Effects Of Root Exudates On The Bioavailability Of PAHs And Their Metabolites In Soils

Polycyclic aromatic hydrocarbons(PAHs)are a class of persistent organic pollutants that were often detected in the environment and have received widespread attention because of their carcinogenicity,teratogenicity and mutagenicity.Oxygenated PAHs(OPAHs),as metabolites of PAHs,were often detected with PAHs.OPAHs may have similar or even higher toxicity than parent PAHs and their environmental effects cannot be ignored.Phytoremediation for PAHs and OPAHs contaminated soil has become a research hotspot,because of their aesthetics,low cost and environmental friendliness.Plant root exudates play a major role therein.Current researches focused on the promoting effect of plant root exudates on microbial degradation of PAHs.However,there are few studies on the abiotic role of root exudates in PAHs removal,and the related research about OPAHs is still at beginning.Therefore,it is necessary to study the effects of root exudates on the bioavailability and total forms of PAHs and OPAHs in soils and to reveal its mechanism.In the study,pyrene,9-fluorenone and 9,10-anthraquinone,artificial root exudates(ARE)were used as representatives of PAHs,OPAHs and root exudates,respectively,to investigate the abiotic effects of root exudates on the bioavailability of pyrene in the fresh and aged contaminated soil.Analytical method for OPAHs in soil was established,and the speciation characteristics of different OPAHs were explored.The effects of root exudates on the bioavailability of OPAHs in soil were studied,and the effects of root exudates on the bacterial community in OPAHs-contaminated soil were explored.The conclusions are as follows:(1)In the absence of indigenous microbial community,the content of bioavailable pyrene increased gradually with the increase of initial pollution levels in soil.The bioavailable form(F1)content of pyrene in soil Y was higher than that in soil J.The addition of ARE did not change this phenomenon.The bioavailability of pyrene in fresh and aged soils J and Y increased with increasing ARE concentration,and the activation effect of ARE on the F1 form was more pronounced at the early stage than latter stage.The addition of ARE also affected the distribution characteristics of other forms of pyrene in contaminated soil besides F1,including non-desorbing form(F2),fulvic acid-bound form(F3),humic acid-bound form(F4),crude humin-bound form(F5),mineral-bound form(F6)and kerogen-bound form(F7).There are different correlations between different pyrene speciation and the bioavailability of pyrene in soil will be affected by F2-F7 to varying degrees.ARE can change the correlation between F1 and other speciation in soil,affecting the bioavailability of pyrene and its ecological risk.(2)The results showed that the recovery of 9-fluorenone and 9,10-anthraquinone in soil was 102.4%～104.2% using methanol-dichloromethane(1:1,v/v)solution as extractant to perform ultrasonic extraction and high-performance liquid chromatography(HPLC)determination.9-fluorenone and 9,10-anthraquinone in aqueous phase and bioavailable phase(hydroxypropyl-β-cyclodextrin)were determined using liquid-liquid extraction with dichloromethane as extractant followed by HPLC analysis.The recoveries were 87.2%～87.8% and 78.7%～86.3%,respectively.At 30 d,the contents of OPAHs in different speciation followed the order of F2 > F5 >F7 > F1 > F4 > F3 > F6.The content ratio of extractable form(sum of F1 and F2)in the total residue was approximately the same as the ratio of the bound form(sum of F3-F7).The content of OPAHs bound to organic matters(F3-F5 and F7)was higher than that bound to minerals(F6).The bioavailability of OPAHs decreased as natural attenuation went on,and the bioavailability of 9-fluorenone decreased faster than that of 9,10-anthraquinone in soil.(3)The bioavailability of OPAHs in the sterilized soil was significantly higher than that in the non-sterilized soil,which ARE addition did not change,but changed the bioavailability of OPAHs in the two soils.In the sterilized soil,the bioavailability of OPAHs increased with the increase of ARE concentration at 15 d and 30 d.The high concentration of ARE treatment significantly increased F1 content.In non-sterile soil,ARE treatments had no effect on the bioavailability of OPAHs at 15 d.At 30 d,ARE treatment with the concentration of 40 mg C/kg had no effect on F1 content,but higher concentrations of ARE treatments improved the bioavailability of OPAHs.The addition of ARE also affected the distribution characteristics of F2-F7 OPAHs in contaminated soil.F1 content of OPAHs was positively correlated with F2-F7 contents in the sterilized and non-sterilized soils.ARE treatments did not change the correlation between F1 and F2,F3,F6,F7 in sterilized and non-sterilized soils,but the presence of indigenous microbial communities affected the influence of ARE on the correlation between F1 and F4,F5.(4)The addition of ARE to OPAHs-contaminated soil did not change the dominant bacterial composition of the community,but different concentrations of ARE resulted in the relative abundance changes of some taxa in the soil to different extents.The response mechanisms of different bacterial taxa were different.There was a concentration effect of ARE on the soil bacterial community.ARE reduced the diversity and richness of bacterial communities in OPAHs-contaminated soil and affected their structure.The effects of different speciation of OPAHs on the dominant bacterial phylum were different.The structure and functional group characteristics of each soil component were different.OPAHs bond to soil components had unique properties and thus had different bioaccessibility compared with free OPAHs.