B2 Horizon Research Articles

Morphological characteristics, humus and nutrient content of recent and buried soils of the Inozemtsevo mound

The main objective of the work was to identify the evolutionary changes in the profile of recent and buried soil in the Holocene, the content of humus and the composition of mobile forms of nutrients in the solonetzol- silted black soil in the insufficiently wetted zone of Stavropol Krai. The research was carried out near the village of Inozemtsevo, Stavropol Krai. The soils of the study area are formed on marine sediments. A 1.5-metre-high grave was erected more than 5,000 years ago. At a distance of 150 metres, an incision was made on recent virgin soils. According to morphological characteristics, recent soils are medium saline fused medium powered black soils, while the buried soils evolved into weakly saline fused low powered black soils. They are distinguished by lower profile thickness and brown, rather than black, colouring. There is a BC horizon in recent soils, which is absent in paleosoils. New formation of carbonates and gypsum is noted only in recent soils in B2 and BC horizons. The organic matter content in recent soils is 5-6% and decreases 6-7 times in paleosoils in the upper horizons. Recent soils are depleted in mobile phosphorus, but enriched in mobile potassium. Differences in the content of mobile sulphur are due to some salinity of the paleosoils. In the content of mobile trace elements, the differences between the studied soils are significant, especially zinc, cobalt and molybdenum. Consequently, during the evolutionary period of 5 thousand years, the soil-absorbing complex was impoverished by these microelements.

Dark CO2 fixation in temperate beech and pine forest soils

Soils are the largest terrestrial organic carbon pool and the largest terrestrial source of atmospheric CO2. Non-phototrophic CO2 fixation by microbes re-fixes and recycles CO2 respired in soils. Our previous study showed that in temperate deciduous forest soil profiles, rates of dark CO2 fixation were proportional to microbial biomass, irrespective of soil depth. However, the amount and quality of organic matter entering different soil depths vary for different temperate forest types and these influence microbial communities with unknown consequences for CO2 fixation rates. To test whether differences in the amount and quality of SOC caused by tree species affect dark CO2 fixation rates with depth, we conducted a study using acidic soils from two forest plots from the Schorfheide-Chorin Exploratory, Germany. These soils, dominated by either beech (deciduous) or pine (coniferous) tree stands differ in their SOC content and quality. We traced the incorporation of 2% (v:v) 13C–CO2 label into microbial biomass and estimated the CO2 fixation rates relative to microbial biomass carbon content (in μg C g MBC−1 d−1) across the soil profiles. The rates of dark CO2 fixation per g MBC were similar across the beech soil profiles, but significantly lower in the pine soils at the B2 and BC horizons, suggesting that while dark CO2 fixation rates are linked to MBC in deciduous forest soils, other factors influence dark CO2 fixation rates in coniferous forest soils. The pine subsoils had low SOC content and quality, with a microbial community enriched with heterotrophic fermenters like Chloroflexi that are predicted to have a lower potential for heterotrophic CO2 fixation compared to the other dominant bacteria phyla. In contrast, the beech soil profiles, characterized by higher SOC inputs, featured higher fractions of copiotrophs like Proteobacteria, Acidobacteria, and Actinobacteria predicted to have high heterotrophic CO2 fixation potential. We thus speculate that in contrast to the beech soils, lower SOC inputs in the pine subsoils affected the community composition, leading to lower CO2 fixation rates. We further made comparisons to soils dominated by mixed deciduous trees and featuring a higher MBC and SOC content. Over this range of temperate forest soils, CO2 fixation rates were highest in the mixed deciduous forest soils, with MBC and Shannon index (used as a proxy parameter for community composition) showing the strongest correlations with the varying CO2 fixation rates. Our study suggests that predictions of dark CO2 fixation rates need to consider tree-species specific or site-specific conditions, as these may alter root carbon inputs and affect microbial community composition and their metabolic CO2 fixation potentials in soil.

