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A field experiment was conducted to study the initial stages of chloritization (aluminization) of smectite (fraction

The main sources of the low-molecular-weight aliphatic and aromatic organic acids (LMWOAs) in soils are plant residues, root exudates, and microbial metabolites. In the solutions of upper soil layers of forest ecosystems, citric, oxalic, and malic acids predominate among aliphatic LMWOAs, while substituted and unsubstituted benzoic and cinnamic acids predominate among aromatic LMWOAs. The LMWOA concentrations in plant tissues, root exudates, and microbial metabolites vary from several to some tens of millimoles per liter. In soil solutions, these values decrease by one–two orders of magnitude because of LMWOA sorption and biodegradation, their migration downward the profile, and dilution effect. The sorption of LMWOA anions increases with a decrease in pH and an increase in the concentrations of iron oxides/hydroxides and amorphous aluminosilicate clays. Polybasic acids are more tightly bound to solid phase as compared with monobasic acids. The sorption mechanisms vary depending on pH and LMWOA molecular structure and include formation of innersphere and outersphere surface complexes, ligand exchange, and cation bridge bond. Aromatic LMWOAs can be involved in a hydrophobic interaction. LMWOAs are very rapidly decomposed in the upper horizons, their lifespan amounting to several hours. Some important functions of LMWOAs in soils and landscapes are discussed, including their involvement in the global carbon cycle, mineral weathering, Al and Fe mobilization and translocation, mobilization of Fe and P compounds in the soils deficient in these elements, formation of acid–base buffering of natural waters, and detoxication of soils with increased contents of aluminum, heavy metals, and organic pollutants in soil solutions.

An experiment on the transformation of test biotite of the clay fraction (

Acid–base characteristics and composition of clay minerals were estimated in the rhizospheres of Norway maple (Acer platanoides) and common spruce (Picea abies) and in the corresponding bulk soil samples taken in five replicates from the AELao horizon of podzolic soils. On the plot under spruce forest, both rhizospheric and nonrhizosheric soils were found to be more acid than those on the plot with a considerable part of maple trees in the forest stand. No reliable differences in pH values were found between the maple rhizosphere and corresponding bulk soil, while the rhizospheric soil under spruce forest had significantly lower pH values as compared with the enclosing soil. The rhizospheric soil under both tree species was found to contain reliably more illite minerals in clay-sized fraction, which could be due to the intensification of illitization and physical disintegration of micas and illites in coarse fractions. Under spruce forest stand, the clay fraction in both rhizospheric and nonrhizosheric soils contained more expandable minerals and less kaolinite and illites as compared with those under maple parcel. These differences can be explained partly by the spatial variability of clay composition in the parent material (mantle sandy loam) and, partly, by the dissimilarities in the functioning of different tree species and associated microbial communities causing lower pH values in soils under spruce forest. In the maple rhizospheric soil, pedogenic chlorites were characterized by a higher degree of aluminization in comparison with the enclosing soil owing to more favorable acid–base conditions. A tendency for a deeper transformation of illites into expandable clay minerals was revealed in the spruce rhizospheric soil as compared with the bulk soil, which can be explained by a more acid reaction facilitating the mobilization of aluminum.

We studied the state of the soil cover in sanitary protection zones of two large oil refineries in Samara region (at distances up to 1000 m from the refineries) using a combination of chemical and microbiological test to identify priority pollutants. The following parameters were used for the sanitary-chemical assessment of soils: soil pH and the concentrations of ammonium, nitrates, mercury, anion-active surface-active agents, phenols, petroleum products, arsenic, copper, lead, zinc, cadmium, nickel, cobalt, and benzo[a]pyrene. The sanitary-bacteriological tests included determination of the coliform index, index of lactose-positive Bacillus coli, enterococcus index, total microbial count, and the quantity of saprophytes, actinomycetes, and enteric pathogens. Petroleum products, benzo[a]pyrene, arsenic, and some heavy metals (cadmium and nickel) were the priority pollutants of sanitary protection zones of oil refineries. The results of microbiological tests demonstrated that the soil self-purification processes in the studied areas are very slow. It was found that the content of hydrocarbons in soils depends on the distance of sampling sites from large highways within sanitary protection zones. The contents of most of heavy metals and arsenic in soils were higher near large highways and parking areas in comparison with those at the sites near small streets with low traffic. The results obtained attest to the unsatisfactory state of the soil cover in the examined sanitary protection zones. These zones require further monitoring and certain remediation actions.

