Depth-resolved microbial controls on iron-arsenic redox transformations in flooded paddy soils

Arsenic (As) mobility in flooded paddy soils is tightly coupled to iron (Fe) redox cycling, but how millimeter-scale vertical gradients regulate coupled Fe-As transformations remains poorly resolved. We incubated flooded paddy-soil microcosms and quantified geochemical and microbial responses across a shallow depth profile (0-25 mm). Flooding rapidly generated a stratified redox environment: Eh declined sharply with depth, while NO₃⁻ and SO₄²⁻ exhibited distinct depth-time trajectories, delineating an evolving electron-acceptor backdrop. Consistent with this stratification, aqueous Fe(II) and As(III) increased markedly with depth and covaried over time, suggesting stronger reductive mobilization below surface layer. 16S rRNA sequencing revealed clear depth structuring of bacterial community composition. qPCR showed strong partitioning of redox-associated markers: Gallionella-related 16S rRNA gene copies (use here as a putative Fe-oxidizing indicator; FeOB) were highest near the surface, whereas Geobacter-related 16S rRNA gene copies (putative Fe-reducing indicators; FeRB) increased with depth. Arsenic redox genes also increased during flooding: arrA (dissimilatory arsenate reductase) rose with depth and incubation time, and aioA (arsenite oxidase) increased overall, indicating depth-structure functional potential for As redox cycling. Depth-stratified path modeling supported tiered controls across the profile, with oxidative retention near the surface, FeRB-associated Fe(III) reduction linked to coupled Fe-As mobilization at mid-depth, and a stronger arrA association with dissolved As dynamics in deeper layers. Together, these results support a depth-tiered control framework for Fe-As coupling within the upper paddy soil profile and provide hazard-relevant context for interpreting flooding-driven As mobilization risks.