<p>In this study, ferrous-based denitrification was combined with Feammox (Fe(III) reduction coupled with anaerobic ammonium oxidation) to trigger NH₄⁺ removal through intermittently adding NO_x⁻ (NO₂⁻ and NO₃⁻) into biogas slurry. The results showed that NO_x⁻ oxidized Fe(II), then the generated Fe(III) was reduced to Fe(II) again, resulting in a continuous iron cycling and nitrogen removal. On day 35, the total nitrogen removal efficiencies in the NO₂⁻ (67.52%) and NO₃⁻-added (52.32%) groups were significantly higher than that of the control (without NO_x⁻) (<i>P</i> &lt; 0.05). Nitrifying and Anammox microorganisms were not detected in the NO_x⁻-added reactors, while Feammox functional microorganisms (iron-reducing bacteria) were enriched (1.08%–1.51%), and the electron transfer capacities were also increased by 7.69%–16.08%. Metagenomic analysis showed that the NO₃⁻ group had more nitrate reductase genes but fewer downstream denitrification genes than the control group, indicating that NO₂⁻ accumulated as a key intermediate. NO₃⁻ could not directly oxidize Fe(II), and no nitrate-dependent Fe(II)-oxidizing microorganisms were detected. Moreover, the Fe(II) oxidation products in the NO₃⁻-added reactors were identical to those generated by abiotic NO₂⁻ oxidation, suggesting that NO₂⁻ produced via partial denitrification was likely responsible for Fe(II) oxidation. Based on this, a possible metabolic pathway coupling nitrogen and iron transformations was proposed, in which partial NO₃⁻ reduction to NO₂⁻ may contribute to Fe(II) oxidation and subsequent Fe(III)-mediated NH₄⁺ removal via Feammox. This study provided a method for dealing with biogas slurry, and also offers a new approach for simultaneously removing NO_x⁻ and NH₄⁺.</p> Graphical Abstract <p></p>