Soil Carbon Stocks under Productive and Degraded Brachiaria Pastures in the Brazilian Cerrado

In the central savannah (Cerrados) region of Brazil approximately 50 Mha are occupied by Brachiaria pastures, most of which are classified as degraded. There are few reliable data on soil C stocks under planted pastures in this region and how soil C has been affected by their establishment and subsequent decline in productivity. This study was performed to compare soil C stocks under native Cerrado vegetation (NV) and productive (PP) and degraded pastures (DP) at four sites (chronosequences). Soil texture, bulk density, and 13 C abundance were investigated as candidate indicators for validation of the chronosequences. Productivity of the pastures at each site was evaluated using forage regrowth, existing and deposited litter, and the light fraction of soil organic matter (SOM). At all sites, the soil C stocks were higher under the PP than under the neighboring NV, and stocks under the DP were intermediate or very similar to the stocks under the NV. Soil 13 C abundance and C to N ratio suggested that SOM derived from NV was lost at a very low rate except in the surface layers (0–20 cm) and that soil C lost as pastures declined in productivity was principally derived from the pasture grass Brachiaria. The difference between soil C stocks under NV and PP was only 6 to 7 Mg C ha ‐1 at two sites with lower clay content (11 and 16%, respectively) but reached 12 Mg C ha ‐1 at Site C (46% clay) and 47 Mg C ha ‐1 at Site D (67% clay).