Pyrogenic transformation of organic matter in soils of forest bogs

Natural fires on forest bogs significantly affect all the groups and fractions of peat organic matter. The type and intensity of the fires are responsible for the depth of the pyrogenic transformation of peat. In the course of thermal destruction of peat organic matter, humus substances (humic acids in particular) are accumulated, which leads to changes in the type of humus; the humus reserves may increase by 1.5–8 times. Several ways of the formation of humus components related to the intensity of a fire are suggested. The regressive evolution of bog ecosystems caused by fires is a reversible process. The humus status of pyrogenically transformed horizons and their morphology are preserved within the peat deposit as a relic characteristic of the discrete metamorphosis of the soils.     

Peat soils: Genesis and classification

This paper considers three topical problems—the definition of peat soils as natural-historical formations and the estimation of their profile thickness, the analysis of the genesis of organic soils, and the principles of the classification of peat soils. Based on the experimental data of long-term studies, it was concluded that peat soils may include the whole peat layer and the upper horizons of the surface mineral soil. The organic and mineral parts of the natural structures were found to be a genetically homogeneous soil profile, which has the same history of development. The upper layer of the peat soils should be considered as the horizon reflecting the contemporary stage of the soil formation. A hierarchy of peat soils is analyzed for developing their classification.     

Specific features of the development of soils of hydromorphic ecosystems in the northern taiga of Western Siberia under conditions of cryogenesis

Differently directed and heterochronous cryogenic processes have contributed to the contrasting soil cover patterns and spatial heterogeneity of the properties of soils in hydromorphic ecosystems of the discontinuous permafrost zone of the northern taiga in Western Siberia. Frost heave and permafrost thawing within ecosystems of highmoor bogs have led to the development of specific cryogenic landforms, such as flat-topped and large peat mounds. A set of cryogenic soils is developed in these ecosystems; it includes different variants of cryozems, gleyzems (Cryosols), and peat soils (Histosols). The distribution of these soil types is controlled by the local topography and thawing depth, other factors being insignificant. Alternation of peat horizons of different types and ages, whirl-like patterns of horizon boundaries, considerable variations in the thickness of soil horizons, and inversions of soil horizons under the impact of frost cracking, frost heave, and cryoturbation are typical of the considered soils. Thawing depth is the most significant factor affecting the thickness of organic horizons, the soil pH, and the degree of decomposition of peat. As a result of the upward movement of bog ecosystems under the impact of frost heave, peat soils are subjected to considerable transformation: peat horizons undergo mineralization, and the thickness of organic horizons decreases; in some cases, eluvial–illuvial differentiation of the mineral horizons takes place, and peat podzols are developed. However, the opposite process of the return of the soils to the bog stage of pedogenesis with peat accumulation may take place in any time in the case of activation of thermokarst processes.     

Rheological Properties of Automorphic and Semihydromorphic Cryometamorphic Northern Taiga Soils in Northeastern European Russia (Komi Republic)

Soil pastes at the water content corresponding to the maximum swelling of samples from different genetic horizons of cryometamorphic soils―surface-gleyic iron-illuvial svetlozem (Folic Albic Stagnosol) and peaty and peat humus-impregnated gleyic svetlozems (Histic Gleyic Stagnosols)―have been studied with an MCR-302 modular rheometer (Anton Paar, Austria). It has been found that the strongest interparticle bonds are formed in the horizons of cryometamorphic soils characterized by high contents of humic substances and organomineral Al–Fe–humus compounds. These are horizons of podzol microprofile (Eg and BHF) in iron-illuvial svetlozem and a humus-impregnated horizon (ELhi,g) in peaty and peat svetlozems. Organomineral Al–Fe–humus compounds, as well as the seasonal freezing of soils, determine the elastic-brittle character of interparticle interactions. The contents of clay fractions, exchangeable bases, and organic and organomineral substances impart viscoelastic properties to these contacts. An enhancement of elastic-brittle properties of soil is observed under the impact of gleying and freezing. The threefold decrease of the structural interaction parameter (∫Z) when going from automorphic to semihydromorphic conditions indicates a decrease in the resistance of peaty and peat svetlozems to mechanical loads under increasing hydromorphism compared to iron-illuvial svetlozems.     

