Spatial Variability Characteristics, Influence Factors And Assessment Of Pb And Cd In The Suburban Cropland Soils In Changsha

The agricultural soil contamination with heavy metals in the suburb area draws great attention because of its potential threat to food safety and detrimental effects on the ecosystem.The research about spatial distribution characteristics, the origins, and the status and ecological risk assessment of soil heavy metals have important significance for farmland ecological risk prevention and controlling in the region.In this study, a survey in agricultural soils of the suburb area of Changsha was conducted. A total of513surface soil samples were collected, and the concentrations of Pb and Cd, the soil physicochemical properties of sampling points were analyzed. The location characteristics of soil points were obtained from land-use map under GIS platform.The spatial distribution Characteristics of soil Pb and Cd in suburb cropland of Changsha city were analyzed by using the buffer analysis, spatial autocorrelation analysis and geostatistical analysis methods. The average concentration of Pb and Cd in soils decreased with the distance from near to far in the buffer area at3ranges (i.e. less than20kilometer,20to40kilometer and40to60kilometer from urban area, respectively). It indicated that anthropic activity in urban has great influence on the concentrations of soil Pb and Cd in suburb cropland at certain spatial distance. The spatial autocorrelations for soil Pb and Cd was over a wide range. The spatial autocorrelation and random both contributed to the spatial structure of soil Pb and Cd.The origins of soil heavy metals in suburb interface are usually controlled by many factors, such as parent material, industry and agriculture activities, etc. To effectively decrease heavy metals pollution risks in suburb area and further establish reliable protection measures, it is quite necessary to understand their sources and spatial patterns. The conventional regression model (OLS) was usually used to analyze the relationship between soil heavy metals with their influential factors. However, OLS is only in a global or an average sense to estimate parameters, and it can not reflect spatial local variation or test spatial non-stationarity. Geographically weighted regression model (GWR) is a powerful tool for exploring spatial heterogeneity. The underlying idea of GWR is that parameters may be estimated anywhere in the study area given a dependent variable and a set of one or more independent variables which have been measured at places whose location is known. Not only can it test spatial non-stationafity, but also it can provide the corresponding solutions. As the local model, GWR model has been applied in researches about urban housing land prices and the spatial factors of economic development, but few to the origins and spatial structure of soil heavy metals. Typical influential factors of Pb and Cd were picked up from the points of soil property and location, such as soil pH, organic matter, alkali-hydro nitrogen, rapidly-available phosphorus, rapidly-available potassium, slowly available potassium, the distance from cropland to town, the distance from cropland to settlement, the distance from cropland to industrial construction land, and the distance form cropland to river. The conventional linear regression model(OLS) and spatial regression model(GWR) were applied to consider the relationship among the variables of influential factors and their spatial structure together. The results indicate that soil Pb was highly significantly positively related with Cd. The concentrations of soil pH, organic matter, alkali-hydro nitrogen and rapidly-available phosphorus were significantly positively related with the content of Pb and Cd. The distance from cropland to river, from cropland to town and from cropland to construction land also had some influence on the concentrations of Pb and Cd in agricultural soil of the suburb area of Changsha. The GWR models for Pb and Cd had a better goodness-of-fit than OLS models respectively as a result of GWR models indicated the same tendency of spatial correlation between the Pb and Cd measured values with their estimated values, and their residuals without spatial autocorrelation. GWR model indicated the dependency between soil Pb and Cd with their influence factor values by estimating local parameters of soil sampling point anywhere, and it can explain better their spatial heterogeneity.The quality status and potential ecological risk degree of Pb and Cd were evaluated by using single-factor assessment and Hakanson methods.. The results showed that the concentrations of Pb and Cd in largely sampling sites were at class I (below their average background values). There were some accumulations of Pb and Cd in soils, and the degree of accumulation of Cd was more serious than Pb. The results of single accumulation index and potential ecological hazard coefficient of soil Pb and Cd showed that the tendency which in near suburban area was more serious than middle suburban area and distant suburban area, while distant suburban area was more serious than middle suburban area. The potential ecological hazard tendency of heavy metal Cd was more serious than Pb in each area. The accumulation extent and pollution risk outstanding area was on South-North of near suburban and local industrial mining area of distant suburban.Fuzzy comprehensive evaluation and analytic hierarchy process were applied to establish ecological risk assessment models of Pb and Cd contamination, and evaluation indies (ERI) were set up based on Pb and Cd concentrations, soil physicochemical properties and soil location characteristics. The indies of soil physicochemical properties were soil pH, organic matter, alkali-hydro nitrogen and available phosphorus, while the indies of soil location characteristics were the distance from cropland to town, industrial construction site, river, and settlement. The ecological risk grade was divided to four ranks according to the values of ecological risk index, ERI>0.70,0.6<ERI≤0.70,0.4<ERI≤0.60, and ERI≤0.40were defined as calss I, II, III and IV respectively. About22.9%sampling points were distributed in class III and IV, most of high ecological risk soil samples were located at the surounding area of riverside, town, industrial construction site and settlement. The ecological risk regions of farmland soil Pb and Cd were divided into high, middle, and low ranks.The research could provide reference for the quantitative analysis of spatial structure and influence factors of soil heavy metals in the region. It played an important role in ecological risk prevention and controlling of soil heavy metals pollution. The high-high spatial cluster districts with high concentrations of Pb and Cd and the regions near town, river, and build where were the important regions for controlling pollution risk of Pb and Cd in agricultural soil of the suburb area of Changsha. Ecological risk assessment,classification and regionalization of heavy metals in agricultural soil had important significance for farmland ecological risk prevention and controlling system construction and soil utilization scientifically.