<p>Degradable microplastics (dMPs) are increasingly involved in soils through agricultural practices, yet their long-term implications for soil organic matter (SOM) dynamics and carbon (C) balance remain unresolved. Unlike conventional microplastics, dMPs are chemically labile and characterized by high C/N ratios, enabling them to actively participate in soil C cycling rather than acting as inert contaminants. Here, we propose a net C balance framework that integrates dMP depolymerization, priming-induced SOM losses, and physicochemical stabilization pathways. Evidence suggests that dMP-derived C can be rapidly mineralized through enzyme depolymerization or stimulate positive priming effects that accelerate native SOM decomposition. Conversely, a substantial fraction of dMP-derived C may be retained in soil through mineral associations, metal bridging, and physical occlusion, contributing to persistent SOM pools. Evidence for negative priming effects induced by dMP is limited and predominantly based on short-term laboratory incubations, highlighting a major gap in field-scale validation. We argue that biodegradability does not necessarily imply net soil C neutrality or gains. Instead, the effects of dMPs on soil storage are conditional, governed by polymer traits, microbial nutrient constraints, and stabilization efficiency. Recognizing dMPs as C-active inputs rather than passive residues is essential for integrating plastic-derived C into soil C models and for developing sustainable agricultural strategies under climate change.</p>