SOIL CARBON CHANGES FROM CONVERSION OF FOREST TO PASTURE IN BRAZILIANAMAZONIA

Soils in Brazilian Amazonia may contain up to 136 Ct of carbon to a de pth of 8 m, of which 47 Gt are in the top meter. The current rapid con version of Amazonian forest to cattle pasture makes disturbance of thi s carbon stock potentially important to the global carbon balance and net greenhouse gas emissions. Information on the response of soil carb on pools to conversion to cattle pasture is conflicting. Some of the v aried results that have been reported can be explained by effects of s oil compaction, clay content and seasonal changes. Most studies have c ompared roughly simultaneous samples taken at nearby sites with differ ent use histories (i.e., 'chronosequences'); a clear need exists for l ongitudinal studies in which soil carbon stocks and related parameters are monitored over time at fixed locations. Whether pasture soils are a net sink or a net source of carbon depends on their management, but an approximation of the fraction of pastures under 'typical' and 'ide al' management practices indicates that pasture soils in Brazilian Ama zonia are a net carbon source, with the upper 8 m releasing an average of 12.0 t C/ha in land maintained as pasture in the equilibrium lands cape that is established in the decades following deforestation. Consi dering the equilibrium landscape as a whole, which is dominated by pas ture and secondary forest derived from pasture, the average net releas e of soil carbon is 8.5 t C/ha, or 11.7 x 10(6) t C for the 1.38 x 10( 6) ha cleared in 1990. Only 3% of the calculated emission comes from b elow 1 m depth, but the ultimate contribution from deep layers may be substantially greater. The land area affected by soil C losses under p asture is not restricted to the portion of the region maintained under pasture in the equilibrium landscape, but also the portion under seco ndary forests derived from pasture. Pasture effects from deforestation in 1990 represent a net committed emission from soils of 9.2 x 106 t C, or 79% of the total release from soils from deforestation in that y ear. Soil emissions from Amazonian deforestation represent a quantity of carbon approximately 20% as large as Brazil's annual emission from fossil fuels. (C) 1998 Elsevier Science B.V. All rights reserved.