Organic nitrogen substitution stabilizes antimony in paddy bulk soil but mobilizes it in the rhizosphere

Rhizosphere processes strongly regulate antimony (Sb) bioavailability and root uptake in paddy soils, but whether organic nitrogen substitution induces divergent Sb transformation pathways across rhizosphere remains unclear. Here, by separating bulk soil, rhizosphere soil, and root iron plaque, we identified a soil-rhizosphere-plaque continuum that determines Sb mobilization, retention, and root uptake under organic nitrogen substitution. Organic nitrogen substitution exerted contrasting, rhizosphere-dependent effects on bioavailable Sb fractions, including non-specifically adsorbed Sb and specifically adsorbed Sb. These fractions decreased in bulk soil by 5.07-26.79% via humic acid complexation and sulfide precipitation, but increased in the rhizosphere by up to 31.45% through competitive adsorption by root-derived low-molecular-weight organic acids, phosphate, and dissolved organic matter. At the root surface, organic nitrogen substitution markedly promoted iron plaque formation by 32.41-75.63%, with amorphous iron (hydr)oxides dominating Sb retention and accounting for 86.31-89.08% of plaque-associated Sb. Iron plaque significantly reduced shoot Sb content by intercepting rhizosphere-mobilized Sb, leading to a 24.48-41.02% decrease in shoot Sb content. Finally, we propose a compartment-specific mechanism of Sb fate in which organic nitrogen substitution drives bulk-soil stabilization, rhizosphere mobilization, and root-surface interception of Sb, supporting organic nitrogen management as a strategy to mitigate rice Sb accumulation and sustain rice growth. SYNOPSIS: By distinguishing the rhizosphere from bulk soil, this study reveals that organic nitrogen substitution creates opposing effects on antimony bioavailability across soil compartments, while root iron plaque limits antimony uptake by rice. These findings advance our understanding of antimony cycling in paddy soils and inform safer rice production strategies.