Spatial distribution of Mn oxides in paddy soil profiles regulates rice grain accumulation of Ni and Cr through adsorption and oxidation.

Paddy soils derived from basalt weathering contain high levels of Fe-Mn oxides, along with elevated nickel (Ni) and chromium (Cr), posing threats to rice safety. Unlike Fe oxides, Mn oxides exhibit both adsorption and oxidation capabilities, creating complex regulatory mechanisms for Ni and Cr. The environmental impacts of these oxides depend on their spatial distribution, though the mechanisms remain unclear. This study investigates the synergistic regulation of δ-MnO2 on the speciation transformation and bioavailability of Ni and Cr. Pot experiments were setup using δ-MnO2 distributed either in the rhizosphere or sub-root layers, combined with continuous or intermittent flooding water management. Results show that δ-MnO2 spatial distribution critically influences the distinct environmental behaviors of Ni and Cr. For Ni, δ-MnO2 exhibits adsorption and immobilization effect, but these effects are strongly dependent on the position: distribution in the rhizosphere reduces the concentration of available forms and decreases Ni accumulation in rice grains, while distribution in the sub-root layer hinders downward Ni migration and increases grain Ni accumulation. For Cr, δ-MnO2 primarily converts inert Cr(III) into highly reactive Cr(VI) through oxidation, resulting in increased Cr accumulation in grains. Water management and the spatial distribution of δ-MnO2 show significant synergistic effects: continuous flooding promotes Ni release and Cr(VI) reduction, while intermittent flooding favors Ni adsorption and immobilization. This study challenges the conventional understanding that "metal oxides universally exhibit immobilization effects on heavy metals", clarifying the differential regulatory roles of Mn oxide spatial distribution in paddy soil profiles on the environmental behaviors of Ni and Cr. It reveals the "double-edged sword effect" of Mn oxides in adsorbing/immobilizing Ni while oxidizing/activating Cr, and elucidates the core principle that neglecting their vertical distribution would lead to counterproductive heavy metal control measures. The findings not only provide new insights into the mechanisms by which Mn oxides regulate Ni and Cr accumulation in rice within basalt weathering zones, but also offer scientific and theoretical support for precise management of rice safety production in geologically high-background regions based on the differential properties of heavy metals.