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

Area of Science:

• Environmental Science
• Soil Science
• Geochemistry

Background:

• Arsenic (As) mobility in flooded paddy soils is linked to iron (Fe) redox cycling.
• The influence of millimeter-scale vertical gradients on coupled Fe-As transformations is not well understood.

Purpose of the Study:

• To investigate the regulation of coupled Fe-As transformations by vertical gradients in flooded paddy soils.
• To elucidate the geochemical and microbial responses across a shallow soil depth profile.

Main Methods:

• Incubation of flooded paddy-soil microcosms.
• Quantification of geochemical parameters (Eh, NO₃⁻, SO₄²⁻, Fe(II), As(III)) and microbial community composition (16S rRNA sequencing).
• Analysis of redox-associated gene abundance (Gallionella, Geobacter, arrA, aioA) and depth-stratified path modeling.

Main Results:

• Flooding induced a stratified redox environment with declining Eh and distinct NO₃⁻ and SO₄²⁻ depth-time profiles.
• Aqueous Fe(II) and As(III) increased with depth and covaried over time, indicating reductive mobilization.
• Bacterial communities, Fe-oxidizing/reducing bacteria (FeOB/FeRB) markers, and arsenic redox genes (arrA, aioA) showed clear depth structuring.

Conclusions:

• A depth-tiered control framework governs Fe-As coupling in the upper paddy soil profile.
• Oxidative retention occurs near the surface, while FeRB-associated Fe(III) reduction drives Fe-As mobilization at mid-depth.
• Deeper layers show a stronger association between arrA and dissolved As dynamics, providing context for flooding-driven As mobilization risks.