Root-derived carbon inputs dominate early soil recovery after grassing

Grassing of arable land is a widely adopted restoration strategy to rebuild soil fertility and enhance soil organic carbon (SOC) stabilization, but the contribution of plant species richness during the initial phases of restoration remains unclear. In a four-year field experiment, we compared a species-poor clover–grass mixture and a species-rich regional mixture, using permanent grasslands as control, to assess changes in soil physical properties, soil biota, and SOC fractions. Root biomass in permanent grassland was initially 2241% higher than in both seed mixtures but rapidly converged, with differences declining to 101% after four years. Both seed mixtures significantly increased soil organic matter content and water-holding capacity while reducing bulk density, indicating rapid recovery of soil structure driven by grass-dominated root systems. Soil microbial activity, microbial biomass carbon, and nitrogen were initially 62%, 286%, and 304% higher in permanent grassland, respectively, but these differences diminished substantially over time, demonstrating rapid microbial recovery independent of species richness. SOC fractionation revealed comparable increases in particulate and mineral-associated organic matter under both mixtures, indicating that early SOC stabilization was primarily controlled by root-derived carbon inputs rather than plant diversity. Strong correlations among root biomass, microbial properties, and SOC fractions highlight the key role of root–microbe interactions in driving early SOC stabilization during grassland restoration. Overall, early soil recovery and SOC stabilization after grassing are driven primarily by continuous root-derived C inputs and biotic transformations, while higher plant diversity may enhance long-term soil multifunctionality and C persistence.