<p>Grassing arable fields is an effective strategy to restore soil fertility and enhance soil organic carbon (SOC) stabilization; yet the role of plant species richness during early restoration remains unclear. This four-year study compared the effects of two seed mixtures – a species-poor clover-grass mixture and a species-rich regional mixture – using permanent grasslands as control, on the soil physicochemical properties, microbial activity and biomass, fauna and SOC fractions. Root biomass in the control grassland was initially 23 times higher than in both seed mixtures, but decreased to twice the difference after four years, indicating rapid convergence. Both seed mixtures increased organic matter content and water-holding capacity while reducing bulk density, reflecting effective soil recovery driven by grass-dominated root systems. Soil microbial activity in the control grassland was initially three times higher than in both seed mixtures, while carbon and nitrogen in the microbial biomass was four times higher. Over time, these differences decreased to roughly twice as high as in both seed mixtures, indicating rapid microbial recovery regardless of species richness. SOC fractionation revealed comparable increases in particulate and mineral-associated organic matter across both seed mixtures, suggesting that early SOC stabilization was largely associated with root-derived C inputs and was not strongly influenced by moderate increases in species richness within grass-dominated seed mixtures. Correlation analyses further demonstrated strong linkages among root biomass, biotic activity and abundance and SOC fractions, emphasizing the central role of root-biota interactions in SOC stabilization. Overall, SOC recovery and 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.</p>