<p>Microbial necromass is increasingly recognized as a major source of stable soil organic matter (SOM), and its persistence is often attributed to interactions with clay-sized minerals. However, the mechanisms underlying this mineral-mediated stabilization remain poorly understood. Here, we conducted an in situ dual-labeled (¹³C and ¹⁵N) microbial necromass experiment across a clay gradient to quantify how clay content and necromass origin (bacterial vs. fungal) regulate necromass persistence. We find that higher clay content markedly enhances necromass retention by strengthening mineral protection, suppressing microbial activity and diversity, and limiting leaching losses. NanoSIMS imaging shows that new necromass preferentially associates with organic matter coatings on the rough mineral surfaces, highlighting organo-organic interfaces as important stabilization pathways. Necromass origin exerts little effect on retention despite marked differences in C:N ratios and bulk chemical composition, indicating that finer-scale molecular features, rather than broad compositional differences, govern necromass stabilization in soils.</p>