Reservoir Distribution Analysis with Seismic Colored Inversion in Fluvial Deposition Environment West Natuna Basin

West Natuna Basin is a fluvial deposition environment with thin-bed sand reservoir overlapping with shale and shaly sand. Inversion is applied using Seismic Colored Inversion which considered to provide with robust result to identify geological features such as sand channels’ continuity with low-acoustic impedance value alongside zone of interest. Seismic Colored Inversion can be made by inverting seismic data into Acoustic Impedance value which can be obtained by deriving data from well, as control, then utilized it to change the seismic data volume to Acoustic Impedance volume. By performing a lateral slicing along the target horizons, the distribution of sand channels can be identified. Therefore, the promising reservoir zone with low Acoustic Impedance value and the non-reservoir zone can be clearly distinguished. Slicing was performed in A1 horizon, showing low AI sand channels ranging from 12.000 up to 15.500 ((ft/s)*(g/cc)) spreading at the center of the map through the East. Slicing in B2 horizon, showing what appears to be sand channels with low AI value ranging from 16.000 up to 17.800 ((ft/s)*(g/cc)) spreading at the center of the map between the U1 and U3 well, and appears at the Northeast of the map.

Quantitative Analysis of Pore Space Structure in Dry and Wet Soil by Integral Geometry Methods

We present a methodology for a numerical analysis of three-dimensional tomographic images in this paper. The methodology is based on integral geometry, topology, and morphological analysis methods. It involves calculating cumulative and non-cumulative pore size distributions of Minkowski functionals and Betti numbers. We investigated 13 samples in dry and wet (saturated beyond the field capacity) conditions within different horizons of the Phaeozem albic. For samples of the arable horizon, an increase in the Euler characteristic was observed in the process of wetting. For samples from the A2, AB and B2 horizons, the Euler-Poincare characteristic decreased during wetting. It has been proven that both Betti numbers (number of isolated pores and number of “tunnels”) decrease with swelling of the AB and B2 horizons at a depth of 20–90 cm. For samples from the arable horizon, another dependence was observed: A Betti number of zero increased first but decreased during wetting. Based on the change in topological characteristics, two methods of changing the topology of the void space of the soil were demonstrated. The above-described quantitative changes of proposed parameters of pore space tomographic images prove the possibility and progressiveness of their usage for the pore space transformation estimate.

Temporal and Spatial Features of Phosphatase Activity in Natural and Human-Transformed Soils

Data on phosphatase activity (PA) in soils of different natural zones, intrazonal soils, buried soils, and cultural layers in archaeological monuments of different age were summarized. The variability of PA in soil complexes was analyzed. The maximum values of PA were recorded in the upper horizon of Eutric Retisols, and minimum values were found in Haplic Calcisols of the Baer hills in the Caspian Lowland. In Solonetzes, a wider variation of PA in the A1 horizon was recorded as compared to that in Cambisols and Calcisols. Mean value of PA in the upper horizon of Solonetz was 1.5 times higher than in the upper horizon of Cambisols and Calcisols. The maximum values of PA in the profile of Solonetz were determined in the uppermost 2-cm-thick layer. The B1 horizon of Cambisols and Calcisols had a higher PA in comparison with that in the A1 horizon of these soils, whereas the B1 horizon of Solonetz had a lower PA than that in the A1 horizon. In the paleosols buried under kurgans of different age, PA was higher in paleosols buried in humid epochs than in paleosols buried in arid epochs. Cultural layers of ancient settlements had the maximum variability of PA values. In some cases, in the cultural layers with low organic matter content, the values of PA correlated with the content of phosphates. In the cultural layers with ash and charcoals, the low values of PA were recorded at high phosphate content.

