Forest fires can change the quality of dissolved organic matter (DOM) in soils, and consequently have a great influence on biogeochemical cycles in forest ecosystems. However, little information is available regarding the effects of fire on the chemical composition of DOM in boreal forest soils. To clarify these effects, the molecular composition of soil DOM was compared between recently-burned and long-unburned boreal forests (6 and 156 years since the last fire, respectively) in Finnish Lapland. Ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry revealed that there were no significant differences in species, average molecular weight or the number of molecular compounds detected between the recently-burned and long-unburned forests. However, the number of compounds with condensed aromatic structures tended to be larger in the recently-burned forest, whereas the numbers of proteins and carbohydrates not shared between the two forests were significantly smaller. Lignin-like molecules accounted for most of the total molecular species in both forests. Our results suggest that fire not only generated several species of dissolved black carbon, but also caused burned plant residues, which supplied diverse lignin-like molecules in the recently-burned forest soils and led to the number of molecular species being comparable to that in the long-unburned forest soils.
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