Accumulation of organic matter in the mineral layers of permafrost-affected soils of coastal lowlands in East Siberia

On the basis of a large volume of literature and original data, the high content (1–7%) of organic matter in the mineral layer of loamy permafrost-affected soils of coastal lowlands in East Siberia (from the lower reaches of the Lena River to the lower reaches of the Kolyma River) has been statistically proved. In most cases, the reserves of C_org in the mineral layer of these soils exceed those in the surface organic horizons and constitute 60–90% of the total soil pool of C_org. The enrichment of the mineral layer with C_org is due to the cryogenic retention (retenization) of humus (the illuviation and accumulation of colorless humic substances above permafrost) and the cryogenic mass exchange (mechanical admixture of organic matter from the upper organic horizons into the mineral layers). The analysis of 60 soil profiles showed that the accumulation of organic matter above the permafrost table is observed in 43% of cases; in general, the organic matter distribution in the soil profiles is highly variable. A specific type of colorless humus is accumulated above the permafrost table. The mechanisms of its precipitation and transformation in the profile require further studies.     

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.     

Clay minerals in a denudation-accumulative soil catena

Chernozems and agrochernozems of the Kamennaya Steppe agroforest landscape have a silty clay or clay texture and similar associations of clay minerals. The plow horizons of the agrochernozems on a slope of 2°–3° to the Talovaya Balka have an increased content of the smectite phase (50–70%) compared to the upper horizons of the chernozems on flat watersheds (30–50%) due to the lithological discontinuity of the soil-forming material and the possible total removal of material on the slope by denudation. On slightly eroded areas, the clay minerals display a more intense disturbance of their crystal lattice structures by pedogenetic processes, which increase the degree of disorder in their layers and the accumulation of fine quartz in the clay fraction. In the areas with more significant erosion of the humus horizon, the clay minerals are characterized by their perfect structure and clean reflections, which are indicative of the outcropping of less weathered material from the middle part of the chernozem profile less transformed by pedogenesis.     

Spatial and temporal variation in podzol organic matter studied by pyrolysis-gas chromatography/mass spectrometry and micromorphology

A well‐developed podzol hydrosequence that has been partially covered with drift sand, and partially subjected to improved drainage, provides new insights into the causes of variation in soil organic matter chemistry in such soils. While E horizons invariably move towards a dominance of aliphatic components reflecting residual accumulation, the chemistry of organic matter in well‐drained B horizons is determined mainly by decaying roots, which are transformed by microorganisms to humus aggregates. In poorly drained, stratified B horizons, humus coatings dominate and the chemistry is very close to that of dissolved organic carbon. When a sand cover inhibits the supply of fresh litter, microbial decomposition in the A horizon causes a shift in chemistry towards that of the E horizon. Similarly, upon improved drainage and removal of complexed metals from the top of the B horizon, microbial decomposition of all palatable organic matter in the top of the B horizon causes a shift towards E‐horizon chemistry. This is probably the mechanism by which most E horizons in podzols are formed, and not by re‐solution. Marked chemical changes upon improved drainage may take only decades. During microbial decay, small polysaccharide‐derived pyrolysis products (mainly furans, furaldehydes and acetic acid) remain abundant due to the contribution of microbial sugars. Both micromorphology and factor analysis on quantified results of pyrolysis‐gas chromatography/mass spectrometry contribute significantly to the interpretation of the humus chemistry of these profiles and thus to our understanding of soil genesis. Organic chemistry of the investigated podzols can be understood only in the context of their genesis.     

