Abstract: The weathering of black shale is a vital contributor to the high background values of heavy metals (HMs) in soils in Southwest China. However, the accumulation characteristics and controlling factors of HMs in soilcrop systems are not well understood. We investigated the bulk contents and bioavailability of HMs and their transfer pattern from soil to maize in a typical black shale area with high geochemical background in Chengkou of Chongqing. The results showed that the accumulation factor of HMs in soils from the study area followed an order of Cd>Zn>Cu>Ni>Cr>Pb, with Cd, Zn, Cu and Ni exceeding the background value. Cd was the main HM contaminant in the study area. Average Cd content in bulk soils was 5.11 mg·kg^-1. The soils were seriously polluted by Cd, withI_geo ranging from 1.35 to 7.04. 82% of soil samples were classified as heavy contamination based on I_geo. The Cd in these soils posted high ecological risk, with an averageE_r of 1534. The contents of bioavailable Cd in the soils ranged from 0.001 to 1.51 mg·kg^-1, with an average extractability of 6.14%. Extractability of Cd decreased with increasing soil pH in acidic soils, and slightly changed with pH in near-neutral soils. The accumulation of HMs in maize was low, with the total Cd contents ranging from 0.001 to 1.36 mg·kg^-1. 14% of maize samples exceeded the safety threshold of cereal. There was no relationship between the contents of HMs in maize grains and their bulk contents in soils. The relationship between Cd content in maize grains and soil available Cd content depended on soil pH conditions.

Key words: black shale, cadmium pollution, soil, food crop, bioavailability.