Effect Of Scirpus Triqueter Inoculated Plant-growth-promoting Bacteria On Remediation Of Pyrene And Nickle Co-contaminated Soil

The co-contamination of heavy metals and polycyclic aromatic hydrocarbons（PAHs）has aroused extensive attention in the world.Phytoremediation attracts scientists because of its economical and environmental protection.Moreover,Phytoremediation has its own shortcomings such as small biomass,low resistence to environmentalstressandinefficiency.Inoculationplantswith Plant-growth-promoting bacteria（PGPR）is a promising technology to improve phytoremediation efficiency due to its abilities to promote the growth of plants.A strain of PGPR was screened from rhizospheric soil of Scirpus triqueter located in Huangpu-Yangtze River estuary,and then roots of Scirpus triqueter were inoculated with the isolated PGPR to remediate pyrene-nickel（Ni）co-contaminated soil.In order to investigate the mechanism of inoculation Scirpus triqueter with PGPR to remediate pyrene-Ni co-contaminated soil,the influence of PGPR inoculation on the growth conditions of Scirpus triqueter and soil enzyme activities;the effect of PGPR inoculation on the accumulation of pyrene and Ni in plants together with the removal of pyrene and Ni in soil;the influence of PGPR inoculation on the transformation of chemical speciations of pyrene and Ni;the effect of PGPR inoculation on soil microbial community structure were investigated.（1）A strain of PGPR was screened from rhizospheric soil of Scirpus triqueter in Huangpu-Yangtze River estuary.This strain had the ability to lower the level of ethylene in plants,whose 1-aminocyclopropane-1-carboxylic acid（ACC）deaminase activity was 0.039U mg^-1.This strain also had the ability to produce indole acetic acid（IAA）and IAA produced in the bacterial broth was calculated to be 22.3mg L^-1.In addition,this strain had the ability to solubilize phosphate and produce siderophores.Thus,this strain could efficiently promote the growth of plants.Remarkably,the PGPR had the ability to degrade pyrene.The degradation rate of pyrene in 7 days reached 37.88%.So,the isolated PGPR was not only able to promote the accumulation of Ni in plants,but also had the ability to directly degrade pyrene in soil.（2）PGPR inoculation had large influence on the growth parameters of Scirpus triqueter and soil enzyme activities.PGPR inoculation obviously increased the resistence of Scirpus triqueter in single Ni contaminated soil and pyrene-Ni co-contaminated soil.PGPR inoculation did not significantly promote the growth of Scirpus triqueter in clean soil or single pyrene contaminated soil,except for the increasing shoot number.PGPR inoculation significantly increased the fluorescein diacetate（FDA）activity in Ni or pyrene-Ni co-contaminated soil,but there was no obvious increase in FDA activities in pyrene contaminated soil.PGPR inoculation even decreased FDA activity in clean soil.Polyphenoloxidase（PPO）activity was significantly inhibited by contaminants in soil,moreover PGPR inoculation obviously increased PPO activity in all the treatments.PPO activities were positively correlated to the removal rate of pyrene.（3）PGPR inoculation promoted the removal of pyrene and Ni in soil.Plant-promoted microbial degradation accounted for the major removal of pyrene in soil.The removal rate of pyrene（59.5%）in pyrene-Ni co-contaminated soil was higher than that（54.9%）in pyrene contaminated soil after inoculation Scirpus triqueter with PGPR.When Scirpus triqueter was not inoculated with PGPR,the removal rates of pyrene in pyrene-Ni co-contaminated soil and pyrene contaminated soil were respectively 33.8%and 41.9%.Ni concentration(267mg kg^-1)in plants（including stems and roots）in Ni contaminated soil was higher than that(179mg kg^-1)in pyrene-Ni co-contaminated soil after PGPR inoculation.When Scirpus triqueter was not inoculated with PGPR,Ni concentration in plants（including stems and roots）in Ni contaminated soil(88mg kg^-1)was less than that(132mg kg^-1)in pyrene-Ni co-contaminated soil.Generally,PGPR inoculation increased the accumulation of Ni in plants.Moreover to some extent,the presence of pyrene prevented the inoculated Scirpus triqueter from accumulating Ni.PGPB inoculation promoted the removal of pyrene in soil,especially in pyrene-Ni co-contaminated soil.（4）PGPR inoculation could have large influence on soil microbial community structure in contaminated soil.Pyrene-Ni combined contamination or single contamination caused by pyrene or Ni changed soil microbial community structure in varying degrees,where pyrene-Ni combined contamination and sole pyrene pollution respectively brought largest and smallest changes to soil microbial community structure.PGPR inoculation was beneficial to the recovery of soil microbial community structure in contaminated soil.Generally,PGPR inoculation increased the activity of soil microorganisms in contaminated soil.Moreover,PGPR inoculation decreased the populations of fungi in Ni contaminated soil together with some bacteria in clean or pyrene contaminated soil.However,PGPR inoculation had few adverse effects on soil microbial community structure in clean soil.（5）PGPR inoculation influenced the transformation of chemical speciations of pyrene and Ni in soil.PGPR inoculation slightly inhibited the transformation of associated pyrene to bioaccessible pyrene in rhizospheric soil,but PGPR inoculation promoted the transformation of bound and associated fraction of pyrene to bioaccessible fraction of pyrene in non-rhizospheric soil.PGPR inoculation obviously decreased the amount of bioaccessible pyrene in both non-rhizospheric and rhizospheric soil.PGPR inoculation increased the bioavailability of Ni in non-rhizospheric and rhizospheric soil and thus increased the accumulation of Ni in plants.In addition,the amount of the total PLFAs（TPLFAs）,the monounsaturated fatty acids（MUFAs）,the straight and saturated fatty acids（SSFAs）and the branched and saturated fatty acids（BSFAs）was positively correlated to the removal rate of Ni.Thus,PGPR inoculation may increase the whole biomass of soil microorganisms together with the populations of Gram-negative bacteria,some bacteria and Gram-positive bacteria to make contributions to the removal of Ni in soil.