Accumulation and transformation of organic matter in different-aged dumps from sand quarries

The accumulation and transformation of organic matter were studied in chronoseries of different-aged (3-, 10-, 20-, 30-, 43-, and 60-year-old) soils and a background (reference) plot. The ecogenetic succession of plants on sand quarry dumps was characterized. It was shown that the pedogenesis rate was closely related to the rate of phytocenosis development, and the thicknesses of organic and mineral horizons increased synchronously. The profile distribution of organic matter in young soils was estimated as an ectomorphic distribution, and the humus reserves in the mineral horizons of the same soils were comparable with the reserves of organic matter in the litters. The illuvial horizons of the soils under study played a significant role in the deposition of carbon dioxide; the resistance of organic matter to mineralization increased with age. In the soil chronoseries, the combustion heat of litter organic matter increased, as well as the content of energy accumulated in the litters. The composition of humus differed strongly between the eluvial and illuvial horizons; in the chronosequence, the relative content of humic acids increased in the E horizon, and that of fulvic acids increased in the B horizon. The effect of the phytocenosis on the soil was increasingly mediated with time. The accumulation and transformation of organic matter were the leading pedogenic processes at all stages.     

Contrasting composition of free and mineral-bound organic matter in top- and subsoil horizons of Andosols

Andosols are characterised by high organic matter (OM) content throughout the soil profile, which is mainly due to the stabilisation of soil organic matter (SOM) by mineral interactions. The aim of the study was to examine whether there were differences in the chemical composition of mineral-associated SOM and free OM in the top A horizon and in the subsoil (horizons below the A11 horizon). Our experimental approach included the replicated sampling of a fulvic and an umbic Andosol under pine and laurel forest located on the island of Tenerife with a Mediterranean sub-humid climate. We determined the extent of the organo-mineral interactions by comparing the sizes of the light (free) and heavy (dense) soil fractions obtained by physical separation through flotation in a liquid with a density of 1.9 g cm^–3. We determined the elemental and isotopic composition of both fractions and analysed their chemical composition by analytical pyrolysis. The elemental and isotopic composition showed similar values with depth despite the different vegetation and climatic conditions prevailing at the two sites. Carbon (C) stabilised by mineral interactions increased with depth and represented 80–90% of the total C in the lowest horizons. The heavy fractions mainly released N-containing compounds upon analytical pyrolysis, whereas lignin-derived and alkyl compounds were the principal pyrolysis products released from the light fractions of the top- and subsoil horizons. Principal component analysis showed that the chemical composition of OM stabilised by mineral interaction differs in the different horizons of the soil profile. In the A horizons, the chemical composition of this OM was similar to those of the light fractions, i.e. litter input. There was a gradual change in the bulk molecular composition from a higher contribution of plant-derived molecules in the light and heavy fractions of the A horizon to more microbial-derived molecules as well as black C-derived molecules at depth. We conclude that transport processes in addition to decomposition and possibly in situ ageing affect the chemical composition of mineral-associated OM in subsoils.     

Transformation of old-plowed gray forest soils under the influence of differently aged pine forests in the middle Angara River region

The long-term (55–85 years) influence of pine forests on old-plowed gray forest soils (in the middle Angara River basin) has been reflected in the character of the biological cycle and intensity of the biological processes. The population of actinomycetes decreased, and that of fungi increased, within the whole profiles of these soils. The soil profiles became more differentiated according to eluvial-illuvial types. The thickness of the humus (former plowed) horizons decreased. The thicker differently decomposed litter with the abundant fungal mycelium was formed. The most conservative were relic morphological characteristics: plow sole, humus tongues, and the illuvial-metamorphic horizon.     

Effects of air drying on the respiration of forest soil microbial populations

Microbial populations from dried, remoistened and undried forest organic horizons were allowed to thoroughly recolonize sterilized forest organic horizons; oxygen uptake was followed during recolonization. Microbial damage due to air drying was in the order litter (L) > fermentation (F) > humus (H). Species diversity, as indicated by diminished O₂ uptake, was reduced in all horizons which had been air dried, with greatest damage in the litter horizon. Samples of these forest organic soil materials, designated for the study of microbially driven processes, should not be air dried.     

