Key message <p>Divergent patterns of small-scale topsoil organic matter (OM) indicators were identified in mixed vs. monospecific pine forests: tree species identity drives OM quantity, while the C/N ratio is additionally shaped by both overstory and understory diversity. The greater diversity of litter inputs in mixed stands likely accounts for their higher topsoil quality.</p> Context <p>Mixed forests are increasingly recognized for their potential to enhance soil carbon storage compared to monospecific stands. However, the underlying mechanisms, particularly the combined roles of overstory and understory vegetation, remain poorly understood, especially in Mediterranean pine forests.</p> Aims <p>We examined how stand type, overstory and understory species composition, and understory richness affect forest-floor biomass and the quantity and quality of topsoil organic matter (OM) in mixed vs. monospecific pine forests at a small scale.</p> Methods <p>Across six triplets in central-northern Spain (44–151&#xa0;years old), we established 180 subplots, including monospecific stands of <i>Pinus sylvestris</i> (PS), <i>P. pinaster</i> (PP), and mixed stands (MM). We measured topsoil properties (0–5&#xa0;cm), forest-floor biomass (25 × 25&#xa0;cm), understory species composition and richness (1 m²), and overstory composition (4&#xa0;m radius).</p> Results <p>OM quantity (total organic carbon) followed the stand-scale pattern (PS &gt; MM &gt; PP), driven primarily by tree species identity. However, OM quality (C/N ratio) differed (PS ~ MM &gt; PP), suggesting a combined effects of overstory and understory diversity at small scale. A priming-effect linked to diverse understory litter inputs in MM may partly explain this pattern.</p> Conclusion <p>Integrated small-scale assessments of all vegetation layers can guide forest management strategies that couple understory conservation with improved soil carbon sequestration in Mediterranean pine forests.</p> Graphical Abstract <p></p>