Water-extractable organic matter is the most active and mobile form of soil carbon. The other active fraction and extremely biolabile is the carbon constituent of microorganisms. Both of these fractions play an essential role in agrocenoses and in the global carbon cycle on our planet. The aim of the work was to estimate the carbon content of water-extractable organic matter as well as that of microbial origin in typical chernozems (Protocalcic Chernozems) of different land-use types. Protocalcic Chernozem samples taken from the fields of long-term experiments with different types of land use were investigated: a permanent bare fallow for 55 years (since 1964); conventional tillage – four-field crop rotation, first rotation; direct seeding – crop rotation similar to direct seeding, first rotation; a 21-year fallow (since 1998) after 34 years of bare fallow (since 1964). We determined the carbon content of water-extractable organic matter and the carbon content of microbial origin. In the studied series of variants, the proportion of carbon of water-extractable organic matter in the total amount of organic matter in the upper horizon (0–15 cm) was 0.69, 0.85, 1.01, and 0.98%, respectively, while that of carbon of microbial origin was 0.27, 0.55, 0.53, and 1.52%. It was noted that against the background of increasing the content of total organic carbon in direct seeding variant, compared with the traditional soil treatment, the microbial biomass in this variant does not increase. The fallow variant, unlike all other types of land use, is characterized by a higher proportion of microbial carbon, in contrast to the carbon of water-extractable organic matter.

Since Chernozems are among the most fertile soils in the world, the study of their degradation is of great interest. However, the microstructure and composition of the soil organic matter (SOM) in eroded Chernozems have not yet been sufficiently studied. We studied the SOM and aggregate states of eroded Chernozems using the example of two catenas with arable Haplic Chernozems in the Kursk region of Russia. In the plow horizons (the part of the soil most susceptible to water erosion), we determined the mean-weighted aggregate diameter (MWD), structure and water stability coefficients (SC and WS; dry and wet sieving, respectively), soil organic carbon (SOC) content, and SOM composition and content (qualitative and quantitative micromorphological analyses, respectively). It was shown that with an increase in the degree of erosion, the content of SOC decreased significantly, according to both chemical and micromorphological methods of evaluation. No significant relationships were found between the degree of erosion and the indicators of the structure (except for WS, which was significantly lower in non-eroded Chernozem than in slightly and moderately eroded soils). With the increasing degree of erosion, the humus state of these soils deteriorates at the microlevel, the intensity of humification decreases, the depth of the appearance of assimilated biogenic aggregates with finely dispersed calcite in the profile increases, the structure is destroyed, lumpy aggregates form, and the proportion of planar voids increases. The downslope transport of the soil solid phase under the impact of erosion is accompanied by the accumulation of the transformation products of carbohydrates in the Chernozems in the lower part of the catena. In the Chernozems located in the transit position of the slope, the composition of SOM is characterized by the predominance of lipids and nitrogen-containing compounds. Our unique results contribute to a deeper understanding of the formation of structure and water resistance in eroded soils.

The paper presents research data on profiles of fatty acids in typical chernozems of various use. It is shown that organic matter degradation in typical chernozems is accompanied by depletion of...

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.

The impact of land use and erosion-deposition processes on some physical, chemical, and biological soil properties in a small agricultural catchment in Kursk oblast is analyzed. Plowed Haplic Chernozems and stratozems (Fluvic Chernic Phaeozems (Loamic, Pachic)), as well as unplowed stratozems of a dry valley bottom have been studied. The proportion of large soil aggregates (clods) >10 mm in soils is high on plowed land and low in the dry valley bottom. Differences in the carbon and nitrogen content and in the C/N ratio have not been detected. The analytical pyrolysis has revealed 26 pyrolysates in the composition of soil organic matter (SOM). The relative abundances of pyrrole, pyridine, toluene, and indan among SOM pyrolysates are higher in plowed soils as compared to the soils at the dry valley bottom. Proportions of furfural and methyl furfural among SOM pyrolysates are higher in soils of the dry valley bottom than in plowed soils. Differences in the content of alkanes and phenol are absent. The biomass of microorganisms is mainly formed by fungi (97–99%), and their abundance is greater in soils of the dry valley bottom than in soils of the plowland. Differences in the respiratory activity of the studied soils have not been revealed. The significant influence of erosion-deposition processes and soil cultivation on the spatial heterogeneity of the SOM composition and microbiological parameters is shown. Plowed soils are characterized by the high relative abundance of nitrogen-containing SOM components, while soils at the dry valley bottom have a relatively high abundance of carbohydrate components of mature SOM. The accumulation of fungal biomass and an increase in the structure coefficient in soils of the dry valley bottom indicate the participation of material redeposited from slopes in soil aggregation.

One of the most justified and applied approaches to isolating pools of soil organic matter is fractionation in heavy liquids. The main problem with this approach is rather large losses in the separation of fractions at the stage of washing fractions from heavy liquids. The paper presents a densimetric fractionation protocol that can significantly reduce these losses. It is suggested to use 0.001 M HCl for washing. This approach, in comparison with distilled water, allows reducing losses of weight from 15 to 5% and of carbon from 7.5 to 2.5%. The paper provides a detailed protocol, used by the Laboratory of Soil Biochemistry of V.V. Dokuchaev Soil Science Institute, to isolate four densimetric fractions using sodium polytungstate solutions: free and occluded SOM with a density of 2.0 g/cm³. In the work we used samples of typical chernozems of different land use types. It was shown that the processes of soil restoration and degradation significantly affect the content of light occluded soil organic matter.