Changes in soil carbon stocks after land-use change from native vegetation to pastures in the Atlantic forest region of Brazil

Millions of hectares (ha) of the Atlantic forest of Brazil have been deforested and replaced by pastures, quite a large proportion of this in the last 60 years. There have been few studies on the impact of this land-use change on stocks of soil organic matter (SOM) only one study reported the state of vigour of the pastures. The aim of this study was to estimate the overall change in SOM stocks 16 years after the removal of forest vegetation in this biome in southern Bahia and the installation of pastures of Brachiaria brizantha fertilized with N and maintained under controlled grazing. Soil samples were taken for evaluation of density and texture and for analyses of C and N total and 13C abundance to a depth of 100 cm at 100 m intervals along four transects of 400 m from the pastures into the forest. Grazing was found not to have any significant effect on soil density (compaction). The live weight gain of the Nellore cattle on both cultivars of B. brizantha, fertilized with 120 kg N ha−1 yr−1 as urea during 12 years, was close to 500 kg ha−1 yr−1. The gain in soil C was similar under the two grass cultivars, being approximately 15 Mg C ha−1 to a depth of 30 cm and 20 Mg C ha−1 to 100 cm. The 13C abundance data showed that the large gain in soil C was due to the slow decomposition of the forest-derived C (total loss 12.6 Mg C ha−1 over 16 years) and the large accumulation of C derived from the Brachiaria (total gain 43.2 Mg C ha−1). These results confirm the potential of productive Brachiaria pastures to accumulate soil C in a tropical climate with year-round rainfall.