The mineralization of organic substances in soil solutions extracted from horizons of a podzolic soil using tension lysimeters was studied. The stability of dissolved organic matter (DOM) varies depending on the horizon. The maximum content of DOM resistant to biodegradation (mineralization degree <50%) was registered in upper organic horizons, and it decreases down the soil profile. The share of mineralized carbon is inversely proportional to the relative content of hydrophobic and phenolic fractions in the DOM. Organic matter extracted from horizon ELhi features the lowest share of mineralizable carbon (23%) amid the highest hydrophobic fraction content (44%). In the course of the oxidative transformation, the DOM aromaticity degree goes up: the extinction coefficient (SUVA280) increases by 2–12 times and the share of phenolic compounds by 2–10 times, while molecular mass decreases down to 5–5.5 kDa. These changes are manifested more intensely in solutions extracted from lower horizons. Therefore, the nondecomposable DOM residue consists of substances most resistant to decomposition.

Mineralization of water-soluble organic substances in forest litters of different compositions under conditions of their pollution with heavy metals from aerogenic emissions is considered. Water-soluble organic substances of the pine forest litters are shown to be less stable than that under the spruce forests irrespective of their pollution; the portion of mineralized at 20°C carbon for a month reached 40%. In the spruce litters, mineralizable amounts of carbon are related to their pollution: in the technogenic areas, compared to the background ones, carbon losses from the litters are twice greater. In the course of decomposition, in the spruce and pine litters, phenol compounds accumulate among the water-soluble organic compounds, and this process is more intense in the contaminated areas. After the decomposition of all the litters, irrespectively of their composition and the distance from the pollution sources, the water-soluble organic substances are characterized by higher indices of humification (HIX), more intense fluorescence, and greater coefficients of specific extinction (E260). In the ultraviolet spectra, bands appear in the region of aromatic amines absorption. These changes indicate the transformation of water-soluble organic substances: the degrees of their condensation and aromaticity increase and nitrogen-containing aromatic compounds are formed.