Mechanisms of poly(butylene adipate-co-terephthalate) microplastic degradation in agricultural soil: Insights from the soybean rhizosphere

The increasing application of biodegradable mulch films in agriculture has raised concerns regarding the potential persistence of biodegradable microplastics (BMPs) in soil and their subsequent ecological impacts. Although these materials are designed to mineralize, their actual rates of breakdown under field conditions vary widely, and the role of plant roots in modulating BMP transformation through the rhizosphere effect remains poorly understood. In particular, how the complex biochemical environment of the rhizosphere influences BMP degradation and byproduct accumulation represents a critical knowledge gap. Here we show that the soybean rhizosphere significantly enhances the degradation of 1% (w/w) large (998.7 ± 74.6 μm) poly(butylene adipate-co-terephthalate) microplastics (PBAT-MPs), whereas small (145.6 ± 3.1 μm) particles remain largely protected within soil aggregates over a 70-day growth cycle. This size-dependent effect is accompanied by preferential hydrolysis of aliphatic adipate units, leading to greater accumulation of degradation monomers in the rhizosphere than in bulk soil. We further demonstrate that PBAT degradation is associated with increased microbial biomass, altered soil carbon pools, and the enrichment of Proteobacteria, particularly Bradyrhizobium and Ramlibacter, which are linked to PBAT hydrolysis and metabolite utilization. These findings redefine the role of plant roots in regulating the fate of biodegradable microplastics in soil and highlight that biodegradable mulches cannot be assumed to degrade benignly under realistic agricultural conditions. Our work underscores the need for rhizosphere relevant criteria when assessing the environmental safety of biodegradable plastics.