Soil aggregate-associated heavy metals subjected to different types of land use in subtropical China

Heavy metal distribution in soils is strongly influenced by soil aggregate size. In this study, we collected surface soil samples from farmlands (FL), woodlands (WL), and bare lands (BL). Air-dried soils were sieved into four size fractions (>3, 1–3, 0.25–1, and <0.25 mm in diameter) to determine the concentrations of eight heavy metals (Cd, Cr, Cu, Ni, Pb, Zn, As, and Mn) in soil aggregates using wet digestion method. Our results found that the amount of macroaggregates (>0.25 mm) was significantly higher than that of microaggregates (<0.25 mm) in all soil samples. Moreover, the proportion of large macroaggregates (>3 mm) showed an order of FL > ML > BL, while the proportion of microaggregates (<0.25 mm) showed an order of FL < ML < BL. The soil Cd and Pb levels in woodlands were significantly higher than those in farmlands and bare lands. In addition, the concentrations of soil Cu and Zn were elevated in farmlands compared to woodlands and bare lands. For the aggregate-associated heavy metal, contents of most metal elements (except As) in soil aggregates slightly decreased with increasing size class in farmlands. Heavy metals were generally depleted in the larger aggregates but enriched in the smaller aggregates. Interestingly, soil organic matter content was positively related to soil Cd, Cr, Ni, and Pb concentrations, but negatively correlated with soil Cu and As levels. The findings suggest that land use changes can affect the concentrations of soil aggregate-associated heavy metals, and these effects vary among different metals.