Biodegradable microplastics trigger agroecosystem trade-offs: Microbial-driven disruption of photosynthesis, carbon fixation, and nitrogen cycling

Biodegradable microplastics (Bio-MPs) are increasingly adopted as sustainable alternatives, yet their ecological impacts in agroecosystems remain poorly understood. This study employs an integrated multi-omics approach to compare the effects of conventional conventional polyethylene microplastics (PE-MPs) and Bio-MPs (poly(butylene adipate-co-terephthalate) (PBAT), poly(butylene succinate) (PBS), and polylactic acid (PLA)) on soil-plant-microbe interactions in sandy soils over a 64-day experiment. Bio-MPs triggered severe vascular obstruction in tomato roots, reducing photosynthetic rates by 28–74 % compared to PE-MPs. Furthermore, Bio-MP pollution enhanced nitrogen transformation in soil, particularly PBS and PBAT Bio-MPs, which promoted denitrification-driven nitrogen transformation. Bio-MPs also appeared to impair microbial carbon sequestration, resulting in elevated CO 2 emissions. PBS-MPs specifically increased biological nitrogen fixation by 28–50 % and boosted Feammox-driven NH 4 + -N conversion by 51–59 %, replenishing soil nitrogen pools. Plants responded to Bio-MP-induced oxidative stress by reallocating metabolic resources toward antioxidant defenses, compromising secondary metabolite synthesis and causing dysregulation of sugar metabolism. Our findings reveal a critical trade-off: while Bio-MPs alleviate plastic pollution, they paradoxically disrupt carbon-nitrogen cycling. This challenges the assumption that "biodegradability" equates to environmental safety and underscores the need for polymer-specific risk assessments. Their microbial-mediated C-N trade-offs challenge the sustainability narrative, calling for a paradigm shift from material-centric to ecosystem-centric evaluations of biodegradable polymers. • Bio-MPs triggered greater agroecosystem trade-offs than conventional PE-MPs. • Bio-MPs reduced microbial carbon sequestration and tomato photosynthesis. • Bio-MPs impacted root development of tomato and induced tomato defense. • PBS-MPs increased bio-nitrogen fixation and soil NH₄⁺-N availability. • PBAT-MPs and PLA-MPs impaired the secondary metabolism of tomato roots.