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Abstract Soil organic matter (SOM) includes many classes of labile compounds available for microbial decomposition or, conversely, stable compounds protected from biodegradation by biological, chemical, and physical stabilization. It is believed that the more thermal energy is spent on the destruction of soil organic matter, the more stable and more resistant for biodegradation it is. We compared the thermal and biological stabilities of organic matter in eleven soil types from deciduous forest, forest-steppe, steppe, and semidesert bioclimatic areas of the European Russia. According to the activation energy (Ea), the highest SOM thermal stability was typical of the ordinary chernozem and meadow vertic soil. The lowest SOM thermal stability was found for gray forest soil; other soil types were characterized by an intermediate resistance towards thermal oxidation. The thermally labile pool (390–400°C) was on the average 59% (40–68%). The SOM biological stability estimated by the ratio of potentially mineralizable organic matter to that resistant to mineralization (biological stability index) decreased in the following order: ordinary chernozem (Haplic Chernozem (Loamic, Pachic)) > meadow vertic soil (Pellic Vertisol (Gleyic, Humic)) > gray forest soil (Luvic Greyzemic Phaeozem (Loamic)) = meadow chestnut soil (Gleyic Kastanozem (Chromic)) > meadow solonetz (Endosalic Gleyic Solonetz (Loamic, Cutanic)) > alluvial meadow soil (Eutric Fluvisol (Humic, Oxyaquic)). The potentially mineralizable SOM pool in the studied soils was 6–27-fold lower as compared with the pool of thermally labile SOM, and the parameters that characterize SOM thermal stability did not correlate with the biological stability index. Thus, SOM thermal lability is not identical to its biodegradability.

The distribution of total organic matter (Corg), particulate organic matter (CPOM), and potentially mineralizable organic matter (C0) in mega- (10–5 and 5–2 mm), macro- (2–0.25 mm), and microaggregates (

The decomposition and mineralization of various plant residues (oak and aspen leaves, pine needles, small branches and thin roots of trees, aboveground biomass and roots of meadow grasses, aboveground biomass and roots of clover, and straw and roots of barley) were investigated in the laboratory experiments by quantitative measurement of produced C–CO2. The plant residues were mixed with vermiculite or gray forest soil (Greyzemic Phaeozems Albic) and incubated under constant temperature and moisture conditions. After a year of incubation, 25–67% of Corg in plant residues were mineralized. Oak leaves, aboveground mass of meadow grasses, and aboveground mass and roots of clover were characterized by a three-pool structure of organic matter with moderate (0.1 > k1 > 0.01 day–1), slow (0.01 > k2 > 0.001 day–1), and very slow (k3 < 0.001 day–1) mineralization rates, while the other types of plant residues had only a two-pool structure with slow and very slow mineralization rates. An opposite relationship between the decomposition rate and the C : N ratio in the plant residues was found. Poorly decomposable types of plant residues were the main source for particulate organic matter (CPOM) in the soil, while highly decomposable types were the main source for microbial biomass (Cmic). The content of potentially mineralizable organic matter in the soil with plant residues correlated positively with CPOM and with Cmic.

The aim of this study was to develop an approach to assessing the impact of rill erosion on aggregates of typical chernozem aggregates in a laboratory experiment with flume. Aggregates from the plow horizons of noneroded and moderately eroded typical chernozem (Haplic Chernozem (Loamic, Aric, Pachic) and Haplic Chernozem (Loamic, Aric)) of Kursk region (Russia) were analyzed. Micromorphometric parameters of the aggregates of the same sizes untreated by water and transported by water flow were statistically compared. These aggregates were initially air-dry or capillary-wetted. Photos of thin sections were processed using Skyscan CT-analyser program to obtain quantitative micromorphometric parameters: form factor (FF) and roundness (Rdn). Then, the coefficient of aggregate surface unevenness (U) was calculated. As a result, it was found that aggregates from the noneroded soil changed stronger under the influence of a shallow-water flow, if the soil before the experiment was dry. This was confirmed by significant statistical differences in all the studied parameters. The change in parameters of aggregates from the moderately eroded typical chernozem depended on the soil structure rather than on the sample moistening before the experiment. A comparison of aggregates treated with shallow-water flow showed that their characteristics are affected both by the initial state of the soil and by the degree of its erosion. The degree of roundness was the most representative parameter, as the largest number of statistically significant differences was found.

Data on the contents of particulate organic matter (POM) in the humus horizons of different soil types were obtained. The influence of the land use system and fertilizers on the carbon of particulate organic matter (CPOM) in gray forest soil and typical chernozem was shown. The mineralization rate of POM under constant conditions of temperature and moisture was estimated, and the ratios between the pools of POM and biologically active organic matter in the soils were calculated. In the soils of natural cenoses, the percentage of POM together with the sand particle-size fraction varied within 12–36% of the soil mass, and the CPOM content varied from 1.66 to 8.03% of the fraction mass. The portion of CPOM in the total Corg ranged from 20 to 48%. Cultivated gray forest soil and typical chernozem were depleted of CPOM by 3.0–3.2 and 2.0–2.8 times, respectively, in comparison with the samples of noncultivated soils. The mineralization rate of POM in the gray forest soil and typical chernozem was about the same (7.9–12.1 and 7.2–8.2% of CPOM, respectively), which was about 1.2–2.4 times higher than the mineralization rate of the total soil organic matter. It was noted that POM could be a sensitive indicator of the Corg dynamics in the soil, a good predictor of changes in the content and quality of soil organic matter, and a significant source of potentially mineralizable organic matter in the soil.