Is the residual ash method applicable to tropical peatlands? A case study from Brunei Darussalam

Tropical peatlands in Southeast Asia are a significant carbon sink, but are under major threat of fire resulting in significant carbon emissions. This study focused on the residual ash method, which has not been applied before for a tropical peatland, to determine the amount of carbon lost due to fire along two transects. To evaluate the method in a tropical peatland, we sampled peat cores to a depth of 150 cm along two transects in a drainage-affected peatland in Brunei Darussalam and analysed Landsat images to determine burn count at individual sampling locations between 1988 and 2020. The ash residue method indicated that the carbon release from the two transects was 3.61 ± 1.08 kg m-2 and 3.77 ± 0.80 kg m-2 respectively due to peat decomposition and fire. However, our results show that although some locations burned up to five times, the expected ash content (of at least 4 %) was not found in the surface peat. Therefore, the majority of the resulting ash from these fires must have been transported out of the peatland, possibly in smoke or washed away via ground and surface water transport. We conclude that the residual ash method to determine carbon loss is not a reliable method to determine carbon loss in degraded tropical peatlands.