Degradation of farmland Red Earth is one of the severe problems facing agriculture in South China.Since 1990's,agronomical
measures such as vegetation recoveries have been taken for its mitigation.In previous studies,role of vegetation recoveries on rapid rebuilding of
soil organic carbon(SOC) and micro\|aggregation has been debated.The objectivity of present study is to describe the redistribution of SOC and heavy metals of human risk concerns and partitioning in the soil particle size fractions as affected by the vegetation recovery practices. The samples were collected at the Grass Extension Station of Jiangxi Province,10 km northwest to Nanchang City.Being typical in central Jiangxi,the soil was Typical Udsiallisols (Chinese Soil Taxonomical Classification,1999)derived from old Quaternary deposits on the rolling slopes.The treatment plots were:A,rotation of corn and cabbage(for 4a);B,continuous white clover(for 15a);C,continuous orchard grass(for 15a) and rotation of Chinese Lespedeze\|Sudan grass(for 3a after 15a grass)(Table 1).A mixed sample of 5 undisturbed sub\|samples was collected randomly in each plot and in 0～10cm,10～20cm respectively. The undisturbed soil clods were dispersed in water using the ultrasonic energy at 21 5Hz and 300mA.Soil particles in size of >2mm,2～0 25mm were separated by wet sieving,those in size of 0 25～0.02mm and 0 02～0 002mm by sedimentation and siphon,and those in size of <0 002mm by centrifuging.The contents of organic carbon,Pb and Cd contents in the particle separates were determined by wet combustion and HF\|HNO 3\|HClO 4 digestion.The concentration of the metals were measured by AAS with the internal standard control. Prominent variation was found in the content of SOC and Pb and Cd contents among the
soil particle fractions,with highest contents in the fraction<0 002mm(Table 2,Fig.1).The fractions of 2～0 25mm and 0 25～0 02mm generally possessed 50% and 20% of the total amount of the analyzed components respectively.Significantly positive correlation was found between the portion of Pb or Cd and that of SOC in the soil particle fractions(Fig.2).Compared with that in the uncultivated soil,the recovery treatments enhanced accumulation of SOC and decreased of Pb and Cd in the predominant particle fraction of 2～0 25mm,whereas elevated Cd level in the 0~10cm layer(Table 4).Under the corn\|cabbage rotation,no remarkable variation was shown of SOC and heavy metals in the soil particle fractions as compared to that under the other treatments.Therefore,the vegetation recovery measures had not only beneficial effects on SOC sequestration(Li, et al .2001),but also affected the distribution pattern of both SOC and heavy metals in soil particle fractions.However,the effects of these measures on bioavailability of the measured components in the particle fractions remain to be determined.