Altitudinal Patterns of Soil Organic Carbon and Its Drivers in the Mountains of Southeastern Tibet

Understanding the distribution and drivers of soil organic carbon (SOC) in mountain ecosystems is essential for evaluating carbon stability and climate change responses. This study investigates the spatial patterns and driving mechanisms of SOC and its two main components—particulate organic carbon (POC) and mineral-associated organic carbon (MAOC)—along altitudinal gradients in five Himalayan valleys. SOC in 0–10 cm soil peaks at mid-elevations (1000–3500 m) and declines at higher elevations. SOC content varies markedly across land cover types, highest in forests (71.34 ± 62.36 mg/g), followed by grasslands, and lowest in deserts (12.40 ± 3.24 mg/g). POC is the main component of SOC in most ecosystems, especially forests, as it is closely influenced by vegetation type, biomass input, and microbial activity. In contrast, MAOC increases with elevation and is primarily controlled by soil mineral interactions and physicochemical properties. SOC components are co-regulated by biotic and abiotic drivers. POC formation is closely linked to plant productivity and microbial processes, whereas MAOC accumulation is largely determined by soil physicochemical properties, including soil texture, pH, moisture, and oxides. Under climate warming, significant vegetation shifts—particularly the encroachment of alpine shrubs into meadow areas—have altered SOC composition. Shrub expansion favors POC accumulation, which is less stable and more prone to decomposition, whereas alpine meadows support the formation of more stable MAOC. Although meadows are at risk of degradation, longer growing seasons may enhance SOC storage. These findings reveal the spatial dynamics and controls of SOC in the Himalayas, offering crucial insights for understanding mountain carbon cycles and informing climate adaptation and carbon management strategies.