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 forest was studied both before and after reburning in Brazilian Amazonia. In a 7-year-old pasture studied in Apiau, Roraima, the aboveground dry 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 successional vegetation (woody invaders in the pasture), and 8.0 t ha(-1) of pasture grass (carbon contents 48.2%, 45.4%, and 42.2%, respectively). In terms of carbon, burning efficiencies for these three categories 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, while the total accumulated since conversion (including the initial burn) is estimated at 2.3 t ha(-1) (1.82% of the predeforestation aboveground biomass carbon stock). The dynamics of the original forest remains were represented in simulations that included parameters such as charcoal formation, burning efficiency and carbon concentration in different biomass components. Releases from initial burning of the cleared forest (44.0 t C ha(-1)) plus releases over the course of the succeeding decade 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 reburns, while 1.2% is net conversion to charcoal in the reburns. These results indicate an amount of charcoal accumulation that is smaller than some carbon calculations have assumed, therefore suggesting a greater impact on global warming from conversion of forest to pasture.