Dissolved organic matter (DOM) plays a fundamental role for many soil processes. For instance, production, transport, and
retention of DOM control properties and long-term storage of organic matter in mineral soils. Production of water-soluble
compounds during the decomposition of plant litter is a major process providing DOM in soils. Herein, we examine processes
causing the commonly observed increase in contribution of aromatic compounds to WSOM during litter decomposition, and unravel
the relationship between lignin degradation and the production of aromatic WSOM. We analysed amounts and composition of water-soluble
organic matter (WSOM) produced during 27 months of decomposition of leaves and needles (ash, beech, maple, spruce, pine).
The contribution of aromatic compounds to WSOM, as indicated by the specific UV absorbance of WSOM, remained constant or increased
during decomposition. However, the contribution of lignin-derived compounds to the total phenolic products of 13C-labelled
tetramethylammonium hydroxide (13C-TMAH) thermochemolysis increased strongly (by >114%) within 27 months of decomposition.
Simultaneous changes in contents of lignin phenols in solid litter residues (cupric oxide method as well as 13C-TMAH thermochemolysis)
were comparably small (−39% to +21% within 27 months). This suggests that the increasing contribution of lignin-derived compounds
to WSOM during decomposition does not reflect compositional changes of solid litter residues, but rather the course of decomposition
processes. In the light of recently published findings, these processes include: (i) progressive oxidative alteration of lignin
that results in increasing solubility of lignin, (ii) preferential degradation of soluble, non-lignin compounds that limits
their contribution to WSOM during later phases of decomposition.