Diverse microplastics reprogram rhizosphere processes and constrain photosynthetic performance in barley through coordinated changes in oxidative regulation, nutrient cycling, enzyme hotspots, and microbial organization

This study examined short-term responses of barley to polyethylene and polyvinyl chloride microplastics in a controlled 60-day greenhouse rhizobox experiment with three replicates per treatment. Microplastics were applied at 0, 2, and 4% of dry soil mass, corresponding to high experimental loadings of 20,000 and 40,000 mg kg-1, respectively. Bulk and rhizosphere soils were evaluated separately using enzyme zymography, nutrient analysis, amplicon sequencing, microbial network analysis, Mantel tests, and structural equation modeling. Higher polyethylene exposure and both polyvinyl chloride treatments reduced superoxide dismutase activity by 16.27-39.12%, increased peroxidase activity by 33.09-67.07%, and reduced net photosynthetic rate by 18.02-24.79%. Microplastic treatments also altered the availability of ammonium, nitrate, and Olsen phosphorus and reorganized the spatial distribution of extracellular enzyme activity. Selected β-1,4-glucosidase, β-1,4-xylosidase, and 4-N-acetyl-glucosaminidase hotspots increased, whereas phosphatase hotspots declined across all microplastic treatments. Changes were also detected in bacterial and fungal diversity and microbial network organization, with lower bacterial network complexity and greater fungal network complexity in the rhizosphere. Structural equation modeling explained approximately 60% of the variation in net photosynthetic rate, with barley antioxidant enzyme responses and soil hydrolytic activity showing the strongest negative associations. These results provide short-term evidence that high microplastic loadings can modify interconnected plant and rhizosphere processes under controlled greenhouse conditions. Given the limited replication, short experimental duration, and high exposure levels, these responses should not be interpreted as evidence of field-level causality. Field experiments using environmentally realistic concentrations and longer exposure periods are required to determine their agricultural relevance.