Soils developed on carbonate bedrocks commonly contain high contents of geogenic cadmium (Cd). The bioavailability of Cd greatly varies among such soils and tends to be elevated in paddy soils compared to upland soils. However, the factors controlling the mobilization of geogenic Cd in paddy soils remain unclear. Therefore, this study aimed to reveal the processes and influencing factors of Cd mobilization in soils, by collecting upland soils and paddy soils from a carbonate region in Southwest China and analyzing the integrated soil properties and content, speciation and bioavailability of Cd. The results showed that the in-situ weathering of Cd-rich limestone (average of 0.66 mg/kg) resulted in the enrichment of Cd in overlying soils (4.75 to 13.2 mg/kg). Based on sequential extraction, similar to 43%, 18% and 17% of Cd in paddy soils was retained by Mn oxides, Fe oxides and the residual fraction, respectively. Hence, the transformation of Mn oxides drove the redistribution of Cd. In the upper layers, Cd released from Mn oxides was sequestered by soil organic matter and carbonate. Based on extraction using diethylene triamine pentaacetic acid (DTPA), paddy soils presented a higher Cd bioavailability (27%) than upland soils (18%). Furthermore, both the bioavailability and mobility of Cd gradually increased toward the surface in the profiles. The bioavailability of Cd was significantly correlated with the content of soil organic matter in these soils, indicating that soil organic matter was another crucial factor controlling Cd mobilization in paddy soils. The outcomes of this study provided new evidence for understanding the mobili-zation of Cd in soils and are helpful for developing pollution-control strategies to combat geogenic Cd in paddy soils in carbonate regions.

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