This　study　employed　multispectral　techniques　to　evaluate　fulvic　acid　(FA)　compositional　characteristic　and　elucidate　its　biodegradation　mechanisms　during　partial　nitritation　(PN)　process.　Results　showed　that　FA　removal　efficiency　(FRE)　decreased　from　90.22　to　23.11%　when　FA　concentrations　in　the　reactor　were　increased　from　0　to　162.30　mg/L,　and　that　molecular　size,　degree　of　aromatization　and　humification　of　the　effluent　FA　macromolecules　all　increased　after　treatment.　Microbial　population　analysis　indicated　that　the　proliferation　of　the　Comamonas,　OLB12　and　Thauera　exhibit　high　FA　utilization　capacity　in　lower　concentrations　(　＜　50.59　mg/L),　promoting　the　degradation　and　removal　of　macromolecular　FA.　In　addition,　the　sustained　increase　in　external　FA　may　decrease　the　abundance　of　above　functional　microorganisms,　resulting　in　a　rapid　drop　in　FRE.　Furthermore,　from　the　genetic　perspective,　the　elevated　FA　levels　restricted　carbohydrate　(ko00620,　ko00010　and　ko00020)　and　nitrogen　(HAO,　AMO,　NIR　and　NOR　)　metabolism-related　pathways,　thereby　impeding　FA　removal　and　total　nitrogen　loss　associated　with　N2O　emissions.(c)　2023　The　Research　Center　for　Eco-Environmental　Sciences,　Chinese　Academy　of　Sciences.　Published　by　Elsevier　B.V.