Effects and mechanisms of distinct nitrogen fertilizers on arsenic transformation in a flooded contaminated paddy soil from mining-impacted area

Arsenate (As(V)) can be reduced to arsenite (As(III)) in anaerobic paddiess, and nitrate addition can facilitate As(III) immobilization. However, the effects of different nitrogen fertilizers on As transformation remain unclear. Herein, ammonium fertilizer (AF), nitrate fertilizer (NF), and the ammonium-nitrate fertilizer (ANF) were applied to As-contaminated paddes to investigate their effects on As transformation. Results showed that AF decreased available As by 33.5% and reduced available As(III) from 8.5% to 5.9%, mainly by lowering pH, increasing soil redox potential and nitrate, and promoting nitrate-dependent As(III)/Fe(II) oxidation. Moreover, NF did not significantly change available As, likely due to increased pH, OH⁻ competitive adsorption, but it lowered available As(III) to 3.2% via nitrate-coupled As(III) oxidation. ANF showed a synergistic effect, decreasing available As by 20.7% and reducing As(III) to 3.3%. Overall, the composite ANF can optimally regulate As oxidation and immobilization in polluted paddies compared to the sole ammonium or nitrate.