The conversion of land use is a crucial factor in the dynamics of soil carbon stocks. The widespread abandonment of cropland that began in Russia during the 1990s resulted in vegetation and soil restoration. This led to changes in soil properties and carbon fluxes within the plant-soil-atmosphere system. The aim of the study was to investigate the effects of post-agricultural soil restoration on the rate of changes in soil properties, specifically organic carbon (Corg) and total nitrogen (Ntotal) contents, microbial respiration rate, and activity of hydrolytic enzymes. A post-agricultural chronosequence formed on Haplic Luvisols, including current cropland, 7- and 25-year-old postagrogenic abandoned land, and mesophytic grassland, was studied. The rate of changes in soil properties during post-agricultural restoration was evaluated based on their sensitivity to land use changes, with a comparison of changes in soil organic carbon content. The least sensitive properties to land use change were found to be Corg and Ntotal in the mineral-associated organic matter fraction, and pH values. The content of water-soluble nitrogen, Corg and Ntotal in the free and occluded organic matter fractions, as well as β-glucosidase and chitinase activity, were the most sensitive to land use changes. Consequently, the recovery of sensitive soil properties in the upper 10 cm is complete within the first decade after tillage is stopped. In contrast, the restoration of less sensitive properties requires more than 20 years.

Thalassosols developing on the accumulative coasts of the East Siberian Sea include initial soils of regularly flooded tidal flats with sparse vegetation, episodically flooded marsh soils with different degrees of salinization, and maritime soils that are morphologically close to the zonal soils but are affected by salts transferred by wind with seawater drops and organomineral matter from the non-vegetated seashores. Weakly developed marsh soils have the initial features and structure of the marine sediments combined with the processes of salinization; sulfate reduction; gleyzation; cryogenic mass exchange; as well as the transfer, accumulation, and weak biochemical transformation of the raw organic matter. Soils with different degrees of salinization are formed on tidal marshes and are characterized by some redistribution of salts in the soil profile with weak accumulation of salts in the uppermost organic horizons and in the suprapermafrost layers along with pronounced sulfate reduction. Slightly saline organogenic and peaty gleyic soils predominate in the areas of sedge marshes. The majority of studied marsh soils form under the conditions of distinct cryogenic polygonal microtopography, waterlogging, and relatively shallow permafrost.

The cryogenic mass-exchange processes affect the content and distribution of organic matter in Cryosol profiles enriching the mineral horizons with organic matter. It has been shown that the mineralization capacity of organic materials in Cryosols is low even under optimum conditions of temperature and moisture. Despite the significant variation in the microbial biomass content, the general pattern of its distribution in the profile with a maximum in organic horizons and a minimum in mineral horizons is preserved in all studied profiles. The fraction of the microbial biomass carbon (C mb ) in the total organic carbon is less than 1%. The microbial respiration quotient ( Q r ) varies from <0.1 to 0.3. The most significant influence on the microbial biomass and changes in its respiration activity in the profiles of Cryosols is recorded for the contents of total organic carbon (TOC) and total nitrogen (TN) and for the soil porosity.

The amount of active (potentially mineralizable) organic carbon (C 0 ) in the 1-m-deep layer of typical chernozem, dark-gray forest soil, and gray forest soil was estimated for virgin plots and arable land. It was shown that C 0 is mainly found in the topsoil (0–20 cm), where its pool reaches 32–60% of the total amount of C 0 in the layer of 0–100 cm. The C 0 content and its portion in the total organic carbon decrease down the soil profiles. The disturbance of the structure of the pool of active organic carbon—the loss of the moderately mineralizable (0.1 > k 2 > 0.1 day –1 ) fraction—takes place in the upper horizon of plowed soils. The total pool of C 0 in the upper meter of typical chernozem under cropland and under meadow-steppe cenosis comprises 2.8 and 5.2 t/ha, respectively; for the dark gray forest soil under cropland and forest, it reaches 5.5 and 9.8 t/ha, respectively; and for the gray forest soil under cropland and forest, 2.4 and 3.4 t/ha, respectively. The pools of C 0 in the typical chernozem. dark gray forest, and gray forest soils are comparable with the values of the annual C–CO 2 emission from the soils of these zones.

In the humus horizon of soddy-podzolic soils of postagrogenic cenoses and primary forests, the contributions of the fungi and bacteria were determined by the selective inhibition of the substrate-induced respiration (SIR) by antibiotics; the basal (microbial) respiration and the net-produced nitrous oxide (N 2 O) were also determined. The procedure of the SIR separation using antibiotics (cycloheximide and streptomycin) into the fungal and bacterial components was optimized. It was shown that the fungi: bacteria ratio was 1.58, 2.04, 1.55, 1.39, 2.09, and 1.86 for the cropland, fallow, and different-aged forests (20, 45, 90, and 450 years), respectively. The fungal and bacterial production of CO 2 in the primary forest soil was higher than in the cropland by 6.3 and 11.4 times, respectively. The production of N 2 O in the soils of the primary and secondary (90-year-old) forests (3 and 7 ng N-N 2 O/g soil per hour, respectively) was 2–13 times lower than in the postagrogenic cenoses, where low values were also found for the microbial biomass carbon (C mic ), its components (the C mic-bacteria and C mic-fungi ), and the portion of C mic in the organic carbon of the soil. A conclusion was drawn about the misbalance of the microbial processes in the overgrown cropland accompanied by the increased production of N 2 O by the soil during its enrichment with an organic substrate (glucose).

The mineralization rate of the organic matter (OM) in the aggregate fractions of a gray forest soil separated by repeated sieving through sieves with different mesh sizes was assessed. The samples of the soil aggregate fractions were incubated for 141 days under constant temperature and moisture, and the C-CO 2 emission rate was measured. The mineralizable OM pool in the aggregates of <0.25, 1–0.25, and 3–1 mm in size from the soil under a forest contained easily (C 1 , k 1 > 0.1 days −1 ), moderately (C 2 , k 2 > 0.01 days −1 ), and difficultly (C 3 , k 3 > 0.001 days −1 ) mineralized compounds; the C 1 and C 2 components were present in the coarser aggregates. In the arable soil, the C 1 , C 2 , and C 3 OM components were separated in the aggregates of <0.25 and 1–0.25 mm; the C 1 and C 3 were separated in the aggregates of 3–1 and 5–3 mm; and the C 1 and C 2 were separated in the coarsest (10–5 mm) aggregates. The highest content of potentially mineralized OM (C 0 ) occurred in the aggregates of 1–0.25 and 3–1 mm, but the size of the mineralizable OM pool was more dependent on the portion of the aggregate fraction in the soil than on the absolute C 0 content in the fraction. It was shown that the decrease in the share of coarse structural aggregates is accompanied by a depletion of potentially mineralized OM in the arable soil, and the formation of coarse aggregates is an important condition of the soil carbon sequestration.

In three laboratory experiments with gray forest soils, the rates of mineralization of the bacterial mass, green oat mass, and cellulose applied in increasing doses were determined based on the measurement of the C-CO2 emission. The substances applied were used as biological indicators of the mineralizable organic matter pool in the gray forest soils.