Exchangeable acidity in samples from organic and mineral horizons of taiga and bog soils with strongly acid reaction (pHKCl < 3.3 and $${\text{p}}{{{\text{H}}}_{{{{{\text{H}}}_{2}}{\text{O}}}}}$$ ≤ 4.4) was determined by the Sokolov method of potentiometric titration of KCl extract before and after the addition of a sodium fluoride solution. In the same extracts, the contents of Al, Fe, and Mn ions were determined by the atomic emission method. NaF was added in an amount sufficient for binding all metal ions into fluoride complex compounds. It was found that the content of metals determined by atomic emission is lower than that calculated from titration results in 1.5–2 times in organic horizons and by 20–25% in mineral horizons. Even a higher difference between the two methods was observed for exchangeable protons: from the titration data, their content in organic horizons was 10–12% of the content determined as the difference between total exchangeable acidity and total metal ions determined by atomic emission, and 3–15% in mineral horizons. The revealed differences were attributed to the underestimated results of determining exchangeable protons in the presence of NaF, which was confirmed by the potentiometric titration of model solutions of strong and weak acids prepared from a KCl solution. It was found that the addition of NaF during the titration with a strong acid results in reaching the point of equivalence after the addition of a smaller amount of base. Reasons for the shift of the point of equivalence during titration with a strong acid in the presence of NaF require special investigations. It is recommended to estimate the exchangeable acidity of strongly acid soils due to metal ions by atomic emission or other adequate methods.

Some biological properties and concentrations of extractable Fe, Al, and Si compounds in the samples of the AOEL horizon of podzolic soil taken in five replicates from the rhizosphere of 15- to 20-year-old spruce and from the nonrhizosphere soil are discussed. The soil mass of the rhizosphere is characterized by a significantly higher total number of bacteria, more abundant and diverse saprotrophic bacterial complex, greater length of fungal mycelium, and higher content of benzenecarboxylic acids dominated by benzoic acid in comparison with the nonrhizosphere soil. The soil mass of the rhizosphere and the fraction of 1–5 µm isolated from it are also characterized by significantly higher concentrations of Fe and Al extracted by the Tamm and Bascomb reagents because of the accumulation of Fe-organic and Al-organic complexes. For extractable Al compounds in the rhizosphere, this finding is confirmed by the high coefficient of correlation between the amount of Al in the extracts and the Corg content in the bulk soil mass and in the fraction 1–5 µm. Such a correlation is absent in the nonrhizosphere soil. The content of iron compounds extracted by the Mehra–Jackson reagent from the rhizosphere and nonrhizosphere soils is significantly higher than that extracted by the Tamm and Bascomb reagents; in the nonrhizosphere samples, it is significantly higher than in the rhizosphere. This difference can be explained by the fact that the content of Fe oxides and hydroxides minerals decreased in the soil mass of the rhizosphere due to more active dissolution processes under the conditions of more acid medium and higher concentration of organic ligands, so that the mobilized Fe enters Fe–organic complexes.

The dynamics of desorption of the easily exchangeable and nonexchangeable potassium (K) have been studied in chernozem samples taken from a field trial with a single-time application of different rates of potassium fertilizers after a long period of the absence of fertilizers in Voronezh oblast. The desorption of the easily exchangeable K is adequately described by the zero-order equation of reaction; several mechanisms participate in it. The reaction rate constant regularly increases with an increase in the rate of applied K fertilizer. Taking into account the high correlation between the amounts of K released into the soil solution and absorbed by plants in the course of the dynamic experiments, we may conclude about a significant improvement of plant nutrition conditions even in the case of a single application of K fertilizer at a small rate. The desorption of nonexchangeable K is approximated by the equation of irreversible reaction of the first order. The parameters of this equation, as well as of the equation of diffusion and of the Elovich equation, do not change in dependence on the rate of applied K fertilizer. The reaction rate constants in the equations of the first and zero orders describing the desorption of nonexchangeable and easily exchangeable K from chernozems are three to five times higher than those for soddy-podzolic soils and other soils of humid regions. This can be explained by the differences in the number and nature of exchange centers in the organic matter and on clay minerals of these soils.

An experiment on transformation of biotite (fraction