Biogeochemical process governing cadmium availability in sediments of typical coastal wetlands driven by drying-wetting alternation.

Fluctuations in water levels within coastal wetlands can significantly affect cadmium (Cd) cycling and behavior in sediments. Understanding the effects of drying-wetting cycles on Cd availability and binding mechanisms is crucial. However, information regarding this subject remains limited. This study conducted incubation experiments employing chemical extraction, high-resolution mass spectrometry, and microbiological analysis to investigate the Cd behavior under these conditions. The results from a 40-day anaerobic incubation followed by a 20-day aerobic phase indicated that the drying-wetting cycles triggered fluctuations in physicochemical parameters (e.g., pH, EC, and reactive iron (Fe)), affecting Cd mobility. The mobility of Cd was closely linked to nanozyme activity (R=0.63), exhibiting a strong correlation with Fe (R=0.51). This suggested that the drying-wetting cycles induced Fe changes, which regulated the nanozyme activity, thereby affecting Cd availability. The changes in Cd availability were strongly linked to transformations in iron oxides and organic functional groups (carboxylic-OH and aliphatic C-H), whereas the bacterial community composition, particularly Bacilli and Clostridia, notably influenced Cd accessibility. These findings offer valuable insights into the geochemical dynamics of Cd in coastal wetland sediments under alternating drying-wetting cycles, enhancing our understanding of its biogeochemical cycling and potential risks.