Size effects of microplastics on competition between crop and weed via soil microbes

Microplastics (MPs) are widespread contaminants in agroecosystems, with potential implications for soil microbial communities, plant growth, and crop-weed interactions. This study investigates how MPs of different particle sizes influence crop-weed competition by altering soil microbial communities. Through a controlled greenhouse experiment, we examined the effects of 50 μm and 500 μm polyethylene (PE) MPs on competition between Eruca sativa (crop) and Amaranthus retroflexus (weed). We specifically analyzed shifts in bacterial and fungal community composition, microbial functional groups, and interkingdom microbial networks. Our results indicate that MPs negatively affect plant growth, with larger MPs (500 μm) increasing the competitive advantage of weeds over crops. MPs also alter soil microbial dynamics, leading to a shift in bacterial-fungal interactions from bacterial to fungal dominance with increasing PE particle size. Specifically, smaller MPs (50 μm) promoted bacterial functional groups associated with carbon cycling, whereas larger MPs increased the relative abundance of fungal plant pathogens. Additionally, larger MPs significantly reduced crop biomass, likely due to nutrient cycling disruptions and increased pathogen pressure, ultimately favoring weed competitiveness. This study provides new insights into how MPs pollution influences soil microbial dynamics and crop-weed interactions in agroecosystems.