Study On Soil Heavy Metal Pollution Characteristics And Soil Cd And Pb Safety Thresholds Based On Field-Scale Farmland In High Background Area In Southwest China

The southwest region of China has sufficient reserves and a wide variety of metal minerals,and is a typical area with a high background of heavy metals in the soil.Heavy metals are characterised by persistent,long-term latent,territorial and not easily remediated pollution.Heavy metals accumulate in the soil environment,damaging the structural composition of soil microbial communities and their activity,making soil fertility and quality decline.Heavy metals are enriched in edible parts of vegetables and lead to health exposure risks when they enter the human body through the food chain.In this study,soil physical and chemical properties,heavy metal content characteristics and spatial distribution patterns were analysed to identify the sources of heavy metal contamination and ecological and environmental risks.The ecological safety thresholds for Cd and Pb in the vegetable production areas of the study area were derived based on the species sensitivity distribution method.The above study is intended to provide a theoretical basis for the risk assessment and safe use of heavy metal contaminated farmland in high background areas.The main conclusions of this study are as follows:（1）The results showed that the contents of these eight heavy metals in surface soils differed significantly;in particular,the over-limit ratios of the location points of As,Cd,and Cr were 61.83%,90.83%,and 92.52%,respectively,whereas the heavy metals Zn,Cu,Cr,and Ni mainly existed in the soil in the form of residues,which accounted for more than 80% of the total Zn,Cu,Cr,and Ni contents.The spatial distributions of Cd,Cr,and Cu contents exhibited the following pattern:high concentrations in the southeastern part,intermediate concentrations in the central part,and low concentrations in the northwestern part.Conversely,the spatial distributions of Pb and Zn contents followed the opposite pattern.APCS-MLR modeling revealed that,on the one hand,the heavy metals As,Cr,Cu,and Ni in the study a rea mainly derived from natural sources,which accounted for 39.34%,47.32%,44.53%,and 50.23% of the heavy metal contents derived from all sources,respectively;on the other hand,Pb and Zn mainly derived from industrial activities（accounting for 31.93%and 30.53% of total contents,respectively）and traffic（accounting for 23.36%and 22.17% of total contents,respectively）;finally,Cd from agricultural activities accounted for 53.63%of the total Cd content.Soil pH was negatively correlated to the available contents of As,Cd,Cu,and Ni（P<0.01）.Analysis of geo-accumulation indices showed that Cr pollution in the soil presented middle and severe levels,followed by Cd pollution;moreover,analysis of ecological hazard indices pointed at Cr and Cd as the main ecologically hazardous elements,with single potential ecological hazard index values ranging from 5.20 to 41.12,and from 11.56 to 113.35,respectively.Altogether,our results revealed relatively serious levels of Cr and Cd pollution in the study area,which entail potential risks;therefore,safe use practices,remediation approaches,and other measures should be adopted to lower the risk level.In conclusion,our study accurately evaluated the source,characteristics,and spatial distribution of soil heavy metal pollution at the field scale in the high geological background area of Southwest China,and supports the formulation of science-based prevention and control measures of soil heavy metal pollution.（2）The physical and chemical properties of soils differ significantly among different types of arable land,with maize soils being neutral and paddy soils being acidic,as well as significant differences in pH between maize and vegetable soils（P<0.05）.The soil organic matter content was ranked as follows:vegetable field（37.53±1.36）>rice field（35.55±1.85）>maize field（34.64±3.11）>bare field（33.76±2.36）cmol·kg^-1.Four types of arable land have an intermediate level of cation exchange capacity.In the surface soils of the different types of arable land,there are significant differences in the levels of heavy metals,with a much higher concentration of As and Pb in vegetable land soils,and the highest concentration of Cd,Cr and Zn in maize soils.The mean levels of As,Cd,Cr,Pb,and Zn in the effective state of maize field soils were significantly higher than those of the other three croplands,and the Cd contamination was more severe.In general,the vertical distribution of the six heavy metals was decreasing with increasing soil depth under the four cultivated land types.The mean number of OTUs in maize,rice,and vegetable fields were significantly higher（3897,4077,and 4052）than in bare ground（3348,P<0.05）.In the paddy soils,the Shannon index,Chao1 index,and ACE index were the highest,indicating that the bacterial diversity was higher in the paddy soils than in the three other arable land types.The PCoA and NMDS analyses revealed that the soil bacterial communities were more similar in the rice and vegetable fields and in the bare ground and maize fields,and that there was a significant difference between the soil bacterial communities of the two groups.Research performed on the diversity of bacterial communities in agricultural soils has identified the dominant phylum of the bacterial community as Proteobacteria,whereas bare ground soils and maize field soils also have the phylum Firmicutes,and rice soils and vegetable field soils also have the phylum Acidobacteria.The diversity of soil bacterial species varies among cropland types,suggesting that cropland use can affect the structural composition of soil bacterial communities.There was a significant difference in the relative abundance of the top 10 phyla between the four agricultural land uses（P<0.05）.Soil bacterial diversity indices were significantly correlated with soil pH,SOM,and CEC,and soil Cd and Zn contents influenced soil bacterial community diversity（P<0.05）.In this study,the redundancy analysis revealed that soil pH（R²=0.95）,soil organic matter（R²=0.84）,soil cation exchange（R²=0.77）,soil Cd（R²=0.88）,Pb（R²=0.79）and Zn（R²=0.72）significantly influenced the microbial community structure.（3）In order to ensure the safety production of vegetable varieties on Cd and Pb contaminated farmland under the geological background high area in the Southwest China,32 kinds of leafy vegetables,root vegetables,solanaceous vegetables and bean vegetables are grown.It was determined that the Cd and Pb concentrations in the edible par ts ranged from 0.01-0.24 mg·kg^-1 and 0.02-0.49 mg·kg^-1,respectively.The average Cd content was in the order of root vegetables(0.14 mg·kg^-1)>leafy vegetables(0.08 mg·kg^-1)>solanaceous vegetables(0.04 mg·kg^-1)>bean vegetables(0.03 mg·kg^-1).On average,leafy vegetables were the highest in Pb(0.28 mg·kg^-1)followed by root vegetables(0.25 mg·kg^-1)then solanaceous vegetables and fruit vegetables(0.24 mg·kg^-1)then bean vegetables(0.08 mg·kg^-1).In correlation analysis,pH,SOM,and CEC were highly significant（P<0.01）for Cd and Pb enrichment,explaining 72.11% and 78.12%respectively of the total variation of accumulated Cd and Pb in vegetables.In general,the Burr type Ⅲ distribution function fits the BCF for Cd well for different vegetable species（R²=0.997;RMSE=0.069）.The SSD curve indicated that different vegetable varieties had different sensitivities to Cd and Pb.The thresholds for Cd and Pb in local farmland soils were deduced to be 1.57,4.15,6.66 mg·kg^-1and 100,211.27 and 353 mg·kg^-1for priority conservation,safe use and strict control zones based on the protection of 95%,50% and 20% of vegetable varieties,respectively.