Effects of plant species diversity on particulate and mineral-associated organic carbon in a subtropical forest

Abstract Plant species diversity (PSD) is known to enhance soil organic carbon (SOC) storage, but how it affects distinct SOC fractions, such as particulate (POC) and mineral-associated organic carbon (MAOC), remains unclear. We aimed to elucidate the mechanisms by which PSD regulates the accumulation of POC and MAOC, specifically by disentangling the biotic and abiotic drivers mediating these effects in a subtropical forest. We assessed the variations in surface soil characteristics (0-10 cm layer) and SOC physical fractions (specifically POC and MAOC), across a natural gradient of PSD in a representative subtropical forest in southwest China. POC content ranged from 8.37 to 87.6 g C kg−1 soil (mean: 31.3 ± 20.4 g C kg−1 soil), and MAOC content ranged from 17.6 to 61.0 g C kg−1 soil (mean: 33.2 ± 11.3 g C kg−1 soil). Increasing PSD significantly enhanced POC accumulation and marginally increased MAOC, resulting in a shift in SOC fractions toward a higher POC proportion. Higher PSD was associated with increased soil exchangeable Ca and Mg, which correlated with greater mineral protection of plant- and microbial-derived carbon compounds (e.g., lignin phenols and amino sugars), subsequently fostering the accumulation of both POC and MAOC. Our findings highlight the importance of mineral protection mechanisms, mediated by divalent cations, in regulating POC and MAOC accrual in subtropical forests. PSD indirectly influences SOC fraction dynamics through enhanced mineral stabilization of biologically derived carbon, underscoring the role of biogeochemical pathways in linking PSD to soil carbon storage.