Spatial distribution and controlling factors of heavy metals in soils from Puding Karst Critical Zone Observatory, southwest China

Soil profile samples under different land-use types were collected at the Puding Karst Critical Zone Observatory to investigate the composition, distribution and controlling factors of heavy metals. The heavy metal contents of Cr, Mn, Fe, Ni, Cu, Zn, Cd and Pb were determined, and their relationships to soil properties were examined. The Mn, Zn, Cd and Pb contents were larger in the topsoil than the deeper layer in all land use. In secondary forest land, the Cr, Fe, Ni and Cu contents in the topsoil were lower than in the deeper layer; however, all eight heavy metal contents were enriched in the topsoil under cropland. The results showed soil organic carbon played a fundamental role in controlling of Cd, while it showed negative relationship with Cr, Fe, Ni and Cu. Soil pH was positively associated with Ni and Cu. The Cr, Fe, Ni and Cu contents were significantly correlated to proportion of micro-aggregates, while the Cd and Pb contents showed a positive correlation with proportion of macro-aggregates. According to principal component analysis, the Cr, Fe, Ni and Cu contents might be affected by soil organic matter; however, the Mn, Zn and Pb contents might be influenced by atmospheric heavy metal deposition, and the Cd content might be controlled by both organic matter and atmospheric deposition. The geoaccumulation index and enrichment factor were calculated to assess the pollution level of soils, and the results showed that most of the selected heavy metals in soils may not be present at sufficient levels for contamination, and Mn and Fe were not from a pollution source and might come from natural weathering processes. This research will help researchers make strategic decisions about food security in the choice of agricultural land.