Mechanism Of Salix Viminalis/Crucibulum Laeve Combination To Remediate Polycyclic Aromatic Hydrocarbon-Contaminated Soil Through Organismal Interactions

Polycyclic aromatic hydrocarbons（PAHs）are widely concerned organic pollutants known for their carcinogenicity,mutagenicity,and toxicity to humans or animals.PAHs accumulate in water,sediments,soil,and absorbed by plants,ultimately resulting in their bioaccumulation in various food chains and causing a high health risk to humans.On account of remarkable advantages such as environmentally friendly,relatively inexpensive,and feasible for large-scale application,bioremediation is particularly attractive for the detoxification of PAH-contaminated soil.Plants or white-rot fungi（WRF）were known to have the ability to eliminate/degrade soil PAHs.However,considerable amounts of unfavorable outcomes were obtained from the phytoremediation and WRF-based bioaugumentation experiments of PAHs remediation.These facts impede their application in remediation of PAHs.A recent study has shown that plant/ WRF combination remediation is a more efficient remediation strategy for PAH-contaminated soil,but the mechanism of this strategy is still unclear,and its application prospects remains questionable.Based on above,a pot experiment was conducted in greenhouse to remedy PAH-contaminated soil and four bioremediation treatments: natural attenuation（NA）,mycoremediation（M）,phytoremediation（P）,and plant-microbial remediation（PMR）were applied.Salix viminalis and Crucibulum laeve were selected as experimental materials.This study aimed to（1）evaluate whether the C.laeve can assist in the phytoremediation of soil PAHs by S.viminalis;（2）explore the dynamic succession of soil microbial communities driven by different biotreatments;（3）determine the effects of C.laeve inoculation on the photosynthesis and metabolic profilles of rhizosphere soil of S.viminalis.Our study shed new light on the organismal interactions and their impacts on the degradation of soil PAHs during plant/WRF combination remediation experiment,which is also of great significance to elucidate the underlying mechanism of this bioremediation strategy,and thus broaden the prospect of bioremediation for remedying PAHs contamination.Results showed that:The effect of C.laeve inoculation（using two inoculation treatments and a non-inoculated control）on the phytoremediation potential,growth,and antioxidant metabolism of S.viminalis cultivated in PAH-contaminated soil were studied.The S.viminalis-C.laeve association synergistically led to the highest PAH removal rate.Under the PMR treatment,80% of bioconcentration and translocation factors for all tissues of S.viminalis were > 1,and this ratio was much larger than that with P treatment（20%）.C.laeve inoculation remarkably enhanced the phytoremediation by promoting the S.viminalis-based phytoextraction of PAHs from soils.Furthermore,although C.laeve inoculation altered the antioxidant metabolism of S.viminalis by inducing oxidative stress,thereby inhibiting plant growth;because of its tolerance and resilience,S.viminalis could survive and grew normally for 60 days after the treatments were applied.Therefore,S.viminalis-C.laeve remediation can be considered a feasible approach for the phytoremediation of PAH-contaminated soil.Compared with NA treatment,M treatment significantly（P <0.05）increased soil fungal biomass,enzyme activities and nutrients content,and thus PAH removal,but inhibited some indigenous PAH-degrading bacteria.P treatment enriched these bacteria,but rapid plant growth depleted soil nutrients,thereby significantly（P <0.05）decreasing the soil bacterial biomass and hampering PAH removal.PMR treatment combined the advantages of M and P treatments,and promoted interactions among microbial taxa including degrading microbes.Consequently,PMR treatment achieved the highest removal rate of soil PAHs.These results indicate that the combination of S.viminalis with C.laeve inoculation contributed synergistically to stimulating the growth of indigenous PAHs degraders and bacterial community,and thus accelerated the dissipation of soil PAHs.Photosynthetic pigment content,light response curve of gas exchange parameters and chlorophyll fluorescence parameters were selected as the photosynthetic physiological indexes of S.viminalis and the effects of C.laeve inoculation on the photosynthesis of S.viminalis were reported.The inoculated C.laeve positively promoted photosynthetic pigment content,net photosynthetic rate（P_n）and transpiration rate（T_r）,maximum photochemical efficiency（F_v/F_m）,potential activity of PSII（F_v/F₀）of S.viminalis,but reduced their stomatal conductance（G_s）,intercellular CO₂ concentrations（C_i）,non-photochemical quenching（NPQ）and photochemical quenching（qP）.Different inoculation methods changed variation trend of light response curve of G_s and T_r.In this study,C.laeve increased the photosynthetic rate and transpiration efficiency of S.viminalis,enhancing the removal of soil-borne phenanthrene and pyrene.In general,our study is significant to reveal the mechanism of plant/WRF combination remediation.Based on liquid chromatography-mass spectrometry（LC-MS）and metabolomics method,we focused on the comparative metabolic profiling analysis of rhizosphere soils from the C.laeve inoculation and non-inoculation S.viminalis in PAH-contaminated soil.Results showed that 881 and 823 compounds were detected and identified in metabolic profiling,respectively,under POS and NEG mode.Among them,18 compounds were identified and assumed to be potential root exudates components,and then their function as well as the effects of them on PAH removal were disscussed.The component number and content of rhizosphere metabolites were significantly reduced by inoculation with C.laeve,including 16 root exudate components.In this study,inoculation with WRF promoted the capability of plant roots to absorb broadspectrum soil metabolites,which was of great significance for improving the PAHs accumulation of plants and revealing the mechanism of plant/WRF combination remediation of PAHcontaminated soils.