Radiocarbon ages of humic substances in chernozems

Data on radiocarbon ages of different fractions of humus (humic acids, fulvic acids, and humin) in the profiles of chernozems are analyzed. A chronoecological grouping of humus in modern and buried (fossil) soils is suggested. An increase in the radiocarbon age of humic substances down the soil profile has a stepwise character. It is shown that the ¹⁴C content in chernozems decreases down the soil profile more somewhat slower than the ¹²C content. The dependence of a decrease in the humus content of buried soils on the age of burying is traced for a time span of 800 ka.     

Specific features of organic matter in urban soils of Rostov-on-Don

Data on the fractional and group composition of humus in urban soils of Rostov-on-Don are discussed. We have compared the humus profiles of chernozems under tree plantations and those buried under anthropogenic deposits (including sealed chernozems under asphalt). It is shown that the type of humus in these soils remains stable despite a decrease in its total content after the long-term burial under asphalt. Under the impact of the trees, the organic matter of the chernozems acquired some features typical of gray forest soils, i.e., the humate-fulvate type of humus in the humus horizon and the sharp drop in the humus content down the soil profile.     

The formation of the humus profile of chernozems in the Azov province

The regularities of the formation of the humus profiles in chernozems of the Azov province (“priazovskii chernozems”) were revealed from studying the paleosols of burial mounds (archeological monuments) of the Bronze Age, Early Iron Age, and Middle Ages. The principal differences in the geneses of different parts of the humus profiles were revealed. From 40 to 70% of the current humus reserves in the 1-m-thick layer were shown to be accumulated by the late 3rd and early 2nd millennia BC. In the following 4000 thousand years, the development of the upper soil layers was cyclic. In favorable climatic periods, the humus content increased and accumulation processes predominated. Under aridization, the humus mineralization was intensified, the humus content decreased to 3%, and its reserves did not exceed 18 kg/m². The fluctuations in the humus reserves in the past were not higher than 30% as compared to the recent reference soils. They were most actively replenished from the 1st to the 8th centuries A.D., and the rate of this process reached 0.5 kg/m² for 100 years. In the same period, the thickness of the layer with the humus content above 1% drastically increased. The illuviation of peptized organic matter and its further bioturbation upon the development of the solonetzic process had the greatest importance in the formation of the lower part of the humus profiles.     

Comparative assessment of the biological tolerance of chernozems in the south of Russia towards contamination with Cr, Cu, Ni, and Pb in a model experiment

The biological properties of chernozems in the south of Russia worsen under the impact of contamination with Cr, Cu, Ni, and Pb compounds. The tolerance of chernozems towards contamination decreases in the following sequence: ordinary chernozems > typical chernozems > southern chernozems > leached vertic chernozems. This sequence is specified by the soil reaction and the organic matter content. The high humus content determines the high buffer capacity of chernozems towards contamination with chromium, whereas the high pH values ensure the soil tolerance towards contamination with copper, nickel, and lead. With respect to their adverse effect on the biological properties of the chernozems, the studied heavy metals can be arranged into the following sequence: CrO₃ > CuO > PbO ≥ NiO.     

Physical properties of ordinary chernozems in steppe landscapes adjacent to forests

The spatial changes in the ecological conditions of the humus formation; the humus content and reserves and its fractional-group composition and amphiphilic properties; and the structure, density, and water absorption coefficient of ordinary chernozems were studied on nine key plots at distances of 34 and 2 km from two forest massifs of different sizes??the Buzuluk Bor (111 000 ha) and a small birch grove (25 ha)??in the steppe zone of the Cis-Ural region. With an increase in the distance from the forests, the soil humus reserves decrease, the C_ha/C_fa ratio becomes larger, and the portion of hydrophilic and amphiphilicneutral fractions in the soil organic matter increases. In the same direction, the structure coefficient decreases (from 2.5?C3.7 to 1.5?C2.2), the soil density gradually increases, and the water permeability decreases and remains within the optimal values range.     

