Effect Of Land Use Pattern Change On The Adsorption Characteristics And Form Distribution Of Heavy Metal In Soil

Recently,China’s soil heavy metal pollution has gotten wide concern of people all over the country,which is worsening and has caused serious consequences.On the other hand,due to the adjustment of agricultural planting structure,lots of paddy fields have been turned into vegetable fields.Part of cultivated fields in Hunan province were seriously polluted by heavy metals,and the impact of land use change on soil quality,biological efficiency and migration and transformation of heavy metals has been ignored.In this paper,taking the vegetable fields which were converted from paddy soils with different degrees of pollution as the research object,the adsorption and desorption of heavy metals in soil aggregates and the transformation of available states and forms of heavy metals were studied,and the effects of soil basic properties and changes in Fe-oxides and Mn-oxides content on the behavior of heavy metals in soil were analyzed.The main conclusions of the study are as follows:(1)After the land use pattern change,the pH decreased,and for the none and the heavy pollution sampling points,the contents of heavy metals Cd,Pb,Cu and Zn and organic matter,total nitrogen,alkaline nitrogen,total potassium and available potassium increased,while for the slight and the moderate pollution pionts,the above substances decreased.The contents of total phosphorus and available phosphorus were significantly increased in the four sampling sites.The contribution of pH and CEC for the contents of heavy metals was the largest.The import of heavy metals is homologous to the input of organic matter and soil nutrient elements.Excessive application of organic fertilizer on vegetable soil caused the more serious accumulation of heavy metals and nutrient elements.(2)After land use pattern change,soil pH,the content of organic matter,free iron oxides,CEC,manganese oxide and clay in soil aggregates decreased.The maximum Cd adsorption capacity of paddy soil and vegetable soil was obtained in clay grade(<0.002mm).On the whole,the adsorption effect of paddy soil was higher than that of vegetable soil,and CEC contributed the largest effort for the adsorption of Cd,followed by free iron oxides.Desorption rate of vegetable soil was higher than that of paddy soil,and manganese oxide and organic matter have maximum promotion effect.After the land use pattern change,the adsorption amount of vegetable soil decreased by 22.72%and the desorption rate increased by 35%on average,which enhanced the mobility of Cd and may pose a greater threat to groundwater a nd plants.(3)After land use pattern change,the content of Fe-oxides and Mn-oxides changed significantly.The activation degree of Fe-oxides decreased by 3%-40%,the activation degree of Mn-oxides increased by 11%-27%.After the change of soil types,the contents of available heavy metals in soils extracted by HCl were decreased.In paddy soils and vegetable fields,Cd mainly exists in exchangeable form,Pb is the binding to Fe-Mn oxides form,Cu is mostly residual form,and Zn is the binding to Fe-Mn oxides and residue form.pH was negatively correlated with the exchangeable form of heavy metals.For the exchangeable and the binding to Fe-Mn oxides form of heavy metals,Fe_o,Fe_c,has an opposite effect with Mn_d,Mn_o.In a word,after land use pattern change from paddy soils to vegetable fields,a lot changes happenedd in the soil basic physical and chemical properties and the accumulation ability of heavy metals in soils increased.Hence,the quality of paddy soils was better than that of vegetable fields.Vegetable soils have a worser ability of heavy metals adsorption and interception than paddy soil.However,as can be seen from the morphological distribution,the effective content of heavy metals decreased in vegetable soils,the biological effectiveness also reduced,and the vegetable soils had a lower risk for plants.