Application of handheld X-ray fluorescence spectrometer for major element analysis and characterization of geological samples in Southern Thailand

The handheld X-ray fluorescence spectrometer (hXRF) was applied to major element analysis in the geological samples and the results were compared with the results of the wavelength dispersive X-ray fluorescence spectrometer (WDXRF). In addition, the analytical compositions were confirmed by X-ray diffractometer (XRD) and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX). In the case of pXRF analysis, eight geological reference materials were used to calibrate the analytical method and four known concentration samples were used to prove the method. In research, ten geological samples from Phuket province, Thailand were collected with different depths corresponding to O horizon (0-0.1 m), A horizon (0.1-0.3 m), B1 horizon (0.3-1.0 m), B2 horizon (1.0-2.0 m), C1 horizon (2.0-3.0 m), C2A horizon (3.0-5.0 m), C2B horizon (5.0-8.0 m), C3 horizon (8.0-12.0 m), D horizon (12.0-20.0 m) and RK horizon (> 20.0 m). The elemental analysis results showed that the pXRF results were matched well with that WDXRF results. The concentration range in ten samples were Si (18-22 wt%), Al (12-15 wt%), Fe (1-4 wt%), K (0.4-3.2 wt%) and Ti (0.2-0.4 wt%). The concentration of some elements such as Ca and Mg on those ten samples are low and they are unable to analysed by hXRF. The XRD patterns revealed that all samples consisted of quartz, kaolinite, halloysite, illite, vermiculite and biotite.

Role of unsaturated soil above a heavily contaminated aquifer in the natural attenuation of arsenic

Large attenuation of arsenic is observed in a shallow aquifer in central Mexico, where the concentration decreases from 91.5 to 11.3 mg/L, over 1.3 km horizontal distance. To investigate possible mechanisms of attenuation of this pollutant, we dug a pit between the surface and the saturated zone. We then described the soil profile and determined arsenic and iron concentrations in soil samples as a function of depth. Next, we determined particle size distribution, bulk density, particle density, soil moisture and porosity of the soil material. We also analyzed arsenic concentration in the groundwater. The 2.2 m deep profile intersected 4 soil horizons (A, AB, B1 and B2). We found arsenic accumulation in the B2 horizon, directly overlying the saturated zone. These accumulations coincide with the thickness of the capillary fringe, revealing that arsenic is drawn up in the soil profile by capillary rise of arsenic-contaminated groundwater. Furthermore, arsenic accumulation showed a direct relationship with iron, likely due to arsenic adsorption on iron oxides. Results from this study contribute to understanding a capillary-driven natural attenuation mechanism that removes contaminants from groundwater by sinking them in the capillary fringe.

Drip irrigation wetting patterns and nitrate distribution: comparison between electrical resistivity (ERI), dye tracer, and 2D soil–water modelling approaches

Despite the widespread use of drip irrigation and fertigation, there is limited research on soil wetting patterns and nutrient distribution from drip emitters in agricultural soils. We compared the use of electrical resistivity imaging (ERI) with dye tracer patterns, measured soil nitrate distribution, and modelled changes in soil moisture and nitrate distribution, in a commercial apple orchard in southern Tasmania. Time-lapse two-dimensional ERI revealed wetting plumes beneath the emitters ranged from nil infiltration, to infiltration beyond 1.0 m depth. Lack of infiltration beneath some emitters was attributed to runoff caused by either water repellence or the surface leaf mulch, whilst infiltration to 1.0 m depth was attributed to preferential flow processes. The uncalibrated ERT was unable to discern the separate contributions from of solute concentration, moisture content and temperature to the change in electrical resistivity. The dye tracer study revealed the majority of the fertigated nitrate was retained in the A1 horizon, whilst the fertigated water infiltrated much deeper into the A2 and B2 horizons. The 2D modelling was not able to replicate the variations in infiltration demonstrated by the ERT, principally due to the lumped nature of the soil parameterisation, and inability of the model to simulate preferential flow. As such, use of modelling tools and texture-based guidelines for design and management of drip emitters is not recommended for the texture contrast soils.

