Tree-microbe-soil interactions affecting soil organic carbon fractions in Mediterranean forest soils

Soil organic carbon (SOC) represents a major terrestrial carbon pool, yet the processes that regulate its storage remain uncertain, particularly in water-limited shallow-soil ecosystems. Partitioning SOC into mineral-associated organic carbon (MAOC), considered more persistent, and particulate organic carbon (POC), which is more labile, can provide insight into the mechanisms controlling soil C storage. We investigated how SOC and its fractions were influenced by tree species composition, soil physicochemical properties, and microbial communities in mature Mediterranean forests dominated by Pinus halepensis (canopy conifer), Quercus calliprinos (sub-canopy broadleaf), and Pistacia lentiscus (understory woody shrub), either in monospecific or mixed stands. We further examined how these relationships varied among forest microsites (i.e., beneath the tree canopies and adjacent forest gaps). Across microsites, SOC concentrations were up to twofold higher under tree canopies compared to forest gaps with Quercus plots storing 10 %–30 % more SOC than Pinus and Pistacia plots. SOC variation was primarily explained by POC, which tended to be higher in mixed compared to monospecific plots. In contrast, MAOC showed an apparent saturation pattern, reaching approximately 45 g C kg −1 soil, and was strongly constrained by clay and silt content, indicating limited potential for additional mineral-associated C storage in soils approaching saturation. Tree-associated changes in soil C were accompanied by shifts in microbial community composition that depended strongly on microsite conditions. In particular, bacterial communities beneath tree canopies represented subsets of the more diverse communities occurring in forest gaps, suggesting that tree-induced changes in soil conditions act as an environmental filter on microbial assemblages. However, variation in microbial richness itself was not consistently associated with changes in SOC, POC, or MAOC. Together, these results show that tree effects on SOC emerge through interactions among species identity, microsite conditions, soil physicochemical constraints, and microbial community composition, with different controls operating on labile and mineral-associated C pools. Mixed forests were estimated to store approximately 6.1 Mg C ha −1 more SOC than monospecific pine stands, with this difference primarily associated with the labile POC pool, particularly where MAOC was already close to saturation.