Mechanism Of Ryegrass Root Exudate-enhanced Bioremediation Of PAHs-contaminated Soils With Exogenous Degrading Bacteria PPZ.1

Polycyclic aromatic hydrocarbons(PAHs)are a type of organic pollutants with carcinogenic,teratogenic and mutagenic effect that are ubiquitous in the environment.In recent years,with the increasing of soil PAHs contamination,the remediation of contaminated soil has become a research hotspot.The plant-microbe joint remediation technology is considered to be one of the most promising remediation technologies,which has the advantages of low cost,environmental friendliness,and no secondary pollution.Studying the mechanism of root exudates intensifying the remediation of contaminated soil by exogenous bacteria will help reveal the mechanism of rhizosphere effect and provide a basis for the application of joint remediation technology.In this study,ryegrass was selected as the experimental plant.The effects of PAHs stress on ryegrass root exudates and important physiological indicators of ryegrass chlorophyll,superoxide dismutase(SOD)and peroxidase(POD)were studied;the effects of ryegrass root exudates and their components on the number of exogenous degrading bacteria PPZ.1 in sterilized soil and the degradation of PAHs in the soil were discussed;the effects of ryegrass root exudates and their components on the structure of indigenous bacterial community and PAHs degradation in non-sterile soil were analyzed;finally,the effects of ryegrass root exudates and their components on the sorption behavior of pyrene in the soil were compared.The main conclusions obtained are as follows:(1)Different types and concentrations of PAHs have different effects on the secretion of amino acids and soluble sugars from the roots of ryegrass.PAHs stress not only changes the amount of amino acids secreted by ryegrass,but also changes the types of amino acids.Pyrene stress causes the biomass of ryegrass to increase with the increase of pyrene concentration.The three plant indicators of chlorophyll,SOD and POD increase first and then decrease with the increase of pyrene concentration.(2)The effect of root exudates on the quantity of PPZ.1 and degradation of pyrene varies with the remediation time and the composition of root exudates.Oxalic acid inhibits the growth of PPZ.1 in the soil as a whole.Glucose,valine,and artificial root exudates show growth-promoting effects.Ryegrass root exudates have no significant effect in the early stage,but inhibits its growth in the middle and late stages.Glucose treatment generally promoted the degradation of pyrene by PPZ.1.The root exudates of ryegrass promoted the degradation of pyrene by PPZ.1 at 30 d.No degradation promotion effect was found on the rest of the components or remediation time.The number of exogenous bacteria PPZ.1 is positively correlated with the removal rate of pyrene in the soil,but the addition of root exudates changes the relationship between the number of exogenous bacteria and pyrene degradation.(3)The addition of root exudates and their components has an impact on the diversity and richness of indigenous bacterial communities in pyrene contaminated soil.The species composition of the communities in each treatment group is similar,with Proteobacteria,Firmicutes and Actinomycetes being the main dominant phyla,while the abundance of related species is different at the phylum and genus level.The influence of ryegrass root exudates and artificial root exudates on the structure of indigenous bacterial community is greater than that of a single component.The addition of root exudates and their components can generally promote the degradation of pyrene for 30 d,inhibit it for 60 d,and have no effect on 90 d.The removal rate of pyrene by indigenous bacteria is higher than that of single exogenous bacteria PPZ.1.(4)The sorption of root exudates and their components on pyrene in the soil shows different effects due to the type and concentration of root exudates and the equilibrium concentration of pyrene in the water phase.With the addition of glucose,the K_d value increased by 21.8%-47.8%.After adding oxalic acid,the range of K_d value is-3.0%-41.5%.After adding valine,the range of K_d value is-4.6%-44.6%.The artificial root exudates changed the K_d value by-8.5%-22.9%.Low-concentration ryegrass root exudates showed a strong sorption promotion effect when the water phase concentration of pyrene was low,and a strong sorption inhibition effect when the pyrene concentration was high.