Changes in the content and quality of humus in leached chernozems of the Trans-Ural forest-steppe zone under the impact of their agricultural use

The results of long-term studies of changes in the content and fractional-group composition of humus in leached chernozems (Luvic Voronic Chernozems, WRB 2006) of the Trans-Ural forest-steppe since the beginning of their plowing are considered. These soils are characterized by the high humus pool (up to 500 t/ha in the virgin state and up to 430 t/ha in the plowed state) and the high degree of the organic matter humification. Humus is of the fulvate–humate type in the upper 30 cm and of the humate–fulvate type in the deeper layers. From 1968 to 2006, the total pool of humus in the 1-m-deep soil layer of the plowed chernozems decreased by 8.5% in comparison with the initial virgin soils. The rate of dehumification reached 1.0–1.4 t/ha per year. The long-term plowing of leached chernozems also led to a smaller content of nitrogen in the organic matter (the C/N ratio increased from 11.5 to 13.6). The relative content of free humic acids (HA-1) and humic acids bound to clay minerals (HA-3) increased, whereas the content of calcium humates (HA-2) decreased. The composition of fulvic acids in the plow horizon was characterized by the rise in the content of aggressive (FA-1a) and mobile (FA-1) fractions, while the amount of fulvates bound to calcium (FA-2) decreased.     

Nonhydrolyzable residue in humus of chernozems

Mismanagement of chernozems results in a decreasing humus content at the expense of the carbon of the nonhydrolyzable residue (the humin fraction). This conclusion is proved by the original data obtained by the authors and by other scientists. It is also shown that the newly formed humus is dominated by the carbon of nonhydrolyzable residue. These facts attest to the presence of labile humus compounds subjected to mineralization in the humin fraction of chernozems.     

Monitoring of the humus status of soils of the Ingulets irrigation system

The results of long-term studies (1957–2007) of the changes in the morphology of soil profiles and in the reserves and fractional composition of the humus in the soils of the Ingulets irrigation system are discussed. After 50 years of irrigation, the boundaries of the genetic horizons shifted downward by 15–30 cm. The redistribution of the humus took place: its content decreased to a low level in the plow layer of the irrigated and rainfed soils and significantly increased in the layer of 60–100 cm so that the reserves of humus in the layer of 0–100 cm somewhat increased and corresponded to a moderate level. The distribution of humus in the soil profiles was characterized by the gradual lowering down the soil profile. The concentration of nitrogen in the humus of the irrigated southern chernozems was very low. The degree of humification of the soil organic matter was high. The humus was of the humate type in the upper horizons and of the fulvate-humate type in the lower horizons.     

Fractional composition of humus in leached chernozems and gleyed podzolized chernozem-like soils of the northern forest-steppe zone

The composition of humus in leached chernozems differs from that in gleyed podzolized chernozem-like soils in the northern forest-steppe zone of European Russia. Leached chernozems have the fulvate-humate humus. Gleyed podzolized chernozem-like soils have the humate-fulvate humus. A more aggressive composition of humus in the latter soils is caused by their overwetting and the development of gley processes under conditions of a stagnant-percolative soil water regime.     

Microaggregation and stability of organic matter in the forest-steppe chernozems of the Volga region

Based on the regularities of changes in the accumulation, composition, and properties of the microstructural components separated from virgin chernozems by granulodensimetric fractionation, an hypothesis relating the mechanisms of the microaggregate formation and stability with the spatial and temporal distribution of the soil organic matter was confirmed. Schemes were composed for describing the relationships of the soil microstructure with the stability of the organic matter and the dynamics of its renewal in chernozems. Parameters were proposed for characterizing the equilibrium status of soil organic matter.     

Biological activity in the profiles of chernozems as dependent on the character of their agricultural use and microtopography

The biological activity of soil samples taken from genetic horizons of plowed and virgin chernozems in the Kamennaya Steppe Reserve was determined as the amount evolved from the soil upon its incubation in laboratory conditions. In the top humus horizons, the biological activity of virgin chernozem was two to three times higher as compared with that in the plowed chernozem. The intensity of biological processes and the microbial biomass in the soil profile was mainly controlled by the quantity and quality of soil organic matter. Long-term agricultural use exerted a weaker effect on the biological properties of chernozems developed on microelevations (blocks) of the paleocryogenic microtopography in comparison with chernozems developed in the microdepressions between the blocks. The factor analysis showed that soil characteristics related to the biological processes comprise more than 50% of the total variance of factor loads and are distinctly differentiated in the soil profile.     

