Microplastics disrupt nitrogen-fixing bacterial abundance with consequences for plant growth

Microplastic pollution is a global environmental concern due to its persistence, toxicity, and widespread presence across ecosystems. While numerous studies have examined the effects of microplastics on plant-soil systems, less is known about microplastic effects on nitrogen-fixing bacterial communities, rhizobia nodulation, and their interactions with 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 in a controlled plant-vessel system, incorporating three types of microplastic fibers (polyamide, polyester, and polypropylene) at a concentration in soil of 0.4% (w/w). At harvest, we assessed soil properties (including water-stable aggregates, pH, carbon and nitrogen contents), bacterial attributes (total and nitrogen-fixing bacterial community, diversity and composition), nodulation (nodule number and nodule mass), and plant growth parameters (root traits, carbon and nitrogen contents in roots and shoots, root and shoot biomass).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 fibers decreased total bacterial abundance but increased nitrogen-fixing bacterial richness, while polyester fibers reduced total bacterial richness and Simpson’s index. Among the predictor variables such as soil nitrogen content, bacterial diversity, the abundance of bacteria including Azospirillum and Pelobacter, root mass, and nodule mass emerged as the most important predictors for shoot variables (mass, carbon and nitrogen contents). Moreover, microplastics altered the strength and direction of correlations between shoot mass and soil and microbial properties and root traits. For instance, microplastic addition strengthened the weak correlation between shoot mass and Azospirillum to strong positive. This study provides new insights into the mechanisms through which microplastics influence bacterial communities, nodulation, and plant performance, highlighting the complexity and context-dependence of microplastic effects in plant–soil–microbe systems.