Heavy metals in homestead soil: metal fraction contents, bioaccessibility, and risk assessment

Rapid urbanization in China has led to the disappearance of countless villages and the transformation of homestead land into cultivated land or grassland. The quality of homestead soil (HS) plays a pivotal role in land-use conversion and reuse strategies, so the current state of heavy metal pollution in HS deserves attention. This study determined the fraction contents, bioaccessibility, risks, and affecting factors of Hg, As, Cd, Pb, Cu, and Zn in HS by comparing them with soil in cultivated land (CS), grassland (GS), homestead-converted cultivated land (HCS), and homestead-converted grassland (HGS). Results demonstrate that the contents of the six metals exceed background values, especially for Cd and Hg, resulting in significant pollution and elevated ecological risk. Distinct from the dominant residual fraction of other metals, the extractable fraction of Cd shows the highest proportion, which also contributes most to the high values of the Risk Assessment Code and extreme pollution conditions in HS, GS, and CS. Moreover, pH shows predominantly negative relations with the effective available and potentially available contents, while the effects of organic carbon fractions are notably the opposite. Furthermore, CS and GS suggest higher non-carcinogenic and carcinogenic risks than in the converted soil. This study indicates that HS has a lower metal accumulation risk compared with cultivated land and grassland, and homestead conversion seems to restrict the bioaccessibility of metals in soil.