Effects of tracked vehicles on the morphological and physical properties of tundra soils

The agricultural lands of typic tundra of the Yamal Peninsula in Russia are pastures for reindeer (Rangifer tarandus sibiricus Murr.) herds. Currently, degradation of tundra soil cover is mainly caused by mechanical impacts of tracked vehicles used in construction operations. The objective of this study was to evaluate changes in morphological, micro-structural, and physical properties of Cryozems and Cryogenic peaty soils affected by these tracked vehicles. Soil samples were taken from the surface and underlying horizons before and 5 years after four and 100 passes of tracked vehicles. Surface horizons (0–10 cm) of the undisturbed Cryozems and Cryogenic peaty soils were organogenic. Passage of tracked vehicles caused mixing of these horizons with lower sandy loam and loam mineral horizons. Properties of the organomineral horizons formed in this way differed essentially from those of the surface horizons of the undisturbed soils. Microaggregates were completely disturbed, even after only four passes of tracked vehicles. Large inter-aggregate pores disappeared and thin pores or cracks formed as a result of vehicle-induced mechanical impacts. Humification of plant residues was observed to be faster in the compacted organomineral horizons of disturbed soils compared with undisturbed ones. The organic substances formed in the compacted organomineral horizons readily moved downward within the soil profile or were lost during runoff events. High correlation coefficients of organic carbon content with both specific surface area and water retention showed that the above-mentioned organic substances were hydrophilic. Specific surface area and water retention of the disturbed soils rose with increasing organic carbon content. The results obtained in this study demonstrated a high susceptibility of Cryozems and Cryogenic peaty soils to mechanical impacts.     

Postpyrogenic transformation of soils under Pinus sibirica forests in the southern Lake Baikal basin

The results of experimental investigations of the postpyrogenic dynamics of the widespread rawhumus podzols under Pinus sibirica forests in the southern Lake Baikal basin are discussed. Ground fires transform the diagnostic surface organic soil horizons into organic pyrogenic horizons (Opir, OL/Opir, and AOpir). The adverse effect of ground fires of different intensities on the changes in the reserves, the fractional composition of the litters, and the chemical composition of the organic horizons is shown. The soils of dark coniferous forests are inclined to long-lasting restoration. The Pinus sibirica regrowth under the canopy of deciduous species serves as the basis for the restoration of native stands and the formation of the soils characteristic of the southern part of the Lake Baikal basin.     

Chemodestructive fractionation of soil organic matter

The method of chemodestructive fractionation is suggested to assess the composition of soil organic matter. This method is based on determination of the resilience of soil organic matter components and/or different parts of organic compounds to the impact of oxidizing agents. For this purpose, a series of solutions with similar concentration of the oxidant (K₂Cr₂O₇), but with linearly increasing oxidative capacity was prepared. Chemodestructive fractionation showed that the portion of easily oxidizable (labile) organic matter in humus horizons of different soil types depends on the conditions of soil formation. It was maximal in hydromorphic soils of the taiga zone and minimal in automorphic soils of the dry steppe zone. The portion of easily oxidizable organic matter in arable soils increased with an increase in the rate of organic fertilizers application. The long-lasting agricultural use of soils and burying of the humus horizons within the upper one-meter layer resulted in the decreasing content of easily oxidizable organic matter. It was found that the portion of easily oxidizable organic matter decreases by the mid-summer or fall in comparison with the spring or early summer period.     

Buffer capacities of podzolic and peat gleyic podzolic soils to sulfuric and nitric acids

Soil samples from the main genetic horizons of pale podzolic and peat gleyic podzolic soils from the Central Forest Reserve were subjected to a continuous potentiometric titration by sulfuric and nitric acids. The sulfate sorption capacity was determined in soil mineral horizons. The buffer capacity of mineral horizons of both soils to sulfuric acid was found to be higher than that to the nitric acid. This is explained by the sorption of sulfates via the mechanism of ligand exchange with the release of hydroxyl groups from the surfaces of Fe and Al hydroxide particles and edge faces of clay crystallites. The buffer capacity of organic horizons of the pale podzolic soil to sulfuric acid proved to be higher than that to nitric acid; in organic horizons of the peat gleyic podzolic soil, the buffer capacity to sulfuric acid was lower than that to nitric acid. The reasons for this phenomenon have yet to be investigated.     

