The composition of water-soluble organic matter (WSOM) of coniferous- deciduous, coniferous, and moss litter was studied. Litter type affects the composition of WSOM. Sphagnum litter contains less N and dissolved organic carbon (DOC), but more carbohydrates, the WSOM from this litter is characterized by the lowest extinction coefficients Е 254 . WSOM from mixed litter with a predominance of birch litter had a maximum content of N and a small amount of soluble phenolic (SP) substances. WSOM from litter of pine needles differ from the rest by having a lower content of carbohydrates, the maximum amount of SP and hydrophobic fractions. All investigated WSOM were characterized by high rates of decomposition: half-decomposition time of their stable fractions was 2–4 months. WSOM from pine litter at pH ~ 6.5 were absorbed by the BF horizon of iron–illuvial podzol, starting from the concentration of DOC of 100 mg/L. Substances from birch–spruce litter were adsorbed insignificantly under similar conditions.

The water-soluble components originated in coniferous litters desorb into solution the native organic substances from the solid phase of the eluvial horizons of podzols and podzolic soil. In podzols, the water-soluble organic matter ( WSOM ) extracted from litter and then passing through the E horizons are sorbed by the BFH horizons if their concentration in solution exceeds 35 mg C/L. The sorption ranges from tens to hundreds of milligrams of carbon per kilogram of the BFH horizon. The WSOM can be sorbed by the BT horizons of podzolic soil when the concentration of organic substances in the liquid phase is higher than in podzols. The model experiments with minerals show that kaolinite and illite interact with the soluble substances percolating from the litter and selectively extract more hydrophilic and less aromatic components with relatively low molecular weights. On the contrary, goethite interacts with the WSOM prevalently sorbing hydrophobic components, substances of phenolic nature, components enriched in aromatic carbon, and substances with molecular weights less than 7 kDa. The spectral characteristics of WSOM change after the sorption interaction with minerals: the fluorescence associated with the humus substances with the longest conjugation chain decreases or disappears, while new fluorophores containing fragments of phenolic and/or protein structures emerge. The presence of kaolinite and illite in the eluvial horizons and their interaction with WSOM most likely contribute to the transformation of WSOM composition and appearance of the properties maximally favorable for their sorption on iron hydroxides in the lower horizons.

Mineralization resistance of water-soluble organic matter (WSOM) extracted from litter of podzol becomes higher upon the interaction with mineral horizons. Both the portion of WSOM susceptible to mineralization and its mean decomposition rate decrease two and three times, respectively, after its migration through the E eluvial horizon. Sorption in the BF horizon enhances the stability of the input organic matter. The relative content of mineralized carbon decreases to 2% (eight times). In natural soils, adsorbed organic matter may be even more resistant to mineralization due to lower temperatures and biological activity and thus represent the basis for the formation of humus reserve. Taking into account the calculated mineralization rate, the organic carbon accumulation in the BF horizon as a result of WSOM sorption may reach 13.5 mg/100 g per year, but it depends on the concentration of organic substances in soil solutions, hydrological regime, and other factors.