State of microbial communities in paleosols buried under kurgans of the desert-steppe zone in the Middle Bronze Age (27th–26th centuries BC) in relation to the dynamics of climate humidity

The size and structure of microbial pool in light chestnut paleosols and paleosolonetz buried under kurgans of the Middle Bronze Age 4600–4500 years ago (the burial mound heights are 45–173 cm), as well as in recent analogues in the desert-steppe zone (Western Ergeni, Salo-Manych Ridge), have been studied. In paleosol profiles, the living microbial biomass estimated from the content of phospholipids varies from 35 to 258% of the present-day value; the active biomass (responsive to glucose addition) in paleosols is 1‒3 orders of magnitude lower than in recent analogues. The content of soil phospholipids is recalculated to that of microbial carbon, and its share in the total soil organic carbon is determined: it is 4.5–7.0% in recent soils and up to three times higher in the remained organic carbon of paleosols. The stability of microbial communities in the B1 horizon of paleosols is 1.3–2.2 times higher than in the upper horizon; in recent soils, it has a tendency to a decrease. The share of microorganisms feeding on plant residues in the ecological–trophic structure of paleosol microbial communities is higher by 23–35% and their index of oligotrophy is 3–5 times lower than in recent analogues. The size of microbial pool and its structure indicate a significantly higher input of plant residues into soils 4600–4500 years ago than in the recent time, which is related to the increase in atmospheric humidity in the studied zone. However, the occurrence depths of salt accumulations in profiles of the studied soils contradict this supposition. A short-term trend of increase in climate humidity is supposed, as indicated by microbial parameters (the most sensitive soil characteristics) or changes in the annual variation of precipitation (its increase in the warm season) during the construction of the mounds under study.

Timescales of carbon turnover in soils with mixed crystalline mineralogies

Abstract. Organic matter–mineral associations stabilize much of the carbon (C) stored globally in soils. Metastable short-range-order (SRO) minerals such as allophane and ferrihydrite provide one mechanism for long-term stabilization of organic matter in young soil. However, in soils with few SRO minerals and a predominance of crystalline aluminosilicate or Fe (and Al) oxyhydroxide, C turnover should be governed by chemisorption with those minerals. Here, we correlate mineral composition from soils containing small amounts of SRO minerals with mean turnover time (TT) of C estimated from radiocarbon (14C) in bulk soil, free light fraction and mineral-associated organic matter. We varied the mineral amount and composition by sampling ancient soils formed on different lithologies in arid to subhumid climates in Kruger National Park (KNP), South Africa. Mineral contents in bulk soils were assessed using chemical extractions to quantify Fe oxyhydroxides and SRO minerals. Because of our interest in the role of silicate clay mineralogy, particularly smectite (2 : 1) and kaolinite (1 : 1), we separately quantified the mineralogy of the clay-sized fraction using X-ray diffraction (XRD) and measured 14C on the same fraction. Density separation demonstrated that mineral associated C accounted for 40–70 % of bulk soil organic C in A and B1 horizons for granite, nephelinite and arid-zone gabbro soils, and > 80 % in other soils. Organic matter strongly associated with the isolated clay-sized fraction represented only 9–47 % of the bulk soil C. The mean TT of C strongly associated with the clay-sized fraction increased with the amount of smectite (2 : 1 clays); in samples with > 40 % smectite it averaged 1020 ± 460 years. The C not strongly associated with clay-sized minerals, including a combination of low-density C, the C associated with minerals of sizes between 2 µm and 2 cm (including Fe oxyhydroxides as coatings), and C removed from clay-sized material by 2 % hydrogen peroxide had TTs averaging 190 ± 190 years in surface horizons. Summed over the bulk soil profile, we found that smectite content correlated with the mean TT of bulk soil C across varied lithologies. The SRO mineral content in KNP soils was generally very low, except for the soils developed on gabbros under more humid climate that also had very high Fe and C contents with a surprisingly short, mean C TTs. In younger landscapes, SRO minerals are metastable and sequester C for long timescales. We hypothesize that in the KNP, SRO minerals represent a transient stage of mineral evolution and therefore lock up C for a shorter time. Overall, we found crystalline Fe-oxyhydroxides (determined as the difference between Fe in dithionate citrate and oxalate extractions) to be the strongest predictor for soil C content, while the mean TT of soil C was best predicted from the amount of smectite, which was also related to more easily measured bulk properties such as cation exchange capacity or pH. Combined with previous research on C turnover times in 2 : 1 vs. 1 : 1 clays, our results hold promise for predicting C inventory and persistence based on intrinsic timescales of specific carbon–mineral interactions.