Morphology and properties of the soils of permafrost peatlands in the southeast of the Bol’shezemel’skaya tundra

The morphology and properties of the soils of permafrost peatlands in the southeast of the Bol’shezemel’skaya tundra are characterized. The soils developing in the areas of barren peat circles differ from oligotrophic permafrost-affected peat soils (Cryic Histosols) of vegetated peat mounds in a number of morphological and physicochemical parameters. The soils of barren circles are characterized by the wellstructured surface horizons, relatively low exchangeable acidity, and higher rates of decomposition and humification of organic matter. It is shown that the development of barren peat circles on tops of peat mounds is favored by the activation of erosional and cryogenic processes in the topsoil. The role of winter wind erosion in the destruction of the upper peat and litter horizons is demonstrated. A comparative analysis of the temperature regime of soils of vegetated peat mounds and barren peat circles is presented. The soil–geocryological complex of peat mounds is a system consisting of three major layers: seasonally thawing layer–upper permafrost–underlying permafrost. The upper permafrost horizons of peat mounds at the depth of 50–90 cm are morphologically similar to the underlying permafrost. However, these layers differ in their physicochemical properties, especially in the composition and properties of their organic matter.     

Polycyclic aromatic hydrocarbons in tundra soils of the Komi Republic

The qualitative and quantitative composition of polycyclic aromatic hydrocarbons (PAHs) in tundra soils has been studied by gradient high-performance liquid chromatography. The differentiation of the PAHs in the soil profile is of clear accumulative character. The pool of polyarenes in the soils is determined by the cryogenesis processes and mainly consists of low-molecular-weight bi- to tetranuclear PAHs, the high-molecular-weight PAHs in the organic horizons make up no more than 20% of the total polyarenes in the soil, and penta- and hexanuclear structures are almost absent in the mineral layer.     

Soil organic matter characterization of temperate peatland soil with FTIR‐spectroscopy: effects of mire type and drainage intensity

Peatlands are an important component of the global carbon cycle because they comprise huge amounts of terrestrial carbon (C). Different conditions during peat formation and secondary peat decomposition affect the quantity and composition of soil organic matter (SOM) in peats. There are few comparative studies on the chemical composition of SOM in temperate peatland soil. This study investigates compositional changes of SOM functional groups in peats and corresponding peat‐forming plants by Fourier transform infrared (FTIR) spectroscopy. Three plant samples and 29 peat samples were taken from seven temperate peatland sites with different genesis and land‐use intensity. Site‐specific differences, such as genesis of the peat, were found to be reflected in the FTIR spectra. In general, there was more variation in FTIR spectra in samples from fens than in those from bogs and peat‐forming plants. The samples from fens have a smaller C–H absorption band than those from bogs and plants, which reflects greater biochemical activity in the minerotrophic than ombrotrophic environments. In addition to peat genesis, drainage and secondary peat decomposition also affect SOM composition substantially. The larger amounts of aliphatic compounds in undrained peats could be explained by selective preservation caused by anaerobic conditions. With increasing drainage of the sites, there was a decrease in the C–H absorption that was accompanied by a relative increase in C=O absorption. These changes in absorption intensities reflect the enhanced aerobic decomposition and mineralization that accompanies drainage and land‐use intensity. However, the ‘degree of peat decomposition’, a diagnostic tool used in the field, is not reflected by OM composition determined by FTIR spectroscopy. Our results contribute to further understanding of changes in SOM composition during peat formation and processes of secondary decomposition caused by drainage.     

