In　mature　landfill　leachate　with　high　ammonia　nitrogen　and　extremely　low　biodegradable　organic　matter,　it　is　difficult　to　achieve　advanced　nitrogen　removal　through　traditional　nitrification–denitrification　processes.　In　this　study,　a　low-　carbon/nitrogen　mature　landfill　leachate　treatment　process　with　an　anaerobic/oxic/anoxic　strategy　was　constructed　by　inputting　co-fermentation　liquid　of　corn　straw　and　excess　sludge.　Under　the　effect　of　the　co-fermentation　liquid,　partial　nitrification　was　enhanced.　Meanwhile,　with　the　cooperation　of　the　heterotrophic　nitrification-aerobic　denitrification　process,　the　nitrite　accumulation　ratio　and　simultaneous　nitrification　and　denitrification　ratio　reached　99.07　±　0.52%　and　23.26　±　1.02%,　respectively.　The　carbon　source　from　the　co-fermentation　liquid　was　efficiently　converted　into　polyhydroxyalkanoates,　which　could　drive　endogenous　denitrification　to　achieve　advanced　nitrogen　removal.　The　nitrogen　removal　efficiency　in　this　system　reached　98.53　±　1.21%.　In　addition　to　effectively　supplying　carbon　sources,　the　co-fermentation　reactor　also　achieved　a　sludge　reduction　ratio　of　24.78　±　1.68%.　Through　metagenomic　analysis,　the　cooperative　mechanism　by　which　denitrifying　glycogen-accumulating　organisms,　heterotrophic　nitrifying-aerobic　denitrifying　bacteria　and　ammonia-oxidizing　bacteria　achieve　nitrogen　removal　and　nitrite　accumulation　in　oxic　stage　was　revealed.　This　study　provides　a　new　and　efficient　treatment　process　for　mature　landfill　leachate　and　sludge　reduction.　©　2023　Elsevier　B.V.