Grazing exclusion serves as a pivotal strategy for restoring degraded grasslands and substantially boosts carbon sequestration. As a major component of the biomass carbon pool, root carbon sequestration (RCS) is closely associated with the functional strategies of dominant plant species and soil development. However, how RCS changes over restoration time and its underlying drivers remain poorly understood. This study examined the variation of RCS across a chronosequence of grazing exclusion duration (0-28 years) in semi-arid grasslands in Northeast China, along with changes in root traits of dominant species and key soil properties. Results showed that belowground biomass and RCS dynamics followed a quadratic trajectory, peaking approximately 16 years after exclusion. Unimodal patterns in root length (RL), root area (RA), root tissue density (RTD), and root nitrogen content (RNC), opposite trends in specific root length (SRL) and specific root area (SRA), and a monotonic increase in root carbon content (RCC) over time collectively indicate that long-term grazing exclusion shifted the root resource strategy of dominant species from conservation to rapid acquisition. Soil properties such as soil water content, bulk density, pH, and electrical conductivity (EC) reached their optimal values at the mid-term exclusion stage, while soil organic carbon and total nitrogen content increased continuously with exclusion duration. Random forest model identified SRL, RTD, RA, RL and SRA as the main driving traits, and soil nutrient content, stoichiometry, and EC as main soil factors driving RCS. Variance partitioning analysis revealed that root functional traits independently explained 30.7% of the variation in RCS, whereas root-soil interactions accounted for 64.9%. The piecewise structural equation modeling delineated two regulatory pathways: a pathway wherein decreased soil phosphorus content elevated soil stoichiometric ratios, directly enhancing RCS; and a more prominent pathway wherein soil nutrients shaped root functional traits, specifically the trade-off of SRL-RTD, which in turn positively affected RCS, with this trait-driven pathway further reinforced by the amelioration of soil salinity-alkalinity. This study demonstrates that long-term grazing exclusion regulates RCS dynamics by driving shifts in root strategies and mediating root-soil interactions, with the core mechanism lying in trait tradeoffs shaped by soil improvement, thereby providing crucial theoretical support for grassland carbon sink restoration.