Relationships between parameters of the humus status of forest and meadow soils and their altitudinal position on the main Crimean range

The influence of bioclimatic conditions related to the elevation above sea level on the quantitative and qualitative parameters of humus in mountain soils has been studied. It is shown that changes in the water and temperature conditions with the altitude do not exert significant effect on the humus content in mountain- forest soils, because the total amount of soil organic matter mainly depends on the composition and state of the vegetation cover. The humus content is the highest in meadow soils formed on mountain plateau with excessive moistening, which determines the formation of dense grass cover and the temperature regime favorable for humification. The percentage of Cha in the composition of Corg and the optical density of humic acids (HAs) are the qualitative parameters of the soil humus status changing with the altitude. The intensity of humus coloring of the soil depends on the content and optical density of HAs. A comparison of color intensity in the mountainous meadow chernozemlike soils and plain chernozems has shown its significant dependence on the hydrothermic conditions.     

Rheological properties of typical chernozems (Kursk oblast) under different land uses

Rheological parameters of humus horizons from typical chernozems under different land use—on a virgin land (unmown steppe) and under an oak forest, long-term black fallow, and agricultural use—have been studied by the amplitude sweep method with an MCR-302 modular rheometer at water contents corresponding to swelling limit and liquid limit. From the curves of elastic and viscous moduli, the ranges of elastic and viscoelastic (plastic) behavior of soil pastes—as well as that of transition from viscoelastic to viscous behavior—have been determined. It has been shown that the rheological behavior is largely determined by the content of organic matter, which can act as a binding agent structuring the interparticle bonds and as a lubricant in the viscous-flow (plastic) state of soil pastes. Soil samples enriched with organic matter (virgin land, oak forest, forest belt) have a more plastic behavior and a higher resistance to loads. Soil samples with the lower content of organic matter (long-term fallow, plowland) are characterized by a more rigid cohesion of particles and a narrower range of load resistance. Soil pastes at the water content of liquid limit have a stronger interparticle cohesion and a more brittle behavior than at the water content of swelling limit. Methodological aspects of testing soil pastes at the constant sample thickness and the controlled normal load have been considered. For swelling soil samples, tests under controlled normal load are preferred.     

Trends of the natural evolution of chernozems on the East European Plain

A generalized chronosequence of changes in the morphometric parameters of chernozems in the past 5100 years is analyzed. It is shown that the development of chernozems in the center of the East European Plain is characterized by both long-term (interglacial, encompassing the entire Holocene) and short-term (within separate climatic periods of the Holocene) trends. The long-term trend of the evolution of ordinary chernozems developed from the loess-like calcareous loam consists of a continuous increase in the thickness of the soil humus profile and an oscillating increase in the depth of the carbonate accumulations. The Late-Holocene short-term trends of the natural evolution of chernozems are characterized by the high spatial heterogeneity related to the patterns of climatic fluctuations and to the lithological composition of parent materials. In that period, the thickness of genetic horizons in the forest-steppe chernozems was increasing faster than that in the steppe chernozems.     

Organic matter characteristics and C and N transformations in the humus layer under two tree species, Betula pendula and Picea abies

The aim of this study was to compare the effects of silver birch (Betula pendula Roth) and Norway spruce (Picea abies (L.) Karst.) on soil C and N transformations and on the characteristics of organic matter. Soil samples were taken from the humus layer of a replicated 35-year-old birch-spruce field experiment growing on Vaccinium myrtillus site type in middle-eastern Finland. The soil was a podzol and humus type mor. Soil pH was higher under birch (4.7) than under spruce (4.1). The C-to-N ratio was lower under birch (17) than under spruce (23). Per unit organic matter, microbial biomass C and N, net N mineralization and net nitrification were all higher in birch soil than in spruce soil. The rate of C mineralization (CO₂ production) was, however, the same regardless of tree species. Water-extracts were analyzed for the concentrations of dissolved organic C (DOC) and N (DON) and characterized according to molecular size distribution by ultrafiltration and according to chemical composition using a resin fractionation technique. The concentration of DON, in particular, was higher in birch soil than in spruce soil. The distribution of DOC and DON into different fractions based on molecular size or chemical composition was rather similar in both soils. The concentration of total phenolics, expressed as tannic acid equivalents, was higher in the humus layer under birch than in the humus layer under spruce, because the birch humus layer contained significantly more low-molecular weight (about <0.5 kD) phenolics than the spruce humus layer did. The concentration of proanthocyanidins (condensed tannins) was higher in spruce soil than in birch soil. The concentrations of the five most abundant phenolic acids showed that ferulic and p-coumaric acids were more abundant in spruce soil. Birch soil tended to contain slightly more nonvolatile sesquiterpenes than the spruce soil. The concentration of diterpenes was similar in both soils; but birch soil contained significantly more triterpenes, mainly sterols, than spruce soil did.     

