance generated from biomass after pyrolysis under anoxic conditions,which has the advantages of high stability and sorption capacity.Nowadays,biochar is widely applied in soil pollution remediation.However,most studies mainly focused on the effect of biochar on the sorption and leaching of atrazine in soil.The effect and related mechanism of atrazine dissipation affecting by biochar in soil is unclear yet.The environmental risk of atrazine in soil includes migration risk and residual risk both.The residual risk of atrazine is determined by its dissipation behavior.However,the complexity of soil structure also causes the uncertain of atrazine dissipation affecting by biochar.The researche based on the effect of biochar on the distribution and dissipation of atrazine in soil fraction with different size is rarely seen.Therefore,this study focused on the effect of biochar on atrazine disspitaiton in sterilized and non-sterilized soil and soil fraction(sand,silt and clay).The effect of biochar on the sorption of soil and soil fractions were also investigated.And the correlation between sorption and dissipation of atrazine were evaluated to elucidate the physicochemical mechanism of atrazine dissipation affecting by biochar in soil.Furthermore,the microbial activity,diversity and composition in atrazine contaminated soil after biochar amendment were determined.And the interactions that occur among biochar,microorganisms,and atrazine,and their combined effects on atrazine disspitation in soil were explored.The aim was to elucidate the biochemical mechanism of atrazine dissipation affecting by biochar in soil.The biochar derived from silkworm excrement(BCse)and sugarcane top(BCst)passed through a 150?m-sieve and named as BCse100 and BCst100,respectively.And the BCse and BCst passed through a 75?m-sieve were named as BCse200 and BCst200,respectively.The results of this study will provide more comprehensive information for the application of BC to remediate AT-contaminated soil and control of its environmental risks,which is of great significance to agricultural production and the control of organic pollutants.The main results were summarized as follows:(1)Effect of biochar on the distribution of atrazine in soil and soil fractions.There was no significant difference on the proportion of soil fraction with different size between the sterillised and unsterillised group after biochar amendment.Howerver,biochar amendment improved the content of non-desorbing atrazine of sand,silt and clay in non-sterilized soil.In non-sterilized soil of 21^th day,application of BCse100 and BCst100 did not change the distribution of non-desorbing atrazine in soil fractions,and the highest distribution of non-desorbing atrazine can be observed in sand.Whereas,after the application of BCse200 and BCst200,the highest distribution of non-desorbing atrazine can be observed in silt with the proportions of 47.69%and 47.74%,respectively.(2)Effect of biochar on the dissipation of atrazine in soil and soil fractions.Biochar amendment retarded the dissipation of atrazine both in sterilized and non-sterilized soil,but their dissipation pathways were totally different.The atrazine dissipation in non-sterilized soil was significantly faster than that in sterilized soil,which suggested that microbial degradation was the main pathway of atrazine dissipation in non-sterilized soil and the chemical dissipation was the main pathway in sterilized soil.In non-sterilized soil,BCse and BCst amendment decreased the dissipation rate of atrazine(21?63 d)by 2.53%?2.85%and 5.10%?8.00%,respectively.However,biochar amendment enhanced the dissipation of non-desorbing atrazine in soil,sand,silt and clay compared with the treatment without biochar amendment.In sterilized soil,the dissipation of desorbing atrazine was suppressed after biochar amendment,while the residual of non-desorbing atrazine was improved in soil and soil fractions,expecially in clay.(3)Chemical mechanism of atrazine dissipation affecting by biochar in soil.Isotherm adsorption experiments were conducted for investigating the atrazine adsorbing kinetics of soil and different size fractions.Freundlich model can fit the sorption behavior of atrazine in soil well.Biochar inproved the sorption capacity of soil and soil fractions,and the better results were observed in BCst amended soil.The correlation analysis suggested that the chemical dissipation amount of desorbing atrazine was significantly negatively correlated with TOC content and sorption capacity(K_f)in soil,which indicated that biochar enhanced the conversion of desorbing atrazine to non-desorbing atrazine due to the higher sorption capacity in soil,and thus inhibited the chemical dissipation of atrazine.Moreover,in sterilized treatment,biochar amendment improved the non-desorbing atrazine residual due to the higher sorption capacity of soil and soil fractions after biochar amendment.Therefore,the physicochemical mechanism of atrazine dissipation in soil affecting by biochar can be summed up as that the improvement of TOC content and sorption capacity of soil and soil fractions after biochar amendment enhanced the transformation and redistribution of atrazine in soil,and thus inhibited the atrazine dissipation in soil.(4)Effect of biochar on the microorganism in atrazine contaminated soil.Atrazine contamination inhibited microbial biomass,and the activities of dehydrogenase,catalase and urease.But atrazine improved the fungal diversity and functional diversity in some extent.Additionally,the higher concentration of atrazine,the stronger inhibition of bacterial diversity.Application of biochar improved microbial activity,functional diversity and bacterial diversity in atrazine contaminated soil,expecially improved the bacterial diversity in clay.Biochar amendment improved the relative abundance of Proteobacteria?Actinobacteriota and Firmicutes,but decreased that of Bacteroidetes in soil contaminated with 5 mg·kg^-1atrazine.In the soil contaminated with 50 mg·kg^-1atrazine,biochar amendment improved the relative abundance of Actinobacteriota but decreased that of Firmicutes.However,in soil fraction with different size,biochar amendment inproved the relative abundance of Firmicutes in clay.(5)Microbial mechanism of atrazine dissipation affecting by biochar in soil.The correlation analysis among biochar,microorganism and atrazine dissipation suggested that it was different that the key microorganisms involved in atrazine degradation in high atrazine(50mg·kg^-1)contaminated and low atrazine(5 mg·kg^-1)contaminated soil.Under the stress of atrazine in low concentration,the relative abundance of Bacteroidetes was positively correlated with microbial dissipation of atrazine.Biochar amendment decreased relative abundance of Bacteroidetes in soils and thus inhibited the dissipation of atrazine,which indicated that Bacteroidetes was the key bacteria involved in atrazine degradation.Under the stress of high concentration of atrazine,the relative abundance of Firmicutes was positively correlated with the dissipation of atrazine in soil and soil fractions.Biochar amendment decreased the relative abundance of Firmicutes in soil,and thus inhibited the atrazine dissipation in soil.However,biochar amendment increased the relative abundance of Firmicutes in soil fractions,and thus enhanced the dissipation of non-desorbing atrazine in soil fractions.The dissipation rate of desorbing atrazine was significantly higher than that of non-desorbing atrazine,which caused the retardation of microbial dissipation of atrazine in soil.Therefore,the microbial mechanism of atrazine dissipation in soil after biochar amendment can be summed up as that biochar inhibited the microbial degradation of atrazine by reducing the relative abundance of key bacteria(Bacteroidetes and Firmicutes)in soil.In conclusion,biochar amendment improved the sorption of atrazine on soil,enhancing transformation and redistribution of atrazine in soil and soil fractions,and decreased the relative abundance of key bacteria involved in atrazine degradation,which both inhibited the chemical and microbial dissipation of atrazine,and correspongdingly decreased migration of atrazine.In this study,the sorption of atrazine significantly improved and the atrazine residual was low after in soil treated with BCst100.Moreover,according to the difference of physical and chemical properties in soil,selecting the biochar with appropriate particle size and adjusting the distribution proportion of biochar in each particle size component of soil can not only effectively reduce the risk of atrazine migration,but also effectively control its residual risk.