Biochar mitigates microplastic‐induced destabilization of soil organic carbon via molecular recalcitrance and microbial process regulation

Abstract Soil organic carbon (SOC) stability is critical to climate mitigation but faces growing threats from microplastic (MP) pollution. Biochar (BC), a carbon‐negative soil amendment, enhances SOC persistence through chemical stabilization and microbial modulation. However, its efficacy under simultaneous MP contamination remains unclear. We conducted a 30‐day greenhouse incubation experiment, adding polypropylene MPs (PP, 1%) and BC (2%) to soil to explore shifts in SOC molecular structure and microbial communities. While the addition of PP alone maintained bulk SOC content, it masked underlying destabilization by reducing aromatic carbon in humin (HM) and fulvic acid. This process also enriched bacteria that degrade aromatic carbon, along with fermentative bacteria, ultimately increasing SOC vulnerability to future mineralization. BC addition increased SOC through dual stabilization mechanisms: (1) directly contributing aromatic structures to HM (23% higher aromatic carbon) and (2) suppressing bacterial groups involved in aromatic carbon degradation. Crucially, BC's effects persisted in soils with coexisting PP, maintaining SOC molecular stability and microbial functionality. These findings underscore the potential of MP to influence long‐term carbon–climate feedbacks by reducing SOC stability and altering microbial dynamics, while also highlighting BC's dual role as a carbon sink and a mitigator of MP effects. Read the free Plain Language Summary for this article on the Journal blog.