Changes in the Elemental Composition of Lignin during Humification

Lignin was extracted with aqueous dioxane in the genetic series of humification: dead nonhumified plants -> plant residues at the initial stage of humification (during the first 1-3 annual cycles) -> weakly decomposed peat -> highly decomposed peat. In the course of the humification and peat formation, the lignin was subjected to profound redox transformation; therefore, the elemental composition of the dioxane lignin in the humified peat-forming plants differed from that in the original plants by the higher contents of hydrogen and nitrogen and the lower contents of oxygen and carbon. As a result of the redox reactions, the lignin was partly oxidized and converted to humic substances during the humification of the dead plants. Therefore, it could not be extracted with aqueous dioxane. The reduced part of the lignin remained in the humified materials. The proportion of aromatic fragments in the molecules of the remaining lignin was smaller and that of aliphatic fragments larger than in the lignin of the original plants.