Dandan Song, Lei He
Clay layers are important reservoirs, reaction interfaces, and potential release sources of arsenic (As) in shallow groundwater systems. However, microbial functions within these layers and their regulatory mechanisms for As migration and transformation remain insufficiently understood. Clay sediments from different depths in a shallow borehole of the Jianghan Plain were selected in this study. The functional potential of microbially mediated C-N-S-Fe-As cycling was systematically evaluated. Its potential influence on As migration and transformation in clay layers was also assessed. The results show that vertical variations in mineral composition and grain-size structure provide differentiated microenvironments. These microenvironments may support microbial colonization, organic matter preservation, and redox reactions. The microbial C-N-S-Fe-As metabolic potential in clay layers shows clear vertical differentiation. Nitrification- and oxidation-related genes are relatively enriched in the upper clay layer. This enrichment may form an oxidative barrier favorable for As adsorption and immobilization. Carbon degradation and fermentation can provide electron donors for Fe(III) reduction, nitrate reduction, and sulfate reduction in the middle clay layer. Within the studied profile, the middle interval may represent a key geochemical zone for As activation and mobilization. Microbial Fe(III) reduction is more likely to be the key process driving reductive dissolution of As-bearing iron oxides and As release. Fe/S reduction-related functions are generally weakened in the lower clay layer. Accordingly, the As transformation flux is relatively limited. A site-specific vertical microbial functional zonation pattern is proposed for the studied clay profile. This pattern consists of an upper oxidative immobilization barrier, a middle reductive release core, and a lower low-activity transformation zone. This site-specific study provides insights into the potential microbial geochemical role of clay layers in high-As groundwater formation in the Jianghan Plain.