Assessment Of Surface Soils Heavy Metal Pollution In The Northern Part Of The Modern Yellow River Delta And Its Effect On Bacterial Community Diversity

The Yellow River Delta is the most complete,extensive and youngest preserved wetland ecosystem in the warm temperate zone of China,and its rich biodiversity plays a vital role in maintaining ecological functions.Because of the ecological importance of the Yellow River Basin,the country has issued an outline plan for ecological protection and high-quality development of the Yellow River Basin,proposing to increase the protection of the wetland ecosystem in the Yellow River Delta.However,with the rapid development of agriculture and industry,especially the development of oil and gas resources in the Yellow River Delta,the soil in the Yellow River Delta has been contaminated with heavy metals to varying degrees.Therefore,in this study,a total of 45 surface soil samples were collected in the Yellow River Delta wetlands,of which 30 surface samples of natural soil were used to determine the heavy metal content in the soil samples,to investigate the spatial distribution and sources of heavy metal contamination in the surface soil of the Yellow River Delta wetlands,and to conduct soil contamination risk assessment;the other 15 soil samples were surface soil samples from five different land use types in the Yellow River Delta,the heavy metal content and soil physicochemical properties of these soil samples were measured to assess the heavy metal contamination,and the soil microbial community composition and structure of these samples were further analyzed to investigate the effects of heavy metals and soil physicochemical properties on the microbial community structure.The main conclusions were as follows:（1）The contents of As and Sb in the surface soils of the Yellow River Delta area were 12.33 mg/kg and 1.36 mg/kg,which were significantly higher than the geochemical background values of Shandong Province,while the concentrations of Ag,Cd,Co,Cr,Cu,Li,Ni,Pb and Zn were comparable to the background values of Shandong Province soils.The content of heavy metals in soils basically complied with the soil environmental quality standard（GB15618-2018）.In the southwest（f37）and northwest（f4,f5,f8,f9）of the study area,the content of heavy metals in the surface soils was higher,which was mainly influenced by oil extraction and agricultural surface source pollution.（2）The EF of As in the surface soil of the Yellow River Delta was 1.63 and the EF of Sb was 2.06,producing a significant enrichment of As and Sb,and the I_geo value of Sb was 0.26,reaching a slight degree of contamination.In addition,Ni,Cr and As have relatively high contributions to the sum of toxicity units（ΣTUs）,which may have adverse effects on soil organisms and should be given priority attention.Further multivariate statistical analysis was used to distinguish the possible sources of these heavy metals,and the results indicated that Ag,As,Cd,Cu,Pb,Sb and Zn may belong to the same source and may be influenced by human activities（industrial and agricultural sources）,while Co,Li,Mn,Ni,Pb,V and U may be mainly from natural weathering processes.（3）Among the surface soils of five different land use types,the I_geo values of Pb,Cr,Cd,Ag and Zn in the surface soils around the oil fields located in the southwestern part of the study area were 1.81,1.08,0.60,0.35 and 0.21,respectively,and the heavy metal pollution in this area was serious,and minor enrichment of As and Ni was found in the surface soils of the maize field and around the farms.The heavy metal contamination status of the surface soils of the remaining land use types was relatively light.This was consistent with the results of spatial distribution of heavy metals in the surface soils of the Yellow River Delta wetlands.（4）At the phylum level,the dominant species in the surface soil samples of the five land use types were Proteobacteria（27.76%）,Actinobacteriota（15.98%）and Chloroflexi（13.06%）.The bacterial community structure differed significantly among different land use types of soils,with significant differences in the abundance of Proteobacteria,Actinobacteriota,Firmicutes,Gemmatimonadota,and Verrucomicrobiota.TN,TP,p H,Ni and As were the environmental factors that had a strong influence on the differences in microbial community structure in soil,and the effect of heavy metal content on bacterial community structure was greater than that of soil physicochemical properties.Most of the phylum level bacteria were negatively correlated with heavy metal concentrations,notably,Actinobacteriota was significantly positively correlated with the contents of Ag,As,Cr,Cu,Ni and Zn;Chloroflexi was significantly positively correlated with the contents of As and Ni.This indicated that Actinobacteriota and Chloroflexi are tolerant to heavy metals and have potential in bioremediation of soil heavy metal contamination.This study provides referenceable basic information for solving the status of soil heavy metal pollution in the Yellow River Delta region.At the same time,it provides new insights into the effects of heavy metal pollution and soil physicochemical properties on soil microbial communities in the Yellow River Delta region,as well as provides some references for microbial remediation of heavy metal pollution.