The Influence Of Vegetation Restoration For The Rare Earth And Heavy Metals Of Soil Aggregate In Granite Erosion Area Of Southern China

Historically,Changting County is a typical area where its degree is the most serious of soil and water erosion in southern China under the combined action of natural factors and human factors.Since 1980s,great efforts in soil and water erosion have been taken by local government.,and ecological environment quality was gradually improved by vegetation restoration.The weathering crust in this area is rich in rare earth elements Where the soil was stemmed from the coarse grained granite.For nearly 30 years,the large-scale mining of rare earth caused the problems of soil and water loss and environmental pollution.However,in the past,the recovery of vegetation and soil nutrient by the soil and water loss control only be concerned,while the soil heavy matals,rare earth and other ecological problems be less concerned,let alone the impact of rare earth and heavy metals in soil aggregate.Soil aggregate is the product of interaction between pedogenesis interaction and external factors.There are quite different between binding capaciy and bioavailablity of heavy metals and rare earth elements in different particle size fractions of soil aggregates.The mechanism that the vegetation restoration affect the characteristics of heavy metal and rare earth element in soil aggregate can provide the basis for control of soil and water loss and soil pollution.In this study,the soil physical and chemical properties and soil aggregates were analyzed for six sampling plots of different vegetation restoration time which include Lai-you-keng(unmanegement,0a),Lai-you-keng(manegemented site,5a),Long-jing(lOa),You-fang(15a),Ba-shi-li-he(30a)and Lu-hu(80a)in Changting County,Fujian Province.Distribution of rare earth and heavy metals in bulk samples and different particle size fractions of soil aggregates was determined by ICP-MS.The main conclusions are as follows:(1)The contents of Cr,Ni,Cu,Zn,Cd and Pb were 7.67-15.64 mg/kg,5.02-10.83 mg/kg,4.82-15.02 mg/kg,66.63-96.40 mg/kg,0.31-0.54 mg/kg and 54.45～149.78 mg/kg,respectively.Compared with the secondary standard of soil environmental quality the content of heavy metal did not exceed the standard except for Cd.Cd content was significantly higher than the background value of Fujian Province and the national soil environmental quality standards,the maximum is 5.7 times and 1.8 times than the background value of Fujian Province and the national soil environmental quality standards.Pb is 4 times than the background value of Fujian Province.The contents of heavy metals decreased with the vegetation restoration time,and it was more active in 0-20 cm than in 20-40 cm soil layer.(2)The content for speciation of heavy metals in the 0-20 cm soil layer was as follows:residual>exchangeable>Fe-Mn oxide-bound>organic-bound>carbonate-bound.In the 20-40cm soil layer,the distribution of speciation of heavy metals is:residual>exchangeable>organic state>Fe-Mn oxide-bound>carbonate-bound.In this two soil layers,the residual state content is the largest and the carbonate binding state is the least.With the increase of the vegetation restoration time,there is a tendency that the exchangeable and Fe-Mn oxide-bound turn to the organic-bound.The bioavailability of soil heavy metals increased with the increase of total heavy metals in soil,the greater the soil pollution,the greater the biological toxicity,the greater the environmental hazard.(3)The enrichment of heavy metals in two soil layers decreased with the increase of particle size fractions of soil aggregates,and all of them increased with the increase of plant restoration time.The enrichment of all heavy metals in the 0-20cm soil layer is greater than that in the 20～40 cm soil layer.The Zn enrichment is the smallest among the heavy metal elements.The heavy metals in the two soil layers were all loaded on soil aggregates of larger size.(4)After the rare earth elements entered the soil,most of them remained in the surface soil(0-20cm),and the content of rare earth in the soil was lower than that of the abandoned mine(655.69 mg/kg)except for Youfang,while all of them exceed the soil background value in Fujian Province(223.47 mg/kg)and nation background value(187.60 mg/kg),respectively.The content of rare earth element show up-down-up trend with the increase of vegetation restoration time.The results showed that the content of rare earth elements:Ce>Y>La>Nd>Sc>Dy>Gd>Yb>Er>Sm>Pr>Ho>Tb>Tm>Eu.Rare earth elements are mainly related to water content,grain,organic matter,total nitrogen,total phosphorus,available phosphorus.(5)The contents of rare earth in different plot soils were mainly in the residue form.The content of The speciation of rare earth show:residual>exchangeable>organic-bound>Fe-Mnoxide-bound>carbonate-bound.With the increase of vegetation restoration years,the speciation changes of rare earth element were relatively consistent.Soil physical and chemical properties and the total amount of rare earth elements were significantly correlated with rare earth exchangeable,carbonate-bound,organic-bound and residue-state correlations.(6)The enrichment coefficient of rare earth elements decreased with the increase of particle size fractions,and the enrichment coefficient also increased with the increase of vegetation restoration time.Rare earth elements show more easily adsorbed than heavy metals in<0.25mm aggregates.Light rare earths and heavy rare earth elements in the spatial differentiation are distributed by the different particle size fractions of soil aggregates.The rare earth elements are mainly loaded on aggregates of 0.5～1mm,1～2mm,2～5mm and>5mm,the mass fraction of which are 16.75%～21.68%,16.24%～22.29%,15.61%～29.38%and 15.44%～33.92%respectively.