Soil mineralogy and temperature modulate the effects of biodegradable microplastics on dissolved organic carbon in soils

Biodegradable plastics are increasingly promoted as sustainable alternatives to conventional polymers, yet their degradation in soils can mobilize dissolved organic carbon (DOC), alter the stability of soil organic matter (SOM) and generate oligomers and nanoplastic particles with high bioavailability. The release of labile carbon forms from biodegradable microplastics can stimulate microbial activity, weaken mineral protection of organic matter, and enhance CO₂ emissions, with effects becoming more pronounced under warmer pedoclimatic conditions. Despite their growing use, the extent to which biodegradable microplastics influence DOC dynamics and priming of native SOM across contrasting soil mineralogies remains poorly constrained. Here, we investigated how biodegradable microplastics affect SOM stability under warming by incubating two contrasting soils, a 2:1 clay-rich Chernozem and a highly weathered Ferralsol dominated by 1:1 clays and Fe/Al oxides, with two biodegradable polymers (polylactic acid, PLA and poly(butylene adipate-co-terephthalate), PBAT) at 22 and 27 ºC. Soil CO₂ emissions and priming of native SOM were quantified using stable carbon isotopes (¹³CO₂) measured by cavity ring-down spectroscopy, while changes in DOC quantity and quality were assessed using DOC concentrations and specific ultraviolet absorbance indices (SUVA₂₅₄, SUVA₂₆₀, and SUVA₂₈₀). PBAT induced substantially higher cumulative CO₂ emissions than PLA, driven by strong positive priming effects, particularly in the Chernozem, where priming accounted for a large fraction of total CO₂-C released at both temperatures. In contrast, PLA showed minor or negligible priming effects. In the Ferralsol, total DOC concentrations were largely unaffected by plastic type, but biodegradable microplastics, especially PLA at 27 ºC, significantly reduced SUVA indices, indicating shifts toward less aromatic and potentially less stable DOM. These contrasting responses reflect differences in mineral protection mechanisms and pH regimes between soils. The contrasting responses of the Chernozem and the Ferralsol demonstrate that soil mineral protection mechanisms fundamentally control how biodegradable microplastics influence soil carbon stability. In the Chernozem, where organic matter stabilization relies primarily on 2:1 clays, cation bridging, and aggregate occlusion under neutral to alkaline pH (7.79), PBAT strongly stimulated microbial activity, resulting in pronounced positive priming and substantial losses of native soil carbon. The absence of concurrent changes in SUVA indices indicates that this destabilization was driven mainly by biological activation of weakly protected carbon pools rather than by disruption of chemically stabilized organomineral associations. In contrast, the Ferralsol, dominated by Fe and Al oxides and characterized by acidic pH (3.98) and strong inner-sphere complexation, showed limited sensitivity in total DOC and CO₂ fluxes but exhibited marked, temperature-dependent shifts in DOC quality, particularly under PLA at 27 ºC. Reductions in SUVA indices point to selective alterations in DOC composition, consistent with modified sorption–desorption equilibria or preferential microbial processing of aromatic fractions without large-scale carbon mobilization. These findings indicate that biodegradable microplastics destabilize SOM through distinct pathways depending on mineralogy, either by enhancing microbial priming where mineral protection is weaker, or reshaping DOC composition where physicochemical stabilization dominates, with temperature further modulating these processes.