| Polycyclic aromatic hydrocarbons(PAHs)are a type of refractory organic pollutants commonly found in contaminated soil,which can migrate through soil-crop system and harm human health through the food chain.Previous studies have found that root exudates of plants play an important role in the rhizosphere remediation of PAH-contaminated soil:roots exudates change the bioavailability of PAHs,provide nutrients for soil microorganisms and affect PAH biodegradation.Microbial community structure is a key factor which can affect the effectiveness of rhizoremediation,and the abundance of PAH-degrading genes can be used as a key indicator to measure the degradation ability of rhizosphere microorganisms.However,how does root exudates affect the bacterial community and PAH-degrading genes in PAH-contaminated soil?There is still a lack of understanding at home and abroad.In this paper,naphthalene(NAP),phenanthrene(PHE)and pyrene(PYR)were representatives of PAHs,and artificial root exudates and collected root exudates of rygrass were selected as exogenous additives to study the effects of root exudates on the degradation of naphthalene,phenanthrene and pyrene;PAH-degrading genes such as nidA?nahAc and phe gene were selected as target genes to reveal the response characteristics of PAH-degrading genes to root exudates by qPCR technology;The influence of root exudates on bacterial community structure in PAH-contaminated soil was analyzed by high-throughput sequencing.The main research results are shown as follows:(1)Clarified the effects of root exudates on the degradation of naphthalene,phenanthrene and pyrene in soil.Artificial or actual root exudates mainly promoted the degradation of naphthalene,phenanthrene and pyrene in soils,and the removal rate of ?PAHs in soils increased first and then decreased with the increase of the concentration of root exudates.In the treatment of artificial root exudates,10-1000 mg kg-1 root exudates were added to the soil.After 32 days of cultivation,the removal rates of naphthalene,phenanthrene and pyrene increased by 7.70%,24.01%and 23.45%,respectively;the removal rate of ?PAHs in soils with 200 mg kg-1 actual root exudates was the highest,which was 29.47%higher than the control without addition.Meanwhile,in the treatment of actual root exudates,10-100 mg kg-1 root exudates were added to the soil.On day 32,the removal rates of naphthalene,phenanthrene and pyrene increased by 25.08%,37.64%and 26.95%,respectively;the removal rate of ?PAHs in soils with 50 mg kg-1 actual root exudates was the highest,which was 39.2%higher than the control without addition.(2)Revealed the response characteristics of PAH-degrading genes in soil to exogenoud root exudates.Artificial or actual root exudates mainly increased the abundance of PAH-degrading genes in soil.On days 16 and 32,the addition of 10-1000 mg kg-1 artificial root exudates significantly increased the of abundance of nidA,nahAc and phe genes in soils.At these two sampling time points,the maxium copy numbers of nidA,nahAc and phe genes can be up to 6.98E+07 copies/g·dws?1.89E+05 copies/g·dws and 2.04E+07 copies/g·dws,respectively;the addition of 10-100 mg kg-1 actual root exudates significantly increased the abundance of nahAc and phe genes in soils.At these two sampling time points,the maxium copy numbers of nahAc and phe genes can be up to 6.34E+05 copies/g·dws and 2.3E+07 copies/g·dws.The abundance of these degradation genes was mainly negatively correlated with the contents of naphthalene,phenanthrene,pyrene and ?PAHs in soils,indicating that the increase in the abundance of the degradation genes may be one of the main reasons for the PAH degradation promoted by root exudates in soils.(3)Analyzed the effects of root exudates on bacterial community structure in PAH-contaminated soil.Artificial or actual root exudates affected the soil bacterial community structure.The results of Alpha diversity showed that the richness and diversity of bacterial community in the treatment group are less than the control group on day 32.For example,the Observed OTUs index and Shannon index of soil samples in the treatment group were respectively 1741 and 9.87,which were lower than 2258 and 10.15 in the control group.Beta diversity analysis(using PCoA analysis as an example)showed that as the concentration of root exudates and the cultivation time increased,the distance and structural differences between the treatment group and the control group increased,indicating that the effect of root exudates on the soil bacterial community structure increased.As the structure of the bacterial community changes,artificial or actual root exudates promoted the growth of some reported PAH-degrading bacteria such as Gemmatimonas,Nocardioides,Arthrobacter and Pseudomonas,etc.Among them,the artificial root exudates significantly promoted the growth of Nocardioides and Arthrobacter harboring nidA gene while actual root exudates significantly promoted the growth of Pseudomonas harboring nahAc gene.The results showed that the increase of the abundance of degradation bacteria was an important reason for the PAH degradation promoted by the root exudates in soils. |
PDF Full Text Request