Microplastics disrupt the nitrogen-fixing bacterial community with consequences for plant growth

Microplastic pollution is a global environmental concern in terrestrial ecosystems, yet less is known about microplastic effects on nitrogen-fixing bacterial communities and rhizobia nodulation, the interactions between bacterial communities, nodulation, and soil properties, and how these effects influence plant growth. To address this gap, we conducted a greenhouse experiment using red clover (Trifolium pratense L.) as a phytometer grown in soils amended with microplastic fibres (polyamide, polyester, and polypropylene; 0.4 % w/w). At harvest, we assessed soil physicochemical properties, total and nitrogen-fixing bacterial communities, nodulation, root traits, and plant growth. Our results show that the effects of microplastics ranged from positive to negative on these parameters, as a function of polymer type. For example, polyamide and polypropylene impacted positively soil carbon (10.38 % and 11.48 %, respectively) but negatively specific root length (51.73 % and 58.63 %, respectively), whereas polyester negatively affected both soil aggregation (17.93 %) and nodule number (71.11 %). Relative importance analysis revealed that soil nitrogen, root and nodule mass, and the abundance of Azospirillum and Pelobacter were the strongest predictors of shoot growth. Microplastics further modified the direction and strength of correlations among root, soil, and microbial variables, for instance, most necessarily converting the weak association between shoot mass and Azospirillum abundance into a strong positive relationship. These findings reveal that microplastics restructure plant–soil–microbe interactions by altering nitrogen-fixing bacterial communities and nodulation, emphasizing the complex, context-dependent interactions within the plant–soil–microbe system and highlighting the importance of considering microplastic polymer-specific effects when assessing their ecological impact.