Long-term dynamics of the properties of leached chernozems under different forest plantations

The long-term dynamics of the main morphological, physical, chemical, and physicochemical properties of leached chernozems under introduced (Weymouth pine and Siberian larch) and aboriginal (English oak, Scotch pine, and whitewood) trees are discussed. The trees were sown in 1928–1930 in the Mokhovoe Forestry of Orel oblast (the Shatilovo Forest). A comparative analysis of published data and the results obtained in our study has shown some positive changes in the humus horizons under the impact of forest vegetation. The thickness of the humus layer (A + AB) has increased. It has become less compact. The soil structure and the water stability of the soil aggregates in the upper 30-cm-deep layer have improved. The total humus content in the A horizon and its reserves in the layers of 0–20, 0–50, and 0–100 cm have increased. In general, the properties of the leached chernozems have improved under the impact of the artificially planted trees.     

Transformation of soil organic matter in leached chernozems under minimized treatment in the forest-steppe of West Siberia

Changes in the contents of total organic carbon and the carbon of easily mineralizable fractions of organic matter (labile humus, detritus, and mortmass) in the layers of 0–10, 10–25, and 0–25 cm were studied in leached chernozems ((Luvic Chernozems (Loamic, Aric)) subjected to deep plowing and surface tillage for nine years. In the layer of 0–25 cm, the content of Corg did not show significant difference between these two treatments and comprised 3.68–3.92% in the case of deep plowing and 3.63–4.08% in the case of surface tillage. Tillage practices greatly affected the distribution of easily mineralizable fractions of organic matter in the layers of 0–10 and 10–25 cm, though the difference between two treatments for the entire layer (0–25 cm) was insignificant. Surface tillage resulted in the increase in the contents of mortmass (by 59%), detritus (by 32%), and labile humus (by 8%) in the layer of 0–10 cm in comparison with deep plowing. At the same time, the contents of these fractions in the layer of 10–25 cm in the surface tillage treatment decreased by 67, 46, and 3%, respectively. The estimate of the nitrogen-mineralizing capacity made according to the data on the uptake of soil nitrogen by oat plants in a special greenhouse experiment confirmed the observed regularities of the redistribution of easily mineralizable organic matter fractions by the soil layers. In case of surface tillage, it increased by 23% in the layer of 0–10 cm; for the layer of 0–25 cm, no significant differences in the uptake of nitrogen by oat plants were found for the two studied treatments.     

Irrigation water with an increased magnesium content and its role in the degradation of chernozems in Southeastern Europe

The reasons for the deterioration of the irrigation water quality and the alkaline degradation of chernozems in the Danube-Dnestr and Azov irrigation systems on the East-European Plain were studied. The water of the accumulation reservoirs in these systems is alkaline (pH up to 8.6), contains 1.5–2.5 g/l total dissolved salts, and is characterized by an increased content of magnesium (35% of the total cations). The irrigation of chernozems with such water results in the alkalinization of the soils and a decrease in the content of exchangeable calcium from 85–90 to 55–65% of the total exchangeable cations. The content of exchangeable magnesium increases to 30–40% and that of sodium to 3–5% of the total exchangeable cations. Problems of soil reclamation that have arisen during the functioning of irrigation systems without solving the main task (the improvement of the irrigation water’s quality) have been considered. Recommendations are given for the conservation of chernozems and the preservation of the quality of the irrigation water from the Kuban and Don rivers during its transportation to the irrigated fields.     

Conditions of the formation of ice barriers in eroded chernozems of Western Siberia

It is shown that the water content in the upper part of chernozems in Western Siberia in the late fall period exceeds the field capacity. An ice-rich barrier is formed in the humus horizon of these soils at temperatures below zero in the cold period. This barrier has both negative and positive effects. Being impermeable for snowmelt water, it favors the development of erosion processes. At the same time, it prevents infiltration of snowmelt water beyond the soil profile with the formation of a percolative water regime (favoring soil leaching) in the chernozems.