Abiotic conversion of nitrate to ammonium enhances nitrogen use efficiency in paddy soils

Nitrogen (N) use efficiency is critical in ensuring food production while reducing nitrate (NO₃^--N) to ammonium (NH₄⁺-N) is widely recognized as a key for reducing N loss from paddy fields. Microbial involvement has long been regarded as the indispensable step for NO₃^--N reduction to NH₄⁺-N in nature; it remains largely unknown whether abiotic processes can drive this. Here, using laser confocal fluorescence microscopy and thermodynamic laws, we find that Fe(II) ions in the rice rhizosphere exist on iron plaque in a specific manner with strong reducing capability. These specifically adsorbed Fe(II) ions (Fe(II)_SA) could mediate the abiotic reduction of NO₃^--N to NH₄⁺-N, accounting for 57% of the total NO₃^--N reduction. When cultured in a sterile nutrient solution containing only ¹⁵NO₃^--N and Fe(II) ions, over 90% of N in the xylem of rice plants exists as ¹⁵NH₄⁺-N, directly proving the occurrence of NO₃^--N reduction to NH₄⁺-N and taken up by rice. Theoretical calculations elucidate the Fe(II)_SA-induced local electron depolarization enhances interfacial charge transfer which drives this reaction with the conversion of *NO to *NOH/*NHO identified as the rate-limiting step. We then propose a regulatory strategy to establish a stable Fe(II)_SA/Fe(III)_SA cycle utilizing FeC for controlled-release of Fe(II) ions. Two-year field experiments confirmed this strategy can increase rice yield by 16.1% while cutting half of the N input. Our work enables the design of novel pathways in improving N use efficiency for sustainable rice production.