The problem of fire control on drained peatlands and its solution

Fires on drained peatlands arise as a result of lowering of the groundwater table and the rupture of its capillary fringe from the peat soil horizons. Fires destroy the most fertile soils of the nonchernozemic region, adversely affect the diversity and species composition of the biota and the work of transport, and cause diseases and the death of people. A set of preventive measures against fires on the drained peatlands is proposed. It is important to use these soils only for meadow grass cultivation with rotations enriched in perennial grasses. No cases of “black” crop growing are possible on peatlands. The reclamation of peat soils should be implemented only with the bilateral regulation of the water regime. An optimal system of increasing the fertility of drained peat soils should be applied; their use should also be accompanied by sanding.     

Spatial variation of peat soil properties in the oil-producing region of northeastern Sakhalin

Morphology and properties of medium-deep oligotrophic peat, oligotrophic peat gley, pyrogenic oligotrophic peat gley, and peat gley soils on subshrub-cotton grass-sphagnum bogs and in swampy larch forests of northeastern Sakhalin have been studied. Variation in the thickness and reserves of litters in the studied bog and forest biogeocenoses has been analyzed. The profile distribution and spatial variability of moisture, density, ash, and pH_KCl in separate groups of peat soils have been described. The content and spatial variability of petroleum hydrocarbons have been considered in relation to the accumulation of natural bitumoids by peat soils and the technogenic pressing in the oil-producing region. Variation of each parameter at different distances (10, 50, and 1000 m) has been estimated using a hierarchical sampling scheme. The spatial conjugation of soil parameters has been studied by factor analysis using the principal components method and Spearman correlation coefficients. Regression equations have been proposed to describe relationships of ash content with soil density and content of petroleum hydrocarbons in peat horizons.     

Pyrogenic polycyclic aromatic hydrocarbons in soils of reserved and anthropogenically modified areas: Factors and features of accumulation

The accumulation features of pyrogenic polycyclic aromatic hydrocarbons (PAHs) in soils were analyzed depending on the type of burning material and the combustion conditions (the temperature, oxygen access, and dispersion of the combustion products). The PAH accumulation features in the soils of three reserved areas of forest, steppe, and peat fires were revealed. The distribution features of the PAHs resulting from the anthropogenic pyrogenic processes (the household combustion of wood and the inflammation of coal dumps) in soils were established. The differences in the qualitative composition of the PAHs arriving to the soils from the different pyrogenic sources were shown.     

Accumulation of polycyclic aromatic hydrocarbons in soils and plants of the tundra zone under the impact of coal-mining industry

Thirteen polycyclic aromatic hydrocarbon (PAH) compounds were identified in organic horizons of tundra surface-gleyed soils ( Histic Stagnosols (Gelistagnic) and plants. The total content of PAHs in contaminated soils exceeded the background values by three times. Concentrations of low-molecular weight hydrocarbons in soils at different distances from the coalmines were relatively stable. Concentrations of highmolecular weight hydrocarbons had a distinct maximum at a distance of about 0.5 km from the source of emission. The increased values of correlation coefficients were found between PAH concentrations in organic soil horizons, plants, and coal of the Vorkutinskaya mine. Mostly low-molecular weight structures predominated in the organic soil horizons and in the studied plant species. The maximum capacity for the biological accumulation of PAHs was displayed by Pleurozium schreberi and the minimum capacity was displayed by Vaccinium myrtillus. Mosses and lichens actively absorbed polyarenes from the surface; most of the PAHs were transported into the plants. This phenomenon was not observed for Vaccinium myrtillus Concentrations of PAHs on the surface and in plant tissues decreased with an increase in the distance from the mine. Distribution of polyarenes in plant organs was nonuniform. Insignificant excess of concentration of polyarenes was found in dead part of Pleurozium schreberi in comparison with its living part. The accumulation of polyarenes in the leaves of Vaccinium myrtillus was higher than that in its stems and roots.     

