Influence of fire on peat organic matter from Indonesian tropical peatlands

Effects of fire on peatlands globally are reasonably well documented, however studies of fire effects on tropical peatlands are limited. Specifically, the influence of fire on the biogeochemistry of tropical peat organic matter (OM) is not well understood and this study is the first to investigate this topic. Peat samples from undrained and degraded (drained and burnt) peatland in Central Kalimantan, Indonesia were analysed using novel chemical techniques. Short-term (one month post-fire) effects on peat OM composition were determined using Pyrolysis-Gas Chromatography/Mass Spectroscopy (Py-GC/MS), allowing for detailed molecular investigation. Results revealed significant compositional differences between burnt and unburnt peat pyrolysates including, dominance of recalcitrant aromatic- and aliphatic-derived compounds in pyrolysates from burnt peat, while those from unburnt/undrained peat contained greater contributions of labile compounds including derivatives of lignin and polysaccharides. At one month-post fire, pyrolysates from the upper 5 cm of peat showed that almost all polysaccharides (0.1±0.1%) and lignin-derived compounds (0.8±1.2%) were depleted, although these compound classes were relatively concentrated in subsurface peat (5-50 cm). Charcoal was a significant fraction of the upper 5 cm up to one month post-fire, but none was detected in significant quantities deeper in the peat column (5–50 cm). One-month post-fire, the burnt peat was hydrophilic in the upper 5 cm but retained its inherent hydrophobic character at greater depth. Similar to fire, peat drainage also reduced OM diversity i.e. the number of identified pyrolysis products. Peat pyrolysates from a drained unburnt site were dominated by aliphatic components (41.5-70.8% in the upper 50 cm). Thus both fire and drainage result in alteration of peat OM composition. The effects of fire were, however, short-lived (persisting up to 1.5 years post-fire), implying that peat OM composition can exhibit some recovery over a relatively short time period. Fire frequency had no significant influence on peat OM composition; thermal alteration occurred during the initial fire with no further modification during subsequent fires. The implications of these results for peatland carbon cycling and post-fire ecosystem recovery are discussed