Mycorrhizal fungi helped soils store more carbon even when polluted with microplastics, largely through increases in glomalin, a sticky fungal protein that stabilizes soil carbon. Biodegradable microplastics boosted soil organic carbon by up to 46%, mainly by increasing arbuscular mycorrhizal fungi activity and glomalin levels. These fungal shifts improved soil carbon retention despite the stress of microplastic contamination.