Abstract: Microbial necromass is a key regulator of soil organic carbon (SOC) stability in terrestrial ecosystems, and its formation is directly linked to microbial life cycles. Soil microbes are closely associated with plant community composition and the related root functional traits. However, how these latter two modulate microbial life-history strategies and thereby affect SOC and microbial necromass formation remains unclear. We conducted a plant functional group removal experiment (graminoids, forbs, legumes) in an alpine meadow on the Qinghai-Tibet Plateau to reveal the pathways through which root functional traits mediate SOC change, microbial community strategies (r- vs. K-strategists), and the formation of fungal and bacterial necromass C. The results showed that plant communities with thinner roots and higher specific root length exhibited greater net SOC accumulation than those with coarser roots. This high efficiency of increasing SOC of thinner roots supply triggered a dual microbial response, simultaneously increasing the abundance of fast-growing r-strategist bacteria and slow-growing K-strategist fungi. The changes in r-strategist bacteria negatively correlated with bacterial necromass C accumulation, while K-strategist fungi likely contributed to fungal necromass C accumulation. These findings indicated that thinner roots play a critical role in SOC persistence by enhancing net SOC accumulation, probably via root-induced C inputs, and promoting K-strategist fungal necromass. This study highlights the importance of root functional traits in mediating microbial life strategies and the consequent C sequestration, which improves our understanding of soil C dynamics with shifts in vegetation composition and root functional traits under scenarios of global change.