Nitrate reduction coupling with As(III) oxidation in neutral As-contaminated paddy soil preserves nitrogen, reduces...

In arsenic (As)-contaminated paddy soil, microbial-driven nitrate (NO₃^-) reduction coupled with arsenite (As(III)) oxidation can reduce As toxicity, but the whereabouts of NO₃^- remain unclear. In this study, the experiments were established using selective streptomycin (STP) and cyclohexylamine to inhibit bacterial and fungal functional responses, respectively, and metagenomic sequencing techniques were used to explain the biological mechanisms of NO₃^- reduction coupled with As(III) oxidation in neutral As-contaminated paddy soil. The results indicated that fungal denitrification resulted in stronger nitrous oxide (N₂O) emissions (321.6 μg kg^-1) than bacterial denitrification (175.9 μg kg^-1) in neutral As-contaminated paddy soil, but NO₃^- reduction coupled with As(III) oxidation reduced the N₂O emissions. Only adding STP led to ammonium (NH₄⁺) generation (17.7 mg kg^-1), and simultaneously more NH₄⁺ appeared in NO₃^- reduction coupled with As(III) oxidation; this may be because it improved the electron transfer efficiency by 18.2 %. Achromobacter was involved in denitrification coupled with As(III) oxidation. Burkholderiales was responsible for NO₃^- reduction to NH₄⁺ coupled with As(III) oxidation. This study provided a theoretical basis for NO₃^- reduction coupled with As(III) oxidation reducing N₂O emissions, promoting the reduction of NO₃^- to NH₄⁺, and reducing As toxicity in paddy soil.