Factors affecting cadmium accumulation in the soil profiles in an urban agricultural area

Soil Cd pollution is a serious environmental issue associated with human activities. However, the factors determining exogenous Cd dynamics in the soil profile in a complex environment are not well understood. Based on regional observations from 169 soil profiles across the Chengdu Plain, this study explored the key factors controlling Cd accumulation in the soil profile under actual field conditions. Results showed that total soil Cd contents decreased from 0.377 to 0.196 mg kg^-1 with increasing soil depth. The effects of phosphate fertilizer rates, road density and precipitation on the difference in total soil Cd content were only observed in topsoil, while agricultural land-use type and topography had no impact. In contrast, significant differences in the total soil Cd content among different parent material types were found in the 0-20, 40-60 and 60-100 cm soil depths. One sample t-tests showed that significant Cd accumulation occurred in the whole soil profile in soils formed from Q4 (Quaternary Holocene) grey alluvium, while soils formed from Q3 (Quaternary Pleistocene) old alluvium and Q4 grey-brown alluvium showed significant Cd accumulation only in the 0-40 cm soil layers. In the topsoil, acid soluble Cd accounted for the largest proportion of the total Cd in soils formed from Q4 grey alluvium, reducible Cd was the main fraction in soils formed from Q4 grey-brown alluvium, while reducible Cd and residual Cd contributed the largest proportion of the total soil Cd in soils formed from Q3 old alluvium. The above results indicated that parent material was the decisive factor determining the magnitudes and depths of exogenous Cd accumulation in the soil profile due to its impacts on the Cd fraction distributions. These findings suggested that the parent material-induced Cd fraction distributions and accumulation should be considered for effectively exploring targeted remediation strategies for Cd pollution.