Spatial and temporal changes of soil C after establishment of a pasture on a long-term cultivated vertisol (Martinique)

In 1991 in Martinique (F.W.I), a Digitaria decumbens pasture was established on a vertisol that had supported a market-gardening culture for more than 10 years. Organic matter stock restoration was investigated by measuring carbon contents (C contents) and carbon/nitrogen (C/N) ratios each year from 1992 to 1997. Relations between and soil properties (particle-size distribution, soil depth) and C contents were studied. Furthermore, geostatistical analyses of C contents were realised in order to characterise the C storage in soil at plot scale. The increase of C contents from 1992 (Y0) to 1997 (Y5) was 5 g C (kg soil)−1 in the topsoil (0–10 cm) and 2.5 g C kg soil−1, or 7.5 Mg C ha−1, in the 0–30-cm layer. The intensity of organic C storage had a spatial pattern, although the C/N ratio remained homogeneous across the plot. However, there was no correlation between the C increase and the particle-size distribution or the depth of the soil. In the topsoil, the local variability of the C contents increased with time until 1995 and then there was a gradually spreading of this local variability. Plant-cover distribution and physical structure of vertisol could explain the evolution of spatial structure of the soil C content.