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

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.