<b>Does tree species diversity alter soil organic carbon stability?</b>

The stability of soil organic carbon (SOC) and its environmental and ecological determinants, will have a large influence on the future climate. Despite evidence that tree species diversity increases SOC storage, its influence on SOC stability (i.e. its properties that define its sensitivity to perturbation) remains uncertain. Using a natural gradient of tree species diversity in a subtropical forest, we tested the hypothesis that increased tree species diversity affects multiple ecosystem properties (plant-detritus, the soil microbial community and soil physicochemical properties) that together results in altered SOC stability. We tested for changes in three complementary SOC stability indices: i) the ratio of mineral–associated OC (MAOC) to particulate organic C (POC), ii) SOC chemical recalcitrance (i.e., aliphaticity and aromaticity), and iii) SOC mineralization rate (assessed in laboratory incubations). Higher tree species diversity led to increased SOC content but at the same time evidence for altered stability: decreased MOAC:POC ratios likely due to reduced saturation of iron/aluminium (Fe/Al) oxides with MAOC. Concurrently we observed shifts in the soil microbial community structure and accelerated necromass recycling, leading to increased aliphaticity and decreased aromaticity of SOC, further indicative of increased SOC turnover at higher tree species diversity. By contrast, the decrease in SOC mineralization rate in response to greater tree species diversity was associated with weakened clay and Fe/Al oxides protection as well as strengthened calcium–bound OC. Although plant diversity could enhance forest C sink, its impact on SOC stability must also be carefully considered when assessing climate mitigation potential.