Annual input and output fluxes of heavy metals to paddy fields in four types of contaminated areas in Hunan Province, China.

This study focused on the heavy metal concentrations in the soil and their representative input and output fluxes to paddy fields in four types of contaminated areas in Changzhutan urban agglomerations, Hunan, China. Four types of contaminated areas (mining area, livestock breeding area, suburb area and control area) were chosen for monitoring. Three input pathways and three output pathways were considered so as to figure out the heavy metal balance throughout the topsoil. In all study areas, the input fluxes of atmospheric deposition, accounting for 51.21%-94.74% of the total input fluxes, were significantly higher than those of fertilizer and irrigation water. Nevertheless, the manure fertilizer contributes nearly the same proportion as atmospheric deposition in livestock breeding area. In suburb area, only the flux of atmospheric deposition was higher than that in control area, and other input fluxes showed no significant difference. Crop removal, especially straw removal, was the primary output pathway of soil elements, contributing 60.19%-89.37% of the total output fluxes. Annual balance demonstrated that Cd was the metal of the primary concern in study areas. Furthermore, obvious differences were observed among the distribution of heavy metals (Cd, Pb and As) fractions in four types of contaminated areas. In general, the available content was the primary portion of the total contents, mostly up to about 51.70%-87.76%. The information obtained in this paper provides a more comprehensive understanding of the differences in the net inputs of heavy metals among four types of contaminated areas and emphasizes the significance of straw removal for Hunan agricultural soil.