This paper examines the composition and properties of water-soluble organic substances in relation to their localization in different parts of soil pore space, which determines the organic matter’s susceptibility to biodegradation. Water-soluble organic substances contained in the sod and humus horizons of a Mollic Gleysol were extracted from pores more and less than 30 μm in diameter using sequential centrifugation of soil samples saturated to the full moisture capacity. The obtained solutions do not differ statistically significantly in the total concentration of carbon and phenolic compounds or in their absorption and fluorescence spectra; however, a strong variation in parameter values (the maximum and minimum values differ by 4–15 times) indicates that the composition of extracted substances is heterogeneous. Concentrations of simple phenolcarboxylic acids (PCAs) in solutions extracted from large pores in the humus horizon are 4–8 times higher in comparison with those extracted from smaller pores. Salicylic, benzoic, and cinnamic acids predominate in large pores (75% of the total amount); while p -hydroxybenzoic and benzoic acids predominate in small pores (67% of the total amount). Dissolved organic matter present in the ELBTg horizon of a podzolic soil was isolated from pores more and less than 14.7 μm in diameter at a soil moisture level of 22% using tension lysimeters. Dissolved organic matter extracted from large pores contains more C, N, and phenolic compounds; based on the absorption spectra, it has a larger molecular weight and a higher aromaticity degree. Organic matter contained in small pores is more hydrophobic, and its C : N ratio varies in a wider range (i.e., it is more resistant to decomposition). Phenolic compounds are predominantly confined to large pores; this ensures their preservation in soil due to the transportation to lower horizons with water flows and subsequent chemical stabilization in the form of organomineral compounds.

The content of benzoic acid in water extract from podzolic soil is 3.27 and 1.83 mg/kg in the L and F organic horizons, respectively, and decreases to hundredths of mg/kg in mineral horizons. The experiment shows that with an increase in the equilibrium concentration of benzoic acid from 0.02 to 1.45 mmol/L, its sorption on Ca-montmorillonite rises from 5.61 to 15.34 mmol/kg. Taking into account the content and mineralogical composition of the clay fraction, expandable clays in the upper mineral horizons of podzolic soil are capable to retain from 0.06 to 0.15 mmol of benzoic acid per 1 kg of soil within the same range of equilibrium concentrations. The linear equation and the Freundlich model describe best the measured data. It is assumed that the main mechanisms of benzoic acid sorption under the experimental conditions were represented by cation bridging or water bridging and by the electrostatic interaction of benzoate with positively charged silanol and aluminol groups on edges of montmorillonite particles. The absorption of benzoic acid in the interlayer space of the mineral was not observed.

The migration of water-soluble fraction of petroleum hydrocarbons in typical soils of Bashkortostan is studied. The possibility of their water migration in Fluvic Phaeozems, Haplic Phaeozems, and Amphiskeletic Pantocalcic Chernozems is determined by data of a lysimetric model experiment. Dissolved organic substances, which pass into lysimetric waters from oil-contaminated soils, include the components that are not extracted by hexane, but are characterized by fluorescence in water at the wavelengths typical for petroleum products. These substances differ from soluble organic matter of background soils by higher HIX (humification indices) and presumably represent oxidized transformation products of petroleum hydrocarbons. Lysimetric waters from all oil-contaminated soils are biotoxic, regardless of the presence of nonpolar petroleum hydrocarbons. The toxicity T-index is in positive correlation with the content of dissolved organic carbon and chloride ions, accompanying oil pollution of soils.

— The dynamics of the content and properties of water-soluble organic matter and the biological activity of soils of dwarf-shrub heath and graminoid meadow of alpine tundra on slopes of different aspects are estimated. The variation of most of the studied parameters during the growing season is revealed, which confirms the importance of dynamic observations in assessing the role of soils in changing environment. Soils on the southwestern slope are characterized by higher mean daily temperatures and lower water contents throughout the entire growing season. Despite this, slope aspect does not affect the content of water-soluble organic matter and the potential respiratory activity of soil microorganisms. At the same time, under warmer and drier conditions, the organic matter of alpine tundra soils is less resistant to microbiological transformation. The natural gradient of soil temperature and moisture does not significantly affect the efficiency of using carbon sources and the overall functioning of microbial communities. Based on the data obtained, it is predicted that the adaptation of the soil microbial community to a gradual rise in temperatures in the Subarctic region should proceed without a sharp increase in the rate of mineralization of soil organic matter by microorganisms.

