Nitrogen, sulfur, iron, and microbial communities co-shape the seasonal biogeochemical behaviors of As and Sb in coastal tidal flat wetlands associated with rivers.

Arsenic (As) and antimony (Sb) are affected by complex biogeochemical processes in coastal ecosystems. However, the influence of N, S, Fe, and microbial communities on the biogeochemistry of As and Sb in coastal tidal flat wetlands remain uncertain, particularly when rivers flow into these areas. This study combined diffusive gradients in the thin-film technique with high-throughput sequencing to investigate the release and vertical distribution of As and Sb in river and coastal tidal flat wetland sediments. The results indicated a distinct stratification phenomenon in the As release at depths ranging from 20 mm to -150 mm. At river sites, the release of As occurred in the upper layer (above -40 mm), with peak values of 4.3 and 9.3 μg/L at HS and SY sites in summer, respectively, likely due to anaerobic ammonium oxidation. In the lower layer (below -40 mm), both As and Sb were released, and this was possibly due to Fe reduction. However, at the coastal tidal flat sites, the release of As and Sb may have been driven by anaerobic ammonium oxidation, dissimilatory nitrate reduction to ammonium, sulfate reduction, and Fe reduction. At the river sites, As exhibited increased activity during the summer, and the residual forms were converted more easily into mobile forms. Sb remained relatively stable during both winter and summer. Conversely, both As and Sb primarily existed in residual forms and exhibited higher stability during summer in the coastal tidal flat sites. The microbial phyla Nitrospirota (3.6-7.0 %) and Acidobacteriota (9.5-10.2 %) were more prevalent at the river sites, whereas Desulfobacterota (8.8-12.0 %) and Firmicutes (0.13-27.9 %) were more prevalent at the coastal tidal flat sites. The bacterial genera involved in the N, S, and Fe transformation processes differed between the two sites, and they were primarily Thiobacillus, Limnobacter, and Sulfurovum at the river sites and Sva1033, Anaeromyxobacter, and Sva0485 at the coastal tidal flat sites. In this study, the microorganisms that mediated N, S, and Fe complex processes at various depths in the sediment-water interface were decoupled to elucidate the effect of these processes on the biogeochemical behavior of As and Sb as they move from rivers to coastal tidal flat wetlands.