Frontier Studies in Composition of Humic Substances and Soil Organic Matter

Soils of the Arctic sea coasts are one of the least studied due to the complex logistical accessibility of the region, as well as the severe climatic conditions. The genesis of these soils is determined by several factors of soil formation simultaneously—cryogenesis, the influence of river alluvial processes, as well as the tidal influence of the sea. The paper presents data on the morphological structure of soils formed on the seacoast of the East-Siberian Sea (Kolyma Lowland, North Yakutia). Under the influence of cryogenesis and sea water tidal input, marsh soils are formed, with a relatively high level of salinity and the development of gleyization. Autochthonous and allochthonous soil organic matter play a leading role in marsh soil formation here, including the possible accumulation and biochemical transformation of incoming pollutants (e.g., hydrocarbons). The main objective of the study was to evaluate the soil organic matter genesis and alteration under the influence of tidal processes in coastal permafrost-affected soils as well as to obtain the previously unknown characteristics of the structural and elemental composition of different fractions of organic matter. The elemental composition and ¹³C NMR spectroscopy of humic acids were analyzed. It was revealed that humic acids extracted from the studied marsh soils accumulate up to 50% C and 4% N. Active processes of dehydrogenation are noted in HAs molecules, which indicates a relatively low degree of aliphatic structure development. According to ¹³C NMR spectroscopy, it was revealed that up to 45% of aromatic structural fragments accumulate in marsh soils, indicating a relatively high degree of organic matter stabilization and resistance to biodegradation. Full article

The use of environmentally safe products of natural origin is a global trend today. A particular point of interest is the use of humic fertilizers. This is due to the growing awareness of the positive impact of humic substances on plant growth and development as well as on the quality of agricultural products and soil fertility. Humates are physiologically active substances. As a result, they regulate and intensify metabolic processes in plants and soil, and contribute to the bioavailability of nutrients to plants. EldORost is a new-generation humic product that contains humic substances with a high humification degree. In addition to humates, this product contains a complex of amino acids, macro-, and microelements in a bioavailable form for plants. The product is eco-friendly and completely soluble in water, which is a substantial advantage for drip irrigation systems. It can be used for all types of agricultural crops on a wide diversity of soils and climatic zones. It displays the properties of plant hormones while its optimum concentration is as low as 0.0001% (wt). The efficiency of this novel humic product was tested in laboratory and field tests conducted on potatoes and vegetable crops (tomatoes, cucumbers, cabbage, carrots, onions, beets). The obtained results showed high efficiency displayed in the significantly improved sowing quality of vegetable seeds, nominally increased the germination degree and seed germination energy, intensively stimulated the side root development in plants, accelerated the growth of biomass, increased the fruiting period, and reduced maturation on the yield of potatoes and vegetable crops. The obtained data allowed us to characterize this novel humic product from the perspective of an eco-friendly fertilizer and growth promoter. Full article

Pyrogenic components of soil organic matter are practically not taken into account when assessing the implications of global climate changes on the SOM sequestration rate and its stability. In this regard, both the amount and structural stability of SOM were investigated for postpyrogenic forest–steppe environments. Mature unaffected by fire Psamment Entisols were investigated in comparison with same soil strongly affected by surface and crown fires in 2010 in Tolyatti city. This article discusses postpyrogenic succession when comparing the results of field works in 2010 with 2020. The elemental composition of humic acids as well as ¹³C NMR spectra were investigated for the studied humic acids. Moreover, for the first time ever, integrated indicators of HAs’ molecular composition were presented for postpyrogenic soils. The data obtained showed that there are significant changes in the structural organization of organic matter as a result of forest fires. A simultaneous decrease in the H/C and O/C atomic ratios was identified as a result of fire, which indicates a slight loss of oxygen-containing functional groups, while the aromaticity degree of molecules increases. An increase in the content of oxygen-containing groups in the HA molecules was shown for 10 years after the fires, which was accompanied by the oxidation of the HA molecule with the same increase in the aromaticity degree of the HA molecule. The HA molecules of 2020 turned out to be more resistant to environmental factors compared to analogues in 2010. As a result of fires, the aromaticity degree of HAs molecules significantly increases. Full article

Dry direct-seeded rice cultivation is a simple and labor-saving planting method wherein the combined application of organic and inorganic fertilizers can improve yield. However, the effects of combined fertilizers on soil properties and bacteria in dry direct-seeded rice paddy soil are unclear. Here, four treatments, conventional fertilization (NPK), seaweed bio-organic fertilizer + NPK, Jishiwang bio-organic fertilizer + NPK, and attapulgite organic fertilizer + NPK applied for three consecutive years were tested to explore their effects on soil physical, chemical, and bacterial community characteristics in a dry direct-seeded rice paddy field. The combined fertilizers increased alkaline hydrolysis-nitrogen and available potassium while decreasing the bulk density and pH; in addition, a marked increase in the relative abundance of soil macroaggregates (>5 mm) and clay particles and a decrease in that of sand was observed. Urease and neutral phosphatase activities were the highest with the Jishiwang organic fertilizer + NPK, whereas invertase and catalase activities were the highest with attapulgite organic fertilizer + NPK. All combined fertilizers considerably increased the bacterial richness index (ACE) and Chao index; Jishiwang bio-organic fertilizer + NPK also increased the Simpson index, whereas the seaweed bio-organic fertilizer + NPK reduced it. Proteobacteria and Acidobacteria accounted for 54.25–70.49% of the total bacterial relative abundance. The relative abundance of Verrucomicrobia, Chloroflexi, Firmicutes, and Nitrospirae increased with the combined fertilizers. The correlation network analysis showed predominant antagonistic relationships. A redundancy analysis demonstrated that total nitrogen, soil organic matter, urease, and invertase were the main environmental factors affecting bacterial composition. Combined fertilizers may improve soil physical and chemical properties, fertility, and bacterial richness. Full article

Paramagnetic activity is a fundamental property of humic substances (HSs). The agricultural use of soils does not only affect the qualitative and quantitative compositions of HSs, but also the content of free radicals (FRs) in their structure. Changes in the composition of incoming plant residues and hydrothermal conditions have an effect on soil humification rates and the paramagnetic properties of humic (HAs) and fulvic acids (FAs). Data on the influence of various factors on the content of FRs in HAs and FAs are not enough. Therefore, the purpose of this study was to examine the influence of the degree of hydromorphism and agricultural use on the paramagnetic properties of HA and FA samples obtained from taiga and tundra soils. Studies have shown that the increased hydromorphism in taiga soils leads to the growing concentration of FRs in the HA molecular structure. HAs in virgin tundra soils exhibit a lower content of unpaired electrons when shifting from automorphic soils to hydromorphic ones. Going from the south to the north, the paramagnetic activity of both HAs and FAs tends to decrease due to the overall reduction of the number of polyconjugated systems in the tundra soil HSs. The comparative analysis of the paramagnetic properties in HAs and FAs of virgin and arable soils revealed that their agricultural use reduces the FR concentration in the structure of HSs, in other words it leads to the accumulation of biothermodynamically stable and more humified compounds in the arable horizons. This contributes to the stabilization of SOM in arable soils. Full article

In this study, we examined the influence of long-term conventional tillage on the water-physical, chemical properties, and composition of the organic matter of chernozems. The study has been conducted on an arable plot subjected to water and wind erosion in the Pre-Ural steppe zone (Republic of Bashkortostan, Russia). Soil samples were collected from