This paper identifies and discusses two trends in the digitalization of soil research data: (1) creation and maintenance of databases containing reflectance spectra of the upper soil horizons in the range of 300–2500 nm and information on the main physicochemical properties of these horizons; and (2) accumulation and actualization of already published spectra in the visible range (400–750 nm) and incorporation of the data on all horizons constituting the soil profile into the spectral databases, which makes it possible to identify both individual horizons and entire profiles. The second trend widely uses color parameters of soils; this applies both to international optical systems and to Russian-specific indicatory systems. The algorithms used in such databases can be applied in studies involving open-access global soil libraries.

The succession of natural vegetation on the former arable soils triggers the processes of postagrogenic restoration of soil ecological functions specific of a particular bioclimatic zone. We analyze the postagrogenic dynamics of a set of soil characteristics in the upper (0–5 and 5–10 cm) layers of former arable horizon by the case study of the chronosequence of Retic Albic Podzols (agrosoddy podzols, southern taiga, Kostroma oblast, Russia) withdrawn from agricultural use 15, 20, and 45 years ago. The following soil characteristics are determined: pHKCl; the contents of soil organic carbon (SOC), total nitrogen (TN), and dissolved carbon and nitrogen (DOC and DON); basal respiration (BR) rate; carbon of microbial biomass (Cmic); and relative indicators of the state of microbial community. In addition, absolute (HIX1) and relative (HIX2) humification indices of dissolved organic matter (DOM) are assessed for the soil layer of 0–5 cm in the studied soil chronosequence, as well as the stratification ratio (SR (0–5 : 5–10)). The following changes are observed in the upper layers of the former arable horizon over 45 years of postagrogenic succession: (1) a statistically significant decrease in the pHKCl and humification index of DOM; (2) an increase in the SOC, TN, and Cmic contents and BR rate; and (3) a considerable stratification according to the SOC and TN contents and microbial properties. The highest values of SR for the SOC and TN are observed in the soil of the old-aged forest; for the microbial properties, in the 45-yr-old abandoned soil. Thus, the main characteristics and ecological functions of Retic Albic Podzol gradually restore during the secondary succession on the former arable land in the southern taiga zone.

There are many approaches of pesticide risk assessment. Despite their variation in difficulty and information complexity, all of them are intended to predict the actual pesticide risk as accurately as possible, i.e., to predict the behavior and hazard of a pesticide in the environment with high precision. The aim of this study was to develop a risk indicator of pesticide's negative impact on soil and aquatic organisms. The developed pesticide risk indicator constitutes the sum of points of acute toxicity exposure ratio, long-term toxicity exposure ratio, and the bioconcentration factor. To develop the indicator, mathematical models were used; the input data included the soil and climate conditions of a specific region. Combining the data of pesticide toxicity in the environment allowed for a more accurate risk assessment in terms of using plant protection products. The toxicity and behavior in soil and water of 200 widespread pesticides were studied. It could be concluded that a mathematical model, PEARL 4.4.4, calibrated for region-specific soil-climate conditions, provides a relevant description of the natural translocation and decomposition of pesticides in soils. In addition, the output data of this model can be applied to calculate the risk indicators. The combination of these parameters with pesticide toxicity for non-target groups of organisms allows the risk indicator to be a universal tool for predicting the negative impact of pesticides on the environment at the regional level.

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.

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.

The applicability of fluorescence spectroscopy for studying the dissolved organic matter in the Moskva River has been shown. The most typical surface water fluorophores—humic and fulvic acids and protein substances—have been revealed in the studied water samples. The fluorescence intensity depends on the sampling site and indicates the contamination rate. The dynamics of the composition and fluorescence parameters of river waters varies with respect to the sampling period. While the concentration of dissolved organic matter and chemical oxygen demand increases from October until November, specific ultraviolet absorbance, biological oxygen demand, and fluorescence of humic acids significantly decrease. The observed dynamics does not depend on the sampling site, which confirms the effect of climatic conditions. Statistically significant (p < 0.05) correlations between spectral and chemical parameters of water contamination have been revealed. Fluorescence intensity, tryptophan-containing organic substances, fulvic acids, and values of biological index correlate (r = 0.63–0.92) with the content of ammonium and phosphate ions. The fluorescent index A may be used to determine the zone of the effect of anthropogenic biological impurities on the status of waters of the Moskva River.

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.

Analytical fractionation conditions on the resin XAD-7H of organic substances from gravitational soil moisture influence the proportion and the yield of the extracted fractions. Increasing the column capacity factor k ’ is accompanied by a decreased yield of hydrophobic fraction and the change in its composition. The sorption column capacity factor of k ’ = 30 is sufficient for the extraction of all hydrophobic components from a soil solution with a concentration of C DOM of ∼25 mg/L.