Hydroxylamine steers nitrogen metabolism toward dissimilatory nitrate reduction to ammonium by suppressing competitive denitrification

Dissimilatory nitrate reduction to ammonium (DNRA) is important for nitrogen conservation and resource recovery in wastewater treatment, but its efficiency is often limited by competition for electrons and substrates from denitrifiers. Although hydroxylamine (NH₂OH) has been shown to modulate various nitrogen transformation processes, its long-term effects on DNRA systems and the underlying microbial ecological responses remain unclear. In this study, the nitrogen transformation performance, electron transfer characteristics, and microbial community succession in DNRA systems were comprehensively investigated under prolonged exposure to 0–5 mg/L NH₂OH. The results demonstrated that, with increasing NH₂OH concentrations, the system consistently achieved near-complete nitrate removal without nitrite accumulation, and the effluent NH₄⁺–N reached up to 51.5 mg/L, indicating a substantial enhancement of DNRA ammonium production. Functional activity analyses and apparent electron-equivalent balance suggested an increased contribution of DNRA to nitrate-reduction-associated electron consumption. Metagenomic analyses further showed that NH₂OH could decrease the relative abundances of denitrification-related genes, including nirS, norB, and nosZ, while increasing those of narG and the nrf gene cluster. Building upon the existing DNRA functionality, NH₂OH selectively enriched a tolerant DNRA population, exemplified by Ignavibacteriota, and facilitated cross-feeding interactions and electron transfer network remodeling involving fermentative bacteria. Collectively, these findings suggest that NH₂OH can weaken denitrification competition and increase the apparent contribution of DNRA to nitrate-reduction-associated electron consumption, thereby enhancing ammonium production. Moreover, these findings may provide a theoretical basis for the future development of DNRA–Anammox coupled processes for high-level nitrogen removal.