Soil with high organic carbon concentration continues to sequester carbon with increasing carbon inputs

Identifying soil with a large potential to accumulate organic carbon (OC) could maximise the mitigation benefits of carbon (C) sequestration and help prioritise resources to achieve increases in soil OC. The purpose of this laboratory incubation experiment was to determine if an upper limit to OC accumulation in soil was approached with increasing C input in basalt- and granite-derived soil. For each parent material, two soil layers were compared to observe OC accumulation in soil with a high OC concentration (0 to 0.10m, A1 horizon) and soil with a low OC concentration (0.40 to 0.50m, B2 horizon). Soil samples were incubated for up to 146days. The experiment consisted of three soil incubation cycles, with four treatments applied at the start of each cycle: soil only (control), soil and nutrients only (nutrients), high organic matter (OM) and nutrients (approximating a field equivalent of 12.4MgDM/ha; HOMN) and very high OM and nutrients (31.1MgDM/ha; VHOMN). At the beginning of cycle one 13C labelled OM was applied. There was no asymptotic behaviour between C inputs and OC accumulation in soil observed in this study. Thus, OC accumulation was not approaching an upper limit for either parent material at OM application rates ranging from field equivalents of 12.4 to 93.3MgDM/ha (equivalent to 5.4 to 40.6MgC/ha). There was no significant increase in OC concentration between cycle 2 and 3 for the VHOMN treatment in the granite-derived 0.40 to 0.50m soil. While this is not conclusive, this may indicate the soil is approaching an upper limit to OC accumulation at a lower OC concentration due to the dominance of 1:1 clays, compared to the 2:1 clay dominated basalt-derived soil. This suggests that mineralogy rather than texture may influence OC accumulation and any potential C saturation behaviour of soil. Despite increasing microbial activity, evidenced by increasing soil respiration (P<0.001) and microbial biomass C (P<0.05), as well as a significant (P<0.05) narrowing of the C:N ratio of soil, there was substantial 13C recovery (mean between 19.8 and 25.9 (1.1 se) % for both parent material) at the end of the soil incubation. This supports the hypothesis that the increases in OC accumulation were at least partly due to the conversion of plant residues into microbial detritus which is a major component of the relatively stable pool of OC in soil.

Thermal properties of typical chernozems in Kursk Oblast

Thermal diffusivity and heat capacity of virgin and plowed heavy loamy typical chernozems of Kursk oblast were studied. Thermal diffusivity was determined in the course of step-by-step drying of the initially capillary-saturated samples to the air-dry state. Specific heat capacity was determined for absolutely dry samples. Volumetric heat capacity was calculated according to the de Vries equation. Thermal diffusivity varied within the ranges of (1.15–3.46) × 10–7 m2/s in the Ap horizon, (1.14–3.35) × 10–7 m2/s in the А1 horizon, (1.49–3.70) × 10–7 m2/s in the АВ horizon, (1.49–3.91) × 10–7 m2/s in the В1 horizon, and (1.60–3.80) × 10–7 m2/s in the Вса horizon. The thermal diffusivity vs. water content dependencies had distinct maximums and were flattened in the range of low water contents. The maximums were most pronounced for the mineral B1 and Bca horizons; for the A1 and Ap horizons, the curves were rather S-shaped. Volumetric heat capacity of the air-dry soils varied from 0.96 J/(cm3 K) in the A1 horizon to 1.31 J/(cm3 K) in the Bca horizon; in the state of capillary saturation, it varied from 2.79 J/(cm3 K) in the А1 horizon to 3.28 J/(cm3 K) in the Вса horizon. Thermal properties of topsoil horizons were higher in the plowed chernozem compared with the virgin chernozem, which is explained by an increase in the bulk density and a decrease in the organic matter content in the plowed soil.

