Microplastics (MPs) are widespread in terrestrial ecosystems, raising global concerns that they may disrupt soil organic carbon (SOC) cycling and undermine agroecosystem contributions to climate mitigation. However, it remains unclear how degradable versus inert MPs affect microbially mediated SOC persistence. We conducted a field experiment in a dryland agroecosystem using polyethylene (PE; inert) and polylactic acid (PLA; degradable) MPs at three residue levels, integrating metagenomics with SOC fractionation, microbial necromass quantification and extracellular enzyme stoichiometry to examine microbial pathways underpinning SOC persistence. PE- and PLA-MPs followed contrasting patterns in SOC persistence. PE-MPs were observed to intensify microbial C limitation, enrich Actinobacteria, and increase the relative abundance of carbohydrate-degrading CAZy families, consistent with greater decomposition potential. In the non-labelled soil, POC declined by 10.5%-16.7% under PE-MPs, while microbial necromass formation was constrained and SOC showed a declining trend. In contrast, PLA-MPs were associated with higher relative abundances of genes involved in C and N metabolism and amino sugar/EPS synthesis. These responses were accompanied by a 3.6%-6.9% increase in microbial necromass and greater potential for EPS-related biosynthesis, together with a tendency towards higher mineral-associated organic C. Collectively, these polymer-specific microbial responses highlight how plastic residues may either weaken or reinforce soil C persistence, with implications for forecasting agroecosystem C sinks and guiding plastic substitution and residue management under climate-relevant land stewardship.Read the free for this article on the Journal blog.