Rhizosphere accelerates breakdown of large biodegradable microplastics in soil

The expanding deployment of biodegradable mulch films in global agriculture aims to mitigate persistent plastic pollution, yet the fate of resulting biodegradable microplastics (BMPs) in soil ecosystems remains poorly characterized. Although these materials are engineered for mineralization, their breakdown rates under realistic field conditions vary substantially, and plant roots fundamentally alter soil biogeochemistry through rhizodeposition and microbial recruitment. Whether the biochemically complex rhizosphere environment accelerates or retards BMP degradation, and how degradation byproducts accumulate, represents a critical knowledge gap for assessing the environmental safety of biodegradable agricultural plastics. Here we show that the soybean rhizosphere exhibits size-selective effects on poly(butylene adipate-co-terephthalate) microplastic (PBAT-MP) degradation. Large particles (998.7 ± 74.6 μm) degrade significantly faster than in bulk soil, whereas small particles (145.6 ± 3.1 μm) remain largely protected within soil aggregates over a 70-day growth cycle. Advanced quantitative proton nuclear magnetic resonance analysis reveals preferential hydrolysis of aliphatic adipate units, resulting in greater accumulation of degradation monomers in the rhizosphere than in bulk soil. Microbial community profiling identifies enrichment of Proteobacteria-particularly <i>Bradyrhizobium</i> and <i>Ramlibacter</i> genera-linked to PBAT hydrolysis and metabolite utilization, alongside increased microbial biomass and altered soil carbon pools. These findings challenge the prevailing assumption that biodegradable mulches degrade uniformly and benignly under agricultural conditions. Rhizosphere-relevant assessment criteria are essential for evaluating the true environmental safety of biodegradable plastics in agricultural systems, with broader implications for sustainable soil management and plastic pollution mitigation strategies worldwide.