Effects of simulated redox cycles on trace metal release in a red paddy soil profile enriched with Fe-Mn nodules from China

Redox fluctuations strongly influence the mobility of potentially toxic elements (PTEs) in paddy soils. However, the behavior of these elements on a profile scale, particularly in soils enriched with Fe-Mn nodules (FMNs), remains poorly understood. This study investigated the redox-driven mobilization of As, Cd, Cr, Cu, Ni, Pb, and Zn along a FMN-rich red paddy soil profile from Southern China using controlled oxic-anoxic incubation experiments. The release mechanisms under contrasting redox conditions and durations were elucidated by integrating chemical extraction (CBD), mineralogical characterization (XRD and XANES), solution chemistry, and aqueous speciation. The results show that most PTEs are geologically sourced and are predominantly incorporated into aluminosilicates, exhibiting generally low mobility. FMNs, which have a high degree of crystallinity and a compact structure, act as stable, long-term reservoirs for PTE. In contrast, the surrounding surface soils, which are enriched in reactive organic matter and poorly crystalline Fe oxides, exhibit higher PTE mobility. Cu, Zn, Cd, and Pb are primarily released under oxic conditions but remain below WHO drinking water thresholds. Ni, Cr, and As exhibit more complex redox responses. Cr remains largely immobile due to Cr-Fe co-precipitates; however, speciation modeling indicates the occurrence of Cr(VI) in deeper horizons during oxic phases via Mn oxide-mediated oxidation. Arsenic exhibits the highest mobility, reaching ∼500 µg/L under prolonged anoxia in surface horizons, due to As(V) reduction to As(III), as well as competition from phosphate and dissolved organic matter (DOM). These results demonstrate that the duration of redox processes, the reactivity of organic matter, and the interplay between minerals collectively govern the mobilization of PTEs on a profile scale, with implications for risk assessment and management of red paddy soils.