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 experi mental design allowed us to determine the respective roles of roots and ear thworms (Polypheretima elongata) in soil C stock restoration in D, decumben s 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 signif icantly 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 n ot discernible. (C) 2001 Elsevier Science B.V. All rights reserved.