FIRE IN THE BRAZILIAN AMAZON 2 - BIOMASS, NUTRIENT POOLS AND LOSSES IN CATTLE PASTURES

Conversion to cattle pasture is the most common fate of the approximat e to 426,000 km(2) of tropical forest that has been deforested in the Brazilian Amazon. Yet little is known about the biomass, C, nutrient p ools, or their responses to the frequent fires occurring in these past ures. We sampled biomass, nutrient pools and their losses or transform ation during fire in three Amazonian cattle pastures with typical, but different, land-use histories. Total aboveground biomass (TAGB) range d from to 53 to 119 Mg ha(-1). Residual wood debris from the forests t hat formally occupied the sites composed the majority of TAGB (47-87%) . Biomass of fine fuels, principally pasture grasses, was approximate to 16-29 Mg ha(-1). Grasses contained as much as 52% of the abovegroun d K pool and the grass and litter components combined composed as much as 88% of the aboveground P pool. Fires consumed 21-84% of the TAGB. Losses of C to the atmosphere ranged from 11 to 21 Mg ha(-1) and N los ses ranged from 205 to 261 kg ha(-1). Losses of S, P, Ca, and K were < 33 kg ha(-1). There were no changes in surface soil (0-10 cm) nutrien t concentration in pastures compared to adjacent primary forests. Fire s occur frequently in cattle pastures (i.e., about every 2 years) and pastures are now likely the most common type of land burned in Amazoni a. The first 6 years of a pastures existence would likely include the primary forest slash fire and three pasture fires. Based upon our resu lts, the cumulative losses of N from these fires would be 1935 kg ha(- 1) (equivalent to 94% of the aboveground pool of primary forest). Post fire aboveground C pools in old pastures are as low as 3% of those in adjacent primary forest. The initial primary forest slash fire and the repeated fires occurring in the pastures result in the majority of ab oveground C and nutrient pools being released via combustion processes rather than decomposition processes.