Postpyrogenic Polycyclic Soils in the Forests of Yakutia and Transbaikal region

Periodical forest fires are typical natural events under the environmental and climatic conditions of central and southern Yakutia and Transbaikal region of Russia. Strong surface fires activate exogenous geomorphological processes. As a result, soils with polycyclic profiles are developed in the trans-accumulative landscape positions. These soils are specified by the presence of two–three buried humus horizons with abundant charcoal under the modern humus horizon. This indicates that these soils have been subjected to two–three cycles of zonal pedogenesis during their development. The buried pyrogenic humus horizons accumulate are enriched in humus; nitrogen; total and oxalate-extractable iron; exchangeable bases (Са⁺² and Mg⁺²); and the fractions of coarse silt, physical clay (<0.01 mm), and clay (<0.001 mm) particles in comparison with the neighboring mineral horizons of the soil profile. The humus of buried pyrogenic horizons is characterized by the increased content of humic acids, particularly, those bound with mobile sesquioxides (HA-1) and calcium (HA-2) and by certain changes in the type of humus.     

Some features of technogenic soil layers and horizons in the zones of underground gas storages

Technogenic soils in underground gas storage areas are formed under the combined impact of natural and technogenic soil-forming factors (pipelining, gas well drilling and exploitation). New layers and horizons appear in the soil profiles. Technogenic layers (drilling technogenic layer (TSd), a chemically polluted loamy layer formed during the period of gas well drilling and exploitation; technogenic layer (TS^..), a periodically restoring chemically polluted sandy layer formed during the period of gas well exploitation); technogenic horizons (mixed drilling horizon (TURd), a natural-technogenic horizon formed during the drilling period because of mixing of natural soil material); and modification horizons (mixed organic horizon (TURAY), an organic horizon formed by soil restoration or organic matter transformation), were distinguished.     

Forest humus forms as potential indicators of soil carbon storage in Mediterranean environments

The aim of this work was to investigate the potential of forest humus forms as indicators of soil C storage. To this purpose, Mediterranean forest soils in Southern and Central Italy were examined. Sites differed for elevation, climate, parent material and vegetation conditions, while summer drought was the common ecological factor. A morpho-functional criterion, based on holorganic layers thickness and A horizon properties, was used to classify humus forms, which ranged from Dysmoder to Eumull. Such variability allowed understanding of factors influencing organic matter storage. The relations between carbon stock and humus form were investigated for the topsoil layer (0–20 cm), which was supposed to contain the soil C pools most sensitive to climate change. We found that humus forms can be grouped in statistically different populations, with respect to topsoil C stocks. The use of A horizon structure was the main diagnostic criterion and represented the most effective approach to humus classification in Mediterranean conditions. It appears that humus forms have a clear potential as indicators of organic carbon status in Mediterranean forest soils.     

Transformation of organic matter and clay minerals in cultivated gray forest soils

Mechanisms of the agropedogenic transformation of cultivated gray forest soils are discussed. It is shown that the loss of organic carbon from these soils upon their cultivation is mainly due to the intense mineralization of an easily decomposable fraction having a density of less than 2 g/cm³ and a characteristic size above 50 μm. Simultaneously, a certain portion of soil microaggregates is destroyed. Several additional indices of the processes of soil erosion and sediment accumulation on the soil surface (soil aggradation) are suggested. These indices take into account the character of soil clay minerals and soil organic matter. Indicative characteristics of the second humus horizon are suggested on the basis of published materials and new analytical data.     

Biological mineralization of organic matter in the modern virgin and plowed chernozems,buried chernozems,and fossil chernozems

