When “Biodegradable” Is Not Benign: Microplastic-Driven Disruption of Soil Processes and Plant–Microbe Interactions

The increasing use of biodegradable plastics (BPs) as alternatives to conventional plastics (CPs) is leading to the accumulation of biodegradable microplastics (BMPs) in terrestrial environments. Contrary to assumptions of rapid degradation, BMPs can persist in soil long enough to interact with key biological processes. This review advances the field by proposing a mechanistic framework linking BMP aging and degradation, soil physicochemical transformations, plastisphere assembly, rhizosphere interactions, and plant responses, and by critically evaluating the sources of inconsistency across studies. We show that divergent effects of BMPs can be best explained by four interacting determinants: polymer chemistry and additive composition, aging-driven surface transformations, soil physicochemical properties, and rhizosphere processes including plant-mediated effects. Through these coupled pathways BMPs can alter aggregation, pore architecture, pH, enzyme activity, and carbon and nutrient cycling, thereby reshaping the soil environment in which microorganisms and roots interact. BMP surfaces further can act as dynamic microbial niches that promote biofilm formation, shift microbial community composition and function, and under certain conditions may facilitate pollutant transport, pathogen persistence, and horizontal gene transfer. Plant responses to BMPs are predominantly indirect and emerge from rhizosphere-mediated processes, which helps explain the wide variability in reported plant responses, ranging from subtle metabolic changes to pronounced growth inhibition. However, current evidence is constrained by short-term studies and insufficient consideration of aged materials. Biodegradability should therefore not be equated with low ecological risk in soils. Progress in this field requires integrative approaches linking BMP properties, plastisphere dynamics, and plant–soil interactions over time. • Integrative framework links BMP effects across soil–microbe–plant systems. • BMP impacts are governed by polymer traits, aging, and soil context. • BMPs reshape soil structure, biogeochemistry, and microbial niches. • Plant responses to BMPs are largely indirect and rhizosphere-mediated. • Standardized, long-term studies are essential for realistic BMPs risk assessment.