High concentrations of the acid-insoluble residue (AIR, also known as "Klason lignin") in decomposing litter are considered to indicate high resistance to decomposition; however, the chemical composition of AIR in different types of litter and soil organic matter is poorly understood. In the present study, we characterized samples of common litter (L) types in boreal forests (needle, coarse tree, and moss), as well as fragmented litter (F), and humus (H) layers in two south boreal forest sites using a combination of sequential fractionation and pyrolysis-GC-MS. The results showed that the unfractionated samples were composed of cellulose-derived carbohydrates, guaiacyl-type lignin and other polyphenolic compounds, and that there was little variation among samples. However, pyrolysis-GC-MS analyses of AIR demonstrated that the composition of the AIR fraction differed among the analyzed litter materials as well as between the layers in the soil organic horizon. In the F and H layers, the AIR fraction contained guaiacyl-type lignin and other polyphenolics, as well as lipophilic compounds, which were indicated by the pyrolysis product methyldehydroabietate and short-chain fatty acids. In the AIR fraction, only small amounts of carbohydrate-derived compounds were detected, confirming that the sequential fractionation method efficiently removes soluble polysaccharides. The AIR fraction was poorly soluble in all solvents. The results presented here confirm that the sequential fractionation method efficiently separates water-, chloroform-, and acid-soluble (72% H2SO4) compounds from acid-insoluble compounds (AIR). However, AIR was shown to be a mixture of polyphenolic (mainly lignin-derived) and lipophilic (including fatty acids and resin acid) structures, and may therefore be a poor indicator of lignin and phenolic compounds when investigating the F and H layers in the organic horizon.
- Fractionation method
- Fast pyrolysis