Available Heavy Metal Concentrations and their Influencing Factors in Cropland and Fallows of Different Age in Tropical Area

To explore the effects of soil properties affected by fallow ages on available heavy metal concentrations in tropical China areas, a total of 144 topsoil samples were investigated from croplands and fallows. The total and available concentrations of chromium (Cr), copper (Cu), zinc (Zn), and lead (Pb) in the soil samples were analysed. Soil characteristics (total concentrations, pH, redox potential (Eh), organic matter (OM), particulate diameter) that may affect the available heavy metals were investigated. Additionally, the relationship between soil characteristics and available heavy metals was analysed. The results showed that the pH was lower and the Eh and OM were higher in cropland with frequent cultivation when compared to fallows which were disturbed rarely by anthropogenic activities. The particulate diameter of cropland was relatively small. The concentrations of total and available heavy metals first increased and then decreased as fallows aged. The available Cr is mainly affected by its total concentration, while the concentrations of available Cu, Zn and Pb are affected by the physicochemical properties. Soil properties of fallows changed, and the concentrations of available heavy metals can be reduced, which can reduce the bioavailability, biological toxicity and environmental impact.