Fe(II)　participates　in　complex　Fe-N　cycles　and　effects　on　the　microbial　metabolism　in　the　anaerobic　ammonium　oxidation　(anammox)　dominated　system.　In　this　study,　the　inhibitory　effects　and　mechanisms　of　Fe(II)-mediated　multi-metabolism　in　anammox　were　revealed,　and　the　potential　role　of　Fe(II)　in　the　nitrogen　cycle　was　evaluated.　The　results　showed　that　the　long-term　accumulation　of　high　Fe(II)　concentrations　(70–80　mg/L)　led　to　a　hysteretic　inhibition　of　anammox.　High　Fe(II)　concentrations　induced　the　generation　of　high　levels　of　intracellular　·O2−,　whereas　the　antioxidant　capacity　was　insufficient　to　eliminate　the　excess　·O2−,　thus　causing　ferroptosis　to　anammox　cells.　In　addition,　Fe(II)　was　oxidized　via　nitrate-dependent　anaerobic　ferrous-oxidation　(NAFO)　process,　and　mineralized　to　coquimbite　and　phosphosiderite.　They　formed　crusts　on　the　surface　of　the　sludge,　leading　to　mass　transfer　obstruction.　The　results　of　the　microbial　analysis　showed　that　the　addition　of　appropriate　Fe(II)　increased　the　abundance　of　Candidatus　Kuenenia,　and　served　as　a　potential　electron　donor　to　enrich　Denitratisoma,　promoting　anammox　and　NAFO　coupled　with　nitrogen　removal,　while　high　Fe(II)　concentrations　reduced　the　enrichment　level.　In　this　study,　the　understanding　of　Fe(II)-mediated　multi-metabolism　in　the　nitrogen　cycle　was　deepened,　providing　the　basis　for　the　development　of　Fe(II)-based　anammox　technologies.　©　2023