The phenomenon of mineralization (biological mineralization) of organic matter in chernozems has been studied. A decrease in the content of C_org with time can be considered an index of the organic matter mineralization. It is suggested that the humus horizons of modern chernozems contain the pools of organic matter of different ages: easily decomposable organic matter, labile biologically active humus, stable biologically active humus, and relatively inert humus. The composition and mean residence times of these pools and their contribution to the total organic matter content have been estimated. The particular types of the biological mineralization have been determined on the basis of the comparison between the velocities of mineralization (M) and humification (H) processes: total unidirectional mineralization (M ≫ H), equilibrium mineralization (M ∼ H), nonequilibrium mineralization (M> <H), and zero mineralization. The separation of subtypes is based on data on the relative rates (%) of the organic matter mineralization. On the basis of available experimental data on chernozems buried under kurgans and in loess sediments (with the age of up to 800 ka), the quantitative relationship of the humus content in the buried soils on their age has been found; it has an exponential shape. During the first 100 ka after the soil burial, the soil humus content gradually (with a slowing intensity) decreases from 100–75 to 6.5% of its content in the virgin chernozems. Then, 100–1000 ka after the soil burial, the soil humus content remains approximately constant (6.5% of the initial level, or 0.3% of the soil mass). The rates of mineralization have been estimated. It is shown that the elemental composition (C, H, N, O) of humic acids remains relatively stable for a long time due to the regeneration of the chemical structure of humus (matric restoration of humus). It is suggested that several different forms of humus related to pedogenesis should be distinguished in the biosphere. The renewable humus in the equilibrium state with the environment is typical of the open biospheric (soil) systems. The fossil humus, whose content decreases with time, and whose composition remains stable, is typical of the semiclosed and closed systems. With time, it transforms into residual humus, whose content and composition remain stable. The fossilized organic matter in the fossil soils and sediments of the past geological epochs (Mesozoic and Paleozoic) considerably differs from the renewable, fossil, and residual humus.     

Soil formation in the quarries for limestone and clay production in the Ukhta region

The soils forming on the overgrowing technogenic dumps of quarries for limestone and clay production were investigated in the northern taiga (the Ukhta region). The soils are formed under sparse herbaceous plant communities. In the soils on calcareous technogenic eluvium and clay dumps, the processes of humus formation and accumulation predominate. In the soils of the clay dumps, the leaching of carbonates is expressed to a greater extent than in the soils of the limestone quarries. The nitrogen content of organic matter is low in the soils on the technogenic lime substrates and very low on the clay ones. Fulvic acids predominate in the humus composition. At the stages of the soil restoration studied, the zonal trend of pedogenesis is manifested only in the humus accumulation. The calcareous technogenic eluvium is found to be more favorable for the development of microbial communities as compared to the clay substrates with their small microbial biomass. However, there are no features pointing to the development of zonal soil profiles. This fact attests that, in the first 20–30 years, the soil-forming potential in the northern taiga is insufficient for the initiation of the zonal processes.     

Thermal analysis of organic matter in cryogenic soils (Central Siberian Plateau)

The thermal degradation of organic matter was studied in cryogenic soils with methods of thermal analysis: differential scanning calorimetry and thermogravimetry (DSC and TG, respectively). The DSC curves of most of the samples within the temperature range from 221–247°C to 600°C were characterized by the presence of one wide exothermic peak (at 311–373°C) with a shoulder (or without it) on the descending branch at a temperature of about 400°C. This was connected mostly with the destruction of thermolabile compounds (oligo- and polysaccharides) and with the oxidation of low-aromatic complexes of plant residues and humus substances. Two exothermic peaks at 337–373°C and 448–492°C were found for some samples from the organic horizons. The high-temperature peaks were caused by the thermal destruction of lignin. The fraction of the thermolabile organic matter of the soil (237–261…331–377°C) reached 59–73% in the organic and 52–59% in the organomineral and mineral horizons.     

Humus pedorelicts in soddy calcareous soils of the Vyatka-Kama interfluve

A plowed soddy-calcareous soil with relict features developed from Permian clayey calcareous soil was described for the first time in the area of soddy-podzolic and gray forest soils of mixed forests in the middle and lower riches of the Vyatka River. This soil was thoroughly examined by physical, chemical, biochemical, radioisotope, and physicochemical methods. A second humus horizon enriched in calcium humates was described in this soil. Its age was determined at about 7200 yrs. This horizon attests to the presence of in situ postcarbonate pedorelicts in the soil cover of the eastern Russian Plain. The studied soil was formed during the Atlantic optimum of the Holocene under conditions more favorable to humus-accumulative processes as compared with those during the subsequent period. The soil evolution in the second part of the Holocene was characterized by the partial preservation of the previously formed features with the development of eluviation features under colder and wetter climatic conditions. This intrazonal soil is genetically close to gray forest and soddy-podzolic soils with residual second humus horizons that are widespread on the Vyatka-Kama interfluve. The evolution of the latter soils in the second part of the Holocene has been driven by eluviation processes that gradually eliminate the features of the second humus horizon from the soil profiles.     

