Response of peanut plant and soil N-fixing bacterial communities to conventional and biodegradable microplastics

Microplastics (MPs) occur and distribute widely in agroecosystems, posing a potential threat to soil-plant systems. However, little is known about their effects on legumes and N-fixing microbes. Here, we explored the effects of high-density polyethylene (HDPE), polystyrene (PS), and polylactic acid (PLA) on the growth of peanuts and soil N-fixing bacterial communities. All MPs treatments showed no phytotoxic effects on plant biomass, and PS and PLA even increased plant height, especially at the high dose. All MPs changed soil NO₃^--N and NH₄⁺-N contents and the activities of urease and FDAse. Particularly, high-dose PLA decreased soil NO₃^--N content by 97% and increased soil urease activity by 104%. In most cases, MPs negatively affected plant N content, and high-dose PLA had the most pronounced effects. All MPs especially PLA changed soil N-fixing bacterial community structure. Symbiotic N-fixer Rhizoboales were greatly enriched by high-dose PLA, accompanied by the emergence of root nodulation, which may represent an adaptive strategy for peanuts to overcome N deficiency caused by PLA MPs pollution. Our findings indicate that MPs can change peanut-N fixing bacteria systems in a type- and dose-dependent manner, and biodegradable MPs may have more profound consequences for N biogeochemical cycling than traditional MPs.