This article studies sorption regularities and evaluates thermal stability and resistance to microbial degradation of humic acid during three sorption cycles on bentonite clay, kaolinite, and muscovite using TGA/DSC, XRD, hydrophobic chromatography, light and electron microscopy, etc. The experiment revealed that kaolinite sorbed more humic acids (HAs) in terms of unit surface area (1.03 × 10⁻³ C, g/m²) compared to bentonite (0.35 × 10⁻³ C, g/m 10⁻³ g/m²). Sorption at pH 4.5 showed HA fractionation in amphiphilicity and chemical composition. HA was sorbed on the surface of all sorbents, mainly via hydrophobic components. No intercalation of HA into the interlayer spaces of montmorillonite was observed during sorption. Sorption via hydrophilic interactions was mostly performed on muscovite and bentonite rather than on kaolinite. Sorption of HA resulted in changes in its chemical composition and decreased C/N compared to free HA, which demonstrated selective sorption of nitrogen-containing compounds more typical of muscovite. All minerals adsorbed only a relatively thermolabile HA fraction, while its thermal stability increased compared to that before the experiment. The thermal stability and ratio of the Exo2/Exo1 peak areas on the DSC curves of sorbed HA increased with each subsequent sorption cycle. We revealed the following relationship between thermal stability and resistance to microbial oxidation of the sorbed HA: The higher the thermal stability, the less available the sorbed HA becomes for utilization by microorganisms.

Soils represent the largest reservoir of organic carbon (OC) in terrestrial ecosystems, storing approximately 1500 Gt C. Forest and grassland ecosystems contribute 39% and 34% to global terrestrial carbon stocks, with soils holding about 44% and 89% of forest and grassland carbon, respectively. Land-use changes, such as the conversions between forest and grassland ecosystems, can strongly influence soil carbon accumulation, though the direction and magnitude remain uncertain. Comparative data from paired-plot studies of forest and grassland soils are still limited. In this study, we conducted pairwise comparisons of total OC and total nitrogen (TN) stocks in mature forest and climax grassland soils along a climatic and pedogenic gradient encompassing Retisols, Luvisols, and Chernozems. Relationships between OC and TN stocks (0–10 cm) and soil physicochemical properties—OC and TN contents, bulk density, pH, clay content, and humus fractional composition, as well as biological indicators—the abundance of culturable fungi and bacteria, microbial biomass carbon, potential metabolic activity, and activities of laccase and dehydrogenase, were evaluated. Strong positive correlations were found between OC and TN stocks and OC and TN contents (r = 0.62–0.99), pH (r = 0.79–0.81), clay content (r = 0.70–0.87), and the fraction of humic acids bound with calcium (r = 0.73). OC stocks also correlated strongly with dehydrogenase activity (r = 0.85–0.95). At 0–10 cm depth, OC stocks were higher in grassland soils than in forest soils by factors of 1.6–1.7 in Retisols and 1.4–1.5 in Chernozems. Similarly, TN stocks were 1.6–2.0 times greater in grasslands across all soil types. Community-level physiological profiling revealed higher potential metabolic activity in forest soils compared with grasslands, with the strongest differences in Retisols and Luvisols, while contrasts were attenuated in Chernozems. Overall, the results highlight the fundamental role of organo-mineral interactions and calcium binding in OC stabilization, as well as the likely involvement of dehydrogenase activity in the biogenic formation of calcium carbonates that contribute to this process.

This work studied sorption regularities and assessed thermal stability and resistance to microbial degradation of humic acid as a result of 3 sorption cycles on bentonite clay, kaolinite, and muscovite using TGA/DSC, XRD, hydrophobic chromatography, light and electron microscopy, etc. The experiment revealed that kaolinite sorbed more humic acids (HAs) in terms of unit surface area (1.03 × 10-3 C, g/m2) compared to bentonite (0.35 × 10-3 C, g/m 10-3 g/m2). Sorption at pH 4.5 was accompanied by HA fractionation in amphiphilicity and chemical composition. HA was sorbed mainly due to hydrophobic components on the surface of all sorbents. HA was not intercalated into the interlayer spaces of montmorillonite during sorption. Sorption due to hydrophilic interactions was mostly performed on muscovite and bentonite than on kaolinite. Sorption was followed by a change in the chemical composition of HA and decreased C/N compared to free HA, indicating selective sorption of nitrogen-containing compounds which is most characteristic of muscovite. Only a relatively thermolabile HA fraction was adsorbed on all minerals while its thermal stability increased compared to that before the experiment. Thermal stability and ratio of the Exo2/Exo1 peak areas on the DSC curves of sorbed HA increased with each subsequent sorption cycle. Thermal stability and resistance to microbial oxidation of sorbed HA showed the following relationship: the higher the thermal stability, the less available is sorbed HA for utilization by microorganisms.

Soil conservation technologies are widely studied for their effects on soil organic carbon (SOC) preservation, yet their impact on the composition of soil organic matter (SOM) remains underinvestigated. This study evaluated the effects of two non-inversion tillage systems, MP and NT, on agro-physical and chemical properties and SOM composition (including water-extractable matter) in Haplic Chernozem Pachic. After 12 years, non-inversion tillage showed no significant differences in SOC, WEOC, and soil structure condition compared to MP. Only NT treatment distinctly enhanced the coefficient of soil structuring (K_str) and mean diameter of water-stable aggregates (MWD_WSA), by 1.5 and 2 times, respectively. Differences in SOM composition were clearly pronounced between treatments in the 0–10 cm layer. Non-inversion tillage favored microbial-derived stable SOM, whereas NT enriched SOM with fresh plant material. Our findings revealed that non-inversion tillage shifts the composition of SOM toward recalcitrant components even more than MP due to limited fresh OM input and enhanced mineralization of unprotected SOM during tillage. This poses carbon loss risks. Periodic moldboard plowing may be a way to improve carbon retention in non-inversion tillage, as it allows plant residues to be incorporated into the soil profile and replenish organic matter.

This paper assesses the thermodynamic stability of clay minerals in the upper organo-mineral horizon of podzolic soil, as well as in the rhizosphere of Norway spruce (Picea abies (L.) H. Karst.) and Norway maple (Acer platanoides L.). Moreover, it determines the impact of soil organic matter on the thermodynamic stability of clay minerals. Calculations of ΔG_f and the saturation index (SI) for clay minerals in laboratory experiments simulating soil conditions without soil moisture outflow allowed us to find out that the thermodynamic stability of clay minerals decreased in the series kaolinite > illite > vermiculite > chlorite. In the rhizosphere of spruce, kaolinite, vermiculite and illite have the lowest, and in the soil under maple-the highest thermodynamic stability, which is associated with differences in the properties of soil organic matter of rhizospheres of different tree species. Laboratory experiments on the sorption of soil humic acid (HA) on clay minerals demonstrated that sorbed HA decreased the thermodynamic stability of biotite and increased the thermodynamic stability of kaolinite and muscovite. Thermodynamic stability of clay minerals decreased with increased proportion of sorbed thermolabile organic matter.