Burning of secondary forest in Amazonia: Biomass, burning efficiency and charcoal formation during land preparation for agriculture in Apiaú, Roraima, Brazil

In a burn of 5-year-old secondary forest cleared for agriculture in Roraima, Brazil, carbon partitioning was measured for above-ground portions of both secondary forest (regrowth) and the remains of original forest, felled and burned six years previously. Above-ground dry weight biomass averaged 43.0 ± 6.5 Mg (megagrams = metric tons) per hectare (Mg ha−1) for secondary forest and 105.8 ± 23.7 Mg ha−1 for original forest remains. Pre- and post-burn above-ground biomass loadings were estimated by cutting and weighing six 60-m2 plots and by line-intersect sampling (LIS) done along the axis of each post-burn plot (three transects), plus two supplementary LIS transects. High variability of initial biomass made LIS more reliable for assessing change in material >10 cm in diameter; quantities for diameter classes <10 cm relied on direct weighing. Above-ground carbon pools were reduced by 67.8% in secondary forest and 32.0% in original forest remains. Burning released 28.8 Mg C ha−1 (original forest remains plus secondary forest biomass), or 41.6% of the pre-burn total carbon stock in biomass. The remainder of the carbon either remained as residual biomass (39.2 Mg C ha−1 or 56.5% of pre-burn C) or as charcoal and ashes (1.4 Mg C ha−1 or 2.0%). Carbon stock in charcoal increased from 0.50 Mg C ha−1 to 1.07 Mg C ha−1, a net gain of 0.57 Mg C ha−1, or 0.8% of the pre-burn above-ground carbon stock. The net gain of charcoal carbon was composed of 0.21 Mg C ha−1 from secondary forest biomass and 0.36 Mg C ha−1 from original forest remains; 1.1% of the above-ground secondary forest carbon was converted to charcoal, while the corresponding percentage for original forest remains was 0.7%. Ashes contained an additional 0.29 Mg C ha−1, of which 0.11 Mg C ha−1 can be attributed to secondary forest biomass and 0.18 Mg C ha−1 to original forest remains. If the carbon in ashes is assumed to be finely powdered charcoal, this stock adds 0.21% to the charcoal formation percentage for secondary forest and 0.36% to that for original forest remains. The overall charcoal-formation percentage was 1.6%, or 2.0% if ashes are included. Charcoal-formation percentages in this study are lower than those sometimes assumed in global carbon models; nevertheless, charcoal can represent an important sink of atmospheric carbon over long time scales.