Soil organic carbon (SOC) dynamics in alpine ecosystems are crucial for global carbon cycling, yet the mechanisms governing the accumulation of its key precursors (plant-derived carbon, PRC, and microbial necromass carbon, MNC) and their partitioning into particulate (POC) and mineral-associated organic carbon (MAOC) pools remain poorly understood. This study investigated these mechanisms across a gradient of alpine grasslands (desert, steppe, meadow) at 40 sites in Ngari Prefecture on the northwestern Qinghai-Tibet Plateau. Our findings confirm that PRC is the dominant contributor to SOC across all vegetation types. However, the accumulation and drivers of carbon components exhibited distinct vertical and landscape patterns. Key results show that lignin phenols, dissolved organic carbon, and MNC were the primary predictors for both MAOC and POC pools, with explanatory variables showing stronger influences on POC and in topsoil layers. Notably, a depth-decoupled climate control was identified: PRC in topsoil was primarily and negatively influenced by precipitation, likely due to enhanced leaching and microbial mineralization, whereas subsoil PRC was mainly and positively driven by temperature, reflecting the strong kinetic constraint of low temperature on decomposition at depth. Furthermore, while the absolute stocks of MNC varied significantly among grassland types (highest in meadows), its relative contribution to SOC remained consistent. This study reveals a complex interplay where plant inputs directly fuel the active POC pool and indirectly support the stable MAOC pool via microbial turnover. These insights are critical for developing accurate models and targeted conservation strategies for vulnerable carbon stocks in high-altitude ecosystems.