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.     

Vegetation and pedogenesis on pyrogenic substrates of former peat soils

The role of vegetation and chemical factors in the development of the primary pedogenesis and evolution of pyrogenic formations resulting from fires on drained peat soils was studied. Over four years after the fire, a shallow (1 cm) humus horizon is formed on the surface of the ashy horizon of the pyrogenic formations. For six years, its thickness increases up to 3–4 cm. The dynamics and productivity of the plant cover on the pyrogenic formations were investigated. The dominant plant species were restricted to certain pyrogenic formations. The formation of stable phytocenoses and chemical transformation of substrates are the factors governing the primary pedogenesis on pyrogenic substrates. Four stages in the evolution of the pyrogenic formations were revealed. At the fourth stage, some features appeared that permit us to recognize the development of soddy soils on the pyrogenic substrates (i.e., soddy pyrogenic-mucky, soddy pyrogenic-sandy soils, etc.).     

Aluminum compounds in soil solutions and their migration in podzolic soils on two-layered deposits

The fractional composition of aluminum compounds was studied in soil solutions obtained using vacuum lysimeters from loamy podzolic soils on two-layered deposits. The concentration of aluminum was estimated in brooks and a river draining the area with a predominance of these soils. In soil solutions, the concentration of aluminum was experimentally determined in the following compounds: (1) organic and inorganic monomers, (2) stable complexes with HAs and FAs together with polymers, and (3) the most stable complexes with HAs and FAs together with fine-crystalline and colloidal compounds. The total Al concentration in soil solutions from forest litter was 0.111–0.175 mmol/l and decreased with depth to 0.05 mmol/l and lower in solutions from the IIBD horizons. More than 90% of the Al in the solutions was bound into complexes with organic ligands. Some amount of Al in solution could occur in aluminosilicate sols. The translocation of Al complexes from the litter through the AE horizon to the podzolic horizon was accompanied by an increase in the ratio between the Al concentration in fraction 2 and the C concentration in the solution. The concentrations of Al_tot in the surface waters varied in the range from 0.015 to 0.030 mmol/l. Most of the Al came to the surface waters from the litter and AE horizons and partially from the podzolic horizon due to the lateral runoff along the waterproof IIBD horizon. Approximate calculations showed that the recent annual removal of Al from the AE and E horizons with the lateral runoff was 7 to 560 mg (3–21 mmol) from 1 m².     

Effect of Contrasting Trophic Conditions on the Priming Effect in Gray Forest Soils

Priming effects initiated by the addition of ¹⁴С glucose have been compared for humus horizons of soils existing under continuous input of fresh organic substrates and for buried soil horizons, in which entering of organic matter has been essentially limited. The effect of microrelief on the manifestation of priming effect in the humus horizons of gray forest soil on microhigh and in microlow has been estimated. Humus horizon in soils on microhigh, not activated by glucose, produced two times more СО₂ in comparison with soils of microlow. However, the introduction of glucose canceled the effect of microrelief on СО₂ emission. The intensity of absolute priming effect correlated with the С_org pool, initial microbial biomass, and enzyme activity, decreasing from humus horizons to the buried ones, and did not depend on microrelief. The effect of microrelief was observed, when assessing the priming effect relative to control (soil not activated by glucose): the value of relative priming effect was 1.5 times greater in А horizon of gray forest soil in microlow in comparison with that on microhigh being the result of increasing activity of enzymes.