PASTURE BURNING IN AMAZONIA - DYNAMICS OF RESIDUAL BIOMASS AND THE STORAGE AND RELEASE OF ABOVEGROUND CARBON

Aboveground biomass in cattle pasture converted from tropical dense fo rest was studied both before and after reburning in Brazilian Amazonia . In a 7-year-old pasture studied in Apiau, Roraima, the aboveground d ry weight of biomass (live plus dead) exposed to burning consisted of 96.3 t ha(-1) of original forest remains, 6.2 t ha(-1) of secondary su ccessional vegetation (woody invaders in the pasture), and 8.0 t ha(-1 ) of pasture grass (carbon contents 48.2%, 45.4%, and 42.2%, respectiv ely). In terms of carbon, burning efficiencies for these three categor ies were 13.2%, 66.7% and 94.6%, respectively. Net charcoal formation was 0.35 t C ha(-1) or 0.63% of the carbon exposed to the reburn, whil e the total accumulated since conversion (including the initial burn) is estimated at 2.3 t ha(-1) (1.82% of the predeforestation abovegroun d biomass carbon stock). The dynamics of the original forest remains w ere represented in simulations that included parameters such as charco al formation, burning efficiency and carbon concentration in different biomass components. Releases from initial burning of the cleared fore st (44.0 t C ha(-1)) plus releases over the course of the succeeding d ecade through combustion (12.5 t C ha(-1)) and decay (51.5 t C ha(-1)) total 92% of the original forest biomass carbon (126 t C ha(-1)). Of biomass carbon remaining after the initial burn (84.3 t C ha(-1)), 76. 0% is released: 61.1% through decay and 14.9% through combustion in re burns, while 1.2% is net conversion to charcoal in the reburns. These results indicate an amount of charcoal accumulation that is smaller th an some carbon calculations have assumed, therefore suggesting a great er impact on global warming from conversion of forest to pasture.