Altitude and Soil Layer Regulate Soil Organic Carbon Stability Via Modulating GRSP and its Carbon and Nitrogen Contributions in a Tropical Montane Forest

Elucidating the divergent patterns and regulatory mechanisms of soil organic carbon (SOC) stability along altitude gradients and soil layers in tropical montane forests is beneficial for mitigating global climate warming. We investigated the responses of particulate organic carbon (POC), mineral-associated organic carbon (MAOC), and glomalin-related soil protein (GRSP) to altitude gradients (low, 400–600 m; middle, 700–900 m; high, 1000–1100 m) and soil depths (topsoil, 0–20 cm, subsoil, 20–40 cm). This study aims to reveal altitude and soil layer induced divergence and regulatory mechanisms of SOC stability in a tropical montane forest of southern China. The POC content was markedly higher at high altitude relative to low altitude, and its values at middle and high altitudes were significantly larger in topsoil than subsoil. By contrast, topsoil MAOC contents increased along the rising altitude gradient, with notably higher levels at low and high altitudes compared with subsoil layers. The MAOC/POC ratios in topsoil declined with increasing altitude. SOC stability was governed jointly by GRSP and soil moisture, while GRSP and soil C/N ratios served as the dominant influencing factors in subsoil. Altitude indirectly suppressed SOC stability via negative regulation on relevant variables. Furthermore, lowered SOC accumulation rate and reduced C/N ratio at high altitude primarily accounted for the reversed altitudinal pattern of SOC stability. The altitude and soil-layer differentiation patterns are essential for evaluating SOC stability, facilitating comprehensive understanding of SOC cycling dynamics and model responses to global warming in tropical forests.