T. E. Khomutova

Estimation of the Total and Active Microbial Biomasses in Buried Subkurgan Paleosoils of Different Age

Microorganisms that were isolated from steppe soils buried below kurgans from 5800 to 750 years ago were analyzed for the completeness of isolation, total biomass (the sum of glucose-reactivated and resting microbial cells), and active biomass (metabolically active cells). The metabolic state of microbial communities in buried and modern background soils was estimated from the proportion of active and total biomasses. The paleosoils were found to be characterized by lower total and active biomasses and a lower proportion of active microorganisms as compared to the modern background soils. The age-dependent decrease in the content of active microorganisms in the microbial communities of paleosoils was not monotonic. For instance, the 4000-year-old paleosoil was characterized by a high total biomass and a relatively low content of active microorganisms, whereas the 1950-year-old paleosoil was characterized by a relatively low total biomass and a relatively high content of active microorganisms. This could reflect the temporal dynamics of paleoclimatic conditions in the geographic region under study.

Age and activation of microbial communities in soils under burial mounds and in recent surface soils of steppe zone

Chestnut paleosols buried under steppe kurgans about 4800, 4000, and 2000 years ago and their background analogues were studied in the dry steppe zone on the Volga-Don interfluve. Morphological, chemical, microbiological, biochemical, and radiocarbon studies were performed. Paleoclimatic conditions in the region were reconstructed on the basis of paleosol data. The ages of microbial fractions isolated from the buried and surface soils were determined using the method of 14C atomic mass-spectrometry. It reached 2100 years in the A1 horizon of the buried paleosol, which corresponded to the archaeological age of the kurgan (1st century AD). The ages of microbial biomass isolated from the B2 horizons of the buried paleosol and the background surface soil comprised 3680 ± 35 and 3300 ± 30 years, respectively. The obtained data confirmed our assumption about preservation of microorganisms of the past epochs in the paleosols buried under archaeological monuments. It is ensured by various mechanisms of adaptation of soil microbial communities to unfavorable environmental conditions (anabiosis, transformation of bacteria into nanoforms, etc.). The possibility to stimulate germination of the ancient dormant microbial pool isolated from the buried paleosols by 2–3 orders of magnitude with the use of β-indolyl-3-acetic acid as a signal substance was demonstrated.

Microbiological investigations of paleosols of archeological monuments in the steppe zone

Microbiological studies of the paleosols of archeological monuments (burial mounds) of the Neolithic, Bronze, Early Iron, and Middle Ages (the fourth millennium BC to the fourteenth century AD) located in the dry and desert steppe of the Lower Volga River basin were conducted. The microbial communities that existed at the time of creating the burial mounds were shown to be preserved up to the present time. This fact was confirmed by the regularities of the distribution of the microorganisms in the “mound-buried soil” system and by the data on the determination of the age for the microbial fraction of the paleosols using the method of 14C atomic mass spectrometry. The total biomass of the microbial communities in the paleosols amounted to 20–40% of the microbial biomass in their background analogues. In all the paleosols, a special pool of viable microorganisms was present. In the microbial community of the paleosols, the biomass of the active microorganisms corresponded to 0.30–41.0% of the biomass in the present-day soil; the content of mycelium of microscopic fungi composed 43–50% of that in the recent soil. In the mycelium structure in the paleosols, the share of the dark-colored mycelium increased to 98–100%. The microbiological parameters that give a contrasting characterization of the state of the microbial communities in the soils during the arid and humid climatic periods were revealed. The changes of the arid and humid climatic epochs were reflected in the structure of the microbial communities in the paleosols at the ecological-trophic, metabolic, and genetic levels.

The morphology of cells and the biomass of microorganisms in the buried paleosols and modern steppe soils of the Lower Volga region

The morphology of microbial cells was studied, and the biomass of microorganisms was estimated in the modern steppe soils and paleosols buried under kurgans in the Lower Volga region with the methods of electron microscopy. The shape and ultrastructure of the cells in the modern soils and paleosols were similar, though their average volumes differed (0.37 and 0.28 μm3, respectively). The portion of cells with a volume above 1 μm3 in the surface soils and paleosols reached 10.9 and 9.2%, respectively, and the portion of cells with a volume less than 0.01 μm3 in the surface soils was 10% lower than that in the buried paleosols. It was found that the cells of the microorganisms have an external organomineral layer, which increases the cell volume by 4.9 times, and this fact was taken into account in the calculation of the microbial biomass. In the chestnut and light chestnut paleosols, the latter comprised 1500 and 230 μg of C/g soil, respectively.

Paleosol studies of burial mounds in the Ilovlya River valley (the Privolzhskaya Upland)

Paleosols buried under kurgans of the Bronze (end of the fourth and the third millennia BC), Early Iron (1st–3rd centuries AD), and Medieval (13th century AD) epochs have been studied on the Ilovlya River (a tributary of the Don River) terrace. The evolution of chestnut soils in the south of the Privolzhskaya Upland during the last 5000 years has been traced. It is shown that the mean weighted contents and distribution of soluble salts, gypsum, and carbonates in the soil profiles have been subjected to cyclic changes. The total microbial biomass and its trophic structure in the A1, B1, and B2k horizons of paleosols of different ages have been determined. A comparative analysis of the morphological, chemical, and microbiological data on the paleosols of different ages has been used to reconstruct the climatic dynamics for the last 50 centuries. The aridity of the climate in the studied region increased at the end of the third-the beginning of the second millennia BC and in the second and third centuries AD. The humidization of the climate took place in the 1st and in the 12th–13th centuries AD.

