Plant　litter　is　an　important　input　source　of　carbon　and　nitrogen　in　soil.　While　microplastics　(MPs)　and　plant　litter　are　ubiquitously　present　in　soil,　their　combined　impact　on　soil　biogeochemical　processes　remains　poorly　understood.　To　address　this　gap,　we　examined　the　soil　changes　resulting　from　the　coexistence　of　plant　litter　(Alfalfa)　and　polyethylene　microplastics　(PE).　The　soil　changes　included　physicochemical　properties,　composition　of　soil　dissolved　organic　matter,　and　structure　of　the　soil　microbial　community.　The　results　showed　that　the　addition　of　polyethylene　(PE)　inhibited　the　degradation　of　humus-like　substances　and　decreased　the　quantity　of　humic　acidlike　compounds　in　soil　dissolved　organic　matter　(DOM).　PE　negatively　impacted　plant　litter　decomposition,　disrupted　soil　organic　carbon　(SOC)　breakdown,　interfered　with　the　nitrogen　cycle,　and　significantly　altered　microbial　community　structures　during　the　process.　By　day　35,　SOC　and　total　nitrogen　(TN)　levels　were　reduced　by　39.8%　and　10.1%,　respectively,　in　the　presence　of　PE.　Furthermore,　PE　significantly　decreased　the　abundance　of　nitrogen-fixing　microbes,　including　Streptomyces　(43.1%)　and　Bacillus　(45.9%),　which　play　key　roles　in　nitrate　reduction　to　ammonium.　This　study　highlights　the　environmental　effects　of　MPs　on　plant　litter　decomposition　and　their　potential　implications　for　soil　biogeochemical　processes.