The natural abundance of 13C was used to estimate the turnover of the soil organic matter in a vertisol re-grassed with Digitaria decumbens (C4 plant) following intensive market gardening (C3 plants). In addition, the experimental design allowed us to determine the respective roles of roots and earthworms (Polypheretima elongata) in soil C stock restoration in D. decumbens pasture. The C stock increased from 31 to 37 Mg C ha-1 in 5 years and the d13C increased from -18.1% in market gardening soil to -15.5% in the 5-year-old pasture soil in the upper 20 cm. Below the 20 cm soil layer, the C stock and the d13C did not change significantly in 5 years. The net gain of 6 Mg C ha-1 was the balance of a loss of 5 Mg C ha-1 derived from market gardening and a gain of 11 Mg C ha-1 derived from D. decumbens. Effects of earthworms on the C dynamics were not discernible.