Cadmium bioavailability during flooding–drainage in paddy soils: Spatiotemporal associations with dissolved organic matter transformation and iron speciation

The mechanism underlying the coupled regulation of Cadmium (Cd) activity by Iron (Fe) oxides and dissolved organic matter (DOM) in farmland soils during flooding followed by drainage remains unclear. This study addressed this issue by combining spatial sampling (n = 94) and time-series experiments to elucidate the Fe–DOM–Cd interaction mechanism using the diffusive gradients in thin films (DGT) technique. Regional surveys showed that 83.6% of soils in the study area were acidic, with an average total Cd content of 0.5 mg kg −1 , indicating a substantial Cd pollution risk. Examination of soil properties revealed that the initial release fraction of Cd was dominated by organic-bound components and that Fe oxides could indirectly inhibit Cd activity. This study further selected two soils (AH and HS, respectively collected from Anhua and Heshan districts of Hunan Province, China) with similar total contents of Fe, Cd, and organic matter to investigate the differential mechanisms underlying Cd activity regulation during flooding and drainage periods. HS soil consistently contained higher amorphous Fe content than AH soil, and its labile Fe content was 18.3 times that of AH soil after 1 d of flooding. The increase in labile and amorphous Fe in HS soil coincided with a marked decrease in Cd availability after 5 d of flooding, whereas the corresponding decrease in AH soil was substantially delayed. DOM characteristics considerably differed between flooding and drainage periods. AH soil DOM showed higher aromaticity and humification, while HS soil DOM exhibited a cyclic transformation from protein-like substances to humus and back to protein-like substances. Parallel factor analysis identified two major fluorescent components, namely, terrestrial humus (C1) and tryptophan-like substances (C2). During HS soil flooding, C1 proportion increased from 26% to 88%, while during the early AH soil flooding stage, it exceeded 85%. The concurrent increase in labile Fe and DOM humification was associated with the stronger decline in Cd availability in HS soil, suggesting a potential coupled influence of Fe transformation and DOM evolution on Cd dynamics. Overall, this study provides a theoretical reference for Cd risk regulation in contaminated paddy fields.