Soils of Mountainous Forests and Their Transformation under the Impact of Fires in Baikal Region

Data on postpyrogenic dynamics of soils under mountainous taiga cedar (Pinus sibirica) and pine (Pinus sylvestris) forests and subtaiga–forest-steppe pine (Pinus sylvestris) forests in the Baikal region are analyzed. Ground litter–humus fires predominating in this region transform the upper diagnostic organic soil horizons and lead to the formation of new pyrogenic organic horizons (Opir). Adverse effects of ground fires on the stock, fractional composition, and water-physical properties of forest litters are shown. Some quantitative parameters of the liquid and solid surface runoff in burnt areas related to the slope gradient, fire intensity, and the time passed after the fire are presented. Pyrogenic destruction of forest ecosystems inevitably induces the degradation of mountainous soils, whose restoration after fires takes tens of years. The products of soil erosion from the burnt out areas complicate the current situation with the pollution of coastal waters of Lake Baikal.     

Application of the methods of thermal analysis for the assessment of organic matter in postpyrogenic soils

Experimental data on the effect of surface fires on the organic matter transformation in the gray-humus soils of pine forests were obtained in the southwestern part of the Baikal region. The application of methods of thermal analysis (such as differential scanning calorimetry and thermogravimetry) made it possible to obtain qualitative and quantitative information about the decomposition (oxidation) of the components of the soil organic matter upon their heating. It was found that the organic matter content in the soils subjected to fires of high intensity decreased by 1.9 times in comparison with the control. In the litter horizons of the undisturbed soils, thermolabile components (mostly, oligo- and polysaccharides) comprised 61% of the organic matter, and the portion of thermostable components (aromatic compounds) was 39%. A significant decrease in the content of thermolabile components and an increase in the content of thermostable components (up to 62%) were observed in the organic matter of the postpyrogenic forest litter as a result of the charcoal formation during the fire.     

SYNOPSIS Polycyclic Aromatic Hydrocarbons (PAHs) in Soil — a Review

PAHs are mainly produced by combustion processes and consist of a number of toxic compounds. While the concentrations of individual PAHs in soil produced by natural processes (e.g., vegetation fires, volcanic exhalations) are estimated to be around 1—10 μg kg⁻¹, recently measured lowest concentrations are frequently 10 times higher. Organic horizons of forest soils and urban soils may even reach individual PAH concentrations of several 100 μg kg⁻¹. The PAH mixture in temperate soils is often dominated by benzofluoranthenes, chrysene, and fluoranthene. The few existing studies on tropical soils indicate that the PAH concentrations are relatively lower than in temperate soils for most compounds except for naphthalene, phenanthrene, and perylene suggesting the presence of unidentified PAH sources. PAHs accumulate in C‐rich topsoils, in the stemfoot area, at aggregate surfaces, and in the fine‐textured particle fractions, particularly the silt fraction. PAHs are mainly associated with soil organic matter (SOM) and soot‐like C. Although the water‐solubility of PAHs is low, they are encountered in the subsoil suggesting that they are transported in association with dissolved organic matter (DOM). The uptake of PAHs by plants is small. Most PAHs detected in plant tissue are from atmospheric deposition. However, earthworms bioaccumulate considerable amounts of PAHs in short periods. The reviewed work illustrates that there is a paucity of data on the global distribution of PAHs, particularly with respect to tropical and southern hemispheric regions. Reliable methods to characterize bioavailable PAH pools in soil still need to be developed.     

Soils of postpyrogenic larch stands in Central Siberia: Morphology,physicochemical properties,and specificity of soil organic matter

Morphological features, physicochemical properties, and specific characteristics of the organic matter of cryozems (Cryosols) under postpyrogenic larch forests affected by fires 2, 6, 22, 55, and 116 years ago are considered. The morphological changes in the soils affected by fires are manifested by the burning of the upper organic horizons with preservation of pyrogenic features in the soils for more than a century after the fire. In the first years (2 and 6 years) after the fire, the acidity of the organic horizons and their base saturation become lower. The postpyrogenic soils are characterized by the smaller contribution of the organic horizons to the total pools of soil organic carbon. In the studied cryozems, the organic carbon content is correlated with the contents of oxalate-extractable iron and aluminum. A decrease in the content of water-soluble organic compounds in the soils is observed after the fires; gradually, their content increases upon restoration of the ground cover.