The microbial biomass in paleosols buried under kurgans and in recent soils in the steppe zone of the Lower Volga region

The total microbial biomass (TMB) was assessed in the chestnut and light chestnut soils and in the paleosols under burial mounds (steppe kurgans) in the Lower Volga region on the basis of data on the organic carbon content in the extracted microbial fraction supplemented with the data on the extraction completeness as a conversion coefficient. The completeness of the microbial fraction extraction was determined by direct counting of the microbial cells and colony-forming units (on plates with soil agar). The total microbial biomass varied from 400 to 6600 μg of C/soil. Its values in the buried soils were 3–5 times lower than those in the surface soils. The TMB distribution in the buried chestnut soil profile was close to that in its modern analogue (with the minimum in the B1 horizon). In the buried light chestnut paleosols, the TMB values usually increased down the profile; in the recent light chestnut soils, the maximum TMB values were found in the uppermost horizon.

Soil evolution and climate dynamics in dry steppes of the Privolzhskaya Upland during the last 3500 years

The investigation of paleosols of different ages buried under archaeological monuments of the Bronze (16th–15th centuries BC), Early Iron (2nd–3rd centuries AD), and Medieval (14th century AD) ages demonstrated that the evolution of chestnut soils and solonetzes in the dry steppes of the southern part of the Privolzhskaya Upland during the past 3500 years manifested itself at the level of the genus and species characteristics of the soils, such as the degree of the solonetzicity, the humus content, and the content and composition of the soluble salts. The revealed regularities of the variations of the morphological, chemical, and microbiological soil properties in time allowed reconstructing the secular dynamics of the climatic humidity in the region. It was found that the humidization of the climate with a decrease in the degree of the climatic continentality took place in the middle of the second millennium BC. The Late Sarmatian Time (2nd–3rd centuries AD) was characterized by arid climatic conditions, whereas the Golden Horde time (13th–14th centuries AD) was marked by the general humidization of the climate. A gradual aridization of the climate began in the second half of the 14th century and continued in the 15th century AD.

Assessment of the living and total biomass of microbial communities in the background chestnut soil and in the paleosols under burial mounds

The contents of phospholipids and carbon of the total microbial biomass were determined in the modern chestnut soil and in the paleosols buried under mounds of the Bronze and Early Iron Ages (5000–1800 years ago) in the dry steppe of the Lower Volga River basin. Judging from data on the ratio between the contents of phospholipids and organic carbon in the microbial cells, the carbon content of the living microbial biomass was calculated and compared with the total microbial biomass and total organic carbon in the studied soils. In the background chestnut soil, the content of phospholipids in the A1, B1, and B2 horizons amounted to 452, 205, and 189 nmol/g, respectively; in the paleosols, it was 28–130% of the present-day level. The maximum content was measured in the paleosols buried 5000 and 2000 years ago, in the periods with an increased humidity of the climate. In the background chestnut soil, the total microbial biomass was estimated at 5680 (the A1 horizon), 3380 (B1), and 4250 (B2) μg C/g; in the paleosols, it was by 2.5–7.0 times lower. In the upper horizons of the background soil, the portion of the living microbial biomass in the total biomass was much less than that in the paleosols under the burial mounds; it varied within 8.5–15.3% and 15–81%, respectively. The portion of living microbial biomass in the total organic carbon content of the background chestnut soil was about 4–8%. In the paleosols buried in the Early Iron Age (2000 and 1800 years ago), this value did not exceed 3–8%; in the paleosols of the Bronze Age (5000–4000 years ago), it reached 40% of the total organic carbon.

Characteristics of microbial communities in steppe paleosols buried under kurgans of the Sarmatian time (I–IV centuries AD)

Microbiological studies of paleosols buried under steppe kurgans of different ages of the Middle (I–II centuries AD) and Late Sarmatian (II–IV centuries AD) time in different regions of the Lower Volga steppes were carried out. The regularities of the soil microbial communities’ development were determined in the I–IV centuries AD by the climate dynamics and the replacement of the relatively humid conditions (the I century to the first half of the II century) by dry (the second half of the II century to the first half of the III century) and then again by humid (the end of the III century to the IV century) conditions. In the humid climatic periods, the active biomass of the microorganisms and its portion in the total microbial biomass and the Corg of the soil increased, the portion of microorganisms consuming plant residues increased in the ecological-trophic structure of the microbial community, and the index of oligotrophy decreased. These changes had an opposite direction in the arid climatic periods. The variations of the microbiological parameters relative to the century-long dynamics of the climate over the historical time were synchronous and unidirectional, though the studied soils were found in different soil-geographical zones (dry and desert steppe), natural regions (the Privolzhskaya and Ergeni uplands and the Caspian Lowland), and landforms (watersheds, river terraces, marine plains).

Phosphatase activity in the surface and buried chestnut soils of the Volga-Don interfluve

The phosphatase activity (PA) was studied in the chestnut paleosols buried in 1718–1720 under the Anna Ivanovna rampart in the southern part of the Privolzhskaya Upland and in the middle of the third millennium BC under the burial mound of the Bronze Age on the Northern Yergeni Upland; the background analogues of these soils were also examined. The PA values in the fresh soil samples varied from 2.5 to 37 mg of P2O5/10 g of soil per h with maximums in the A1 horizon of the surface soils and in the B1 horizon of the paleosols. The PA values depended on the time of storage of the samples: with time, they increased by 2.6–2.9 times in the A1 horizon of the background surface soil and decreased by 20–60% in the other soil samples. The specific distribution patterns of the PA values in the soil profiles remained the same independently of the time of storage of the samples. Relatively small amounts of the soil samples were sufficient for the reliable determination of the PA: 1–2 g for the A1 horizon and 3–5 g for the B1 and B2 horizons. The time of incubation with the substrate had to be increased up to 4 h for the long-stored samples.