Root-borne microbial necromass carbon (MNC) is an under-investigated contributor to soil organic carbon (SOC) in grasslands. Here we conduct a benchmark assessment of root-borne MNC based on amino sugars in the fresh roots of 27 dominant species and mixed roots (including dead ones) of mixed species in Inner Mongolian grasslands. We find that mixed roots contain 9.1−62.4 mg g⁻¹ MNC, which may contribute ∼10% of MNC in surface soils (0−10 cm) if all enters the soil without degradation. Root (and arbuscular mycorrhizal fungi; AMF) turnover rather than AMF biomass controls root-borne MNC accumulation. Based on a microcosm decomposition experiment of mixed roots in model soils under optimal conditions (∼22°C; 60% of maximum water holding capacity), we further estimate that 43%−75% of root-borne MNC remains after 2-year decomposition, implying that root-borne MNC is relatively stable. Hence, root-borne MNC may be an overlooked potential source of soil MNC and SOC in grasslands.