Biochar mitigates microplastic-induced Destabilization of soil organic carbon via molecular Recalcitrance and microbial process regulation

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
biochar (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 (FA). 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 microplastics to influence long-term
carbon–climate feedbacks by reducing SOC stability and altering microbial
dynamics, while also highlighting biochar’s dual role as a carbon sink and
a mitigator of microplastic effects.