Prediction Of Availability And Threshold Derivation Of Soil Heavy Metals Based On Multisurface Model

With the increase in emissions of heavy metals from anthropogenic sources,the levels of heavy metals in soils continue to increase and their contamination risks are of increasing concern.The bioavailability of heavy metals in soil has been used as an important indicator to assess the impact of heavy metals on soil ecosystems due to their easier access to the plant.In this study,a database of soil physicochemical properties and heavy metals was established based on ArcGIS software and a multisurface model.The distribution of reactive and free heavy metals in soils in China was simulated,and the distribution of heavy metals on the surface of active soil components was clarified.A soil threshold derivation method that couples the mechanism of heavy metal adsorption by soil particles and the biological effectiveness of heavy metals was established,and then predicted the heavy metal content of seeds under the scenarios of growing wheat and corn.It is of guiding significance for adjusting the agricultural planting structure of contaminated soil and improving the safety of agricultural products in agricultural field systems.The main results of the study are as follows.(1)A database of soil physical and chemical properties was established.The pH of soils in the country gradually increases from south to north,with the southern and northeastern regions being mainly acidic,the northern and western regions being mainly alkaline,and the central region tending to be neutral.Soil organic matter content is lowest in the north and northwest,intermediate in the east and south,and highest in the northeast.The free iron content is higher in the southern and northeastern regions of China,followed by the western region and lowest in the northern region.Amorphous iron and free iron have a certain correlation and have a similar distribution pattern.The content of clay grains is lower in northern and western China,especially in desert areas,while the opposite is true in southern China.(2)The spatial distribution characteristics of free and adsorbed heavy metals in soil components were clarified.The applicability of the multi-surface model to different soil subclass types and different heavy metal types was verified,and the model prediction performance met expectations.The free Pb,Zn,Cu and Cd contents in 1043 soil sample sites were calculated as 0-14.58,0-324.52,0-0.15 and0-0.11 mg/kg.The distribution of free heavy metals was simulated nationwide,and the areas with high free heavy metal content were mainly located in Hunan,Jiangxi,Guangdong,Yunnan,Anhui and Liaoning.The distribution pattern of the higher value areas of free heavy metals and reactive heavy metals content is basically the same.The multisurface model calculates the distribution of reactive heavy metals in soil active particle fractions,and the magnitude of the adsorption state of each fraction as a percentage of reactive heavy metals is: organic matter > amorphous iron > clay minerals > crystalline iron.(3)The spatial distribution of heavy metal contents in wheat and maize seeds was predicted.In the scenario where wheat was grown,the four heavy metals in wheat seeds were Pb>Cu>Zn>Cd,with more than 75% of wheat seeds containing0.37-6.15,17.6-32.27,4.00-9.58 and 0.06-0.12 mg/kg of Pb,Zn,Cu and Cd.Further wheat seeds from the five main production areas of Hebei,Jiangsu,Anhui,Shandong and Henan did not exceed the standard for Zn and Cu,but exceeded the standard for Cd in the seeds in the northeastern part of Hebei and northeastern part of Henan.In the scenario where maize was grown,the maize kernels contained four heavy metals Pb>Cu>Zn>Cd.More than 75% of the maize kernels contained0.02-10.03,16.24-23.76,1.60-2.86 and 0.01-0.01 mg/kg of Pb,Zn,Cu and Cd.None of the maize seeds exceeded the standard for Cu.However,the Pb content of maize seeds in northeastern Inner Mongolia,Heilongjiang,Jilin,Liaoning,Shandong and Henan;the Zn content of maize seeds in southeastern Jilin and southeastern Liaoning;and the Cd content of maize seeds in northeastern Hebei and southwestern Liaoning showed exceedances.It is therefore recommended that small maize be grown in the northeastern areas of Hebei and Henan,and wheat in the southeastern areas of Jilin,southeastern Liaoning,northeastern Hebei and southwestern Liaoning,where food security production can be achieved.(4)In this study,a threshold derivation method for the bioavailability of heavy metals were proposed.Based on the soil multisurface model and the migration characteristics of heavy metals in plants,the soil heavy metal thresholds applicable to the safe cultivation were derived.The soil Cd thresholds for safe wheat cultivation at the five sample sites in this study were 1.26,1.37,6.29,7.22 and 9.03mg/kg lower than the maize thresholds,respectively.Therefore for medium to high Cd contaminated soils it is recommended to grow maize for food security production.It was also found that the magnitude of sensitivity of different physicochemical properties to soil thresholds was p H > organic matter > clay minerals > crystalline iron > amorphous iron.