Population density and total biomass of microbial communities in chestnut soils and solonetzes of the dry steppe zone in the Lower Volga region

The population density and total biomass of microbial communities were determined in chestnut soils and solonetzes of the dry steppe zone in the Lower Volga region with the use of the methods of sequential fractionation of the soil and direct counting. The mean weighted values of the population density of the microbial communities in the soil profiles (A1 + B1 + B2 horizons) in the studied soils varied within 3.8–8.0 × 1011 cells/g of soil. The total microbial biomass in the soils of the Privolzhskaya Upland reached 0.9–2.4 mg C/g of soil; in the soils of the Ergeni Upland, it was 20 to 75% lower. The microbial cells in the soils of the Privolzhskaya Upland were larger than those in the soils of the Ergeni Upland. Sequential fractionation of the soil prior to direct counting contributed to the more complete assessment of the population density of the microbial communities.

Hydropedology and Preferential Flow in the Tasmanian Texture‐Contrast Soils

The two‐way interaction between soil morphology and the processes governing soil water movement were investigated for a range of texture‐contrast soil profiles. The texture‐contrast soils consisted of a seasonally water‐repellent sandy loam A1 horizon over a bleached silica‐cemented A2e horizon and a mottled vertic clay subsoil. Differences in soil morphology and structure among sites had little influence on the proportion of soil that participated in infiltration or the maximum depth of infiltration; however, differences in subsoil structure influenced the processes by which water infiltrated and was stored within the B2 horizons. The occurrence of preferential flow was largely controlled by the effects of antecedent soil moisture content on water repellence in the A1 horizon, silica bridging in the A2e horizon, and clay shrinkage in the B2 horizons. Under dry soil conditions, infiltration resulted from up to five different forms of preferential flow. When soils were near field capacity, most forms of preferential flow ceased; however, wetting front instability and lateral flow developed in the A1 horizon. Preferential flows are not thought to have contributed to the pedogenesis of the texture‐contrast soils. Development of the contrasting soil texture horizons, sand infills, and bleached A2e horizons developed before and independently of the observed preferential flow processes in which reworked aeolian sands buried previously developed clay columns.

Characterization of the microbial communities in the modern and buried under kurgans soils of solonetzic complexes in the dry steppes of the Lower Volga region

The microbial communities were studied in the modern and buried under kurgans (1st century AD) soils of solonetzic complexes on the dry steppes of the northern part of the Yergeni Upland. It was found that the changes in the numbers of microorganisms from different trophic groups and in the biomass of the fungal mycelium along the profiles of the modern and buried solonetzic chestnut soils and solonetzes do not differ significantly. The quantitative estimate of the impact of the solonetzic process on the spatial variability of the microbiological parameters of the soils was given on the basis of the ANOVA. As a rule, the values of the microbiological parameters in all the horizons of the modern and buried chestnut soils were 1.2–2.8 times higher than those in the modern and buried solonetzes. The influence of the degree of solonetzicity of the buried paleosols on the microbiological parameters manifested itself in the entire profile, though in each particular horizon it was only seen in the numbers of some particular trophic groups of microorganisms. The comparison between the mean weighted values of the microbiological parameters in the entire soil profiles (the A1 + B1 + B2 horizons) demonstrated an inverse relationship between the population density of the microorganisms utilizing easily available organic matter and the degree of solonetzicity of the buried paleosols. The maximum biomass of the fungal mycelium was found in the solonetzic chestnut paleosol; it exceeded the biomass of the fungal mycelium in the other paleosols (which did not differ significantly in that parameter from one another) by 1.5–1.6 times.

Biological activity of modern and buried dry-steppe chestnut soils

A comparative assessment of the biological activity of modern chestnut soils and chestnut soils buried beneath the Caucasian line set by Anna Ioannovna (1718–1720) on the dry steppes of the Volga Upland was carried out based on determination of enzymatic activity and the state of microbial communities. It was found that, in the modern soil, maximal values of microbial parameters and phosphatase activity, but a minimal value of the humification coefficient were observed in the A1 horizon, whereas in paleosoil, these values were found in the B1 horizon. This fact is consistent with the data on more humid and cold climate during the construction of the historical monument.

Aggregate formation in salt-affected soils of the Baer mounds

Particle-size and microaggregate-size distributions and the morphology of aggregates <1 mm have been studied in soils of the Baer mounds. It is shown that the soil aggregates contain specific clayey-salt microaggregates and coatings. The type of the clayey-salt pedofeatures (microaggregates and/or coatings) depends on the clay content in the soils, and the size and shape of the clayey-salt microaggregates depends on the concentration and chemistry of salts participating in their formation. The clayey material of the micro-aggregates is mainly represented by smectitic minerals. Calcareous (calcitic, dolomitic, and iron-bearing) clayey-salt microaggregates have been found in all the horizons of investigated soils, except for the Ap horizon of the soil of the former rice paddy in the hollow between the Baer mounds. Clayey-salt microaggregates with simultaneous participation of several different salts (chlorides and sulfates of Na, Mg, and Ca) have been detected in the solonchakous soil at the foot of the Baer mound. For the first time, quasicrystalline formations—pentagonal NaCl crystals of up to 3 μm in size with participation of phyllosilicates and sulfates of Na and Mg—have been described under native conditions in the B2 horizon of the solonchak. It is argued that the formation of clayey-salt aggregates contributes to the good microaggregation of the soils and to the stability of the Baer mounds under conditions of the arid climate.

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.

Equilibrium composition and properties of soil solutions

The high dynamics of their composition are a characteristic feature of soil solutions, which hampers the use of data on the concentrations and activities of elements in the physicochemical studies of soils. The chemical potentials of independent components, whose values depend of the dynamics of the solution composition only slightly, are more stable and efficient parameters. The calculation of the equilibriums by physicochemical simulation and the data on the composition of the water extracts and soil solutions indicated that, in the A1 horizon of a leached chernozem, the chemical potential of the calcium varied from −173.8 to −174.3 kcal/mol and that of magnesium varied from −150.9 to −151.5 kcal/mol when the calcium and magnesium concentrations changed by more than 20 times. In the B1 horizon of the crusted solonetz, the chemical potential of the sodium varied from −81.3 to −82.0 kcal/mol when its concentration changed by 2–3 times. The determination of the chemical potentials of the independent components (K, Na, Ca, Mg, Mn, Fe, Al, and Si) in the relatively simple basis subsystem of the solution and single-component mineral phases with known solubilities allows calculating the standard Gibbs energies of the other soil phases equilibrated with the solution.

Assessment of the microbial biomass using the content of phospholipids in soils of the dry steppe

Microbiological and biochemical investigations of chestnut soils and solonetzes were conducted in the dry steppe of the southern Privolzhskaya and northern Ergeni uplands. The living biomass of the microbial communities in the soils was estimated based on the content of phospholipids in the soils. Significant correlations were revealed between the contents of phospholipids and the main soil properties (the contents of humus, r = 0.66, P = 0.999; clay, r = −0.41, P = 0.95; physical clay, r = −0.57, P = 0.99; and pH, r = −0.59, P = 0.99). The content of phospholipids varied from 69 to 192 nmol/g of soil in the A1 horizons; with depth it decreased down to 36–135 in the B1 horizon and to 26–79 nmol/g of soil in the B2 horizon. The microbial biomass in the solonetzes was lower by 5 to 38% than that in the chestnut soils. A trend of the decreasing of the microbial biomass in the soils from the north to the south was revealed. Based on the content of phospholipids, the number of living microbial cells was assessed; the weighed averages of their number varied from 0.7–3.2 × 1010 to 7.5–13.6 × 1010.