The role of biological activity (roots, earthworms) in medium-term C dynamics in vertisol under a Digitaria decumbens (Gramineae) pasture

The natural abundance of C-13 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 delta C-13 increased from -18.1 parts per thousand, market gardening soil to -15.5 parts per thousand in the 5-year-old pasture soil in the upper 20 cm. Below the 20 cm soil layer, the C stock and the delta C-13 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. (C) 2001 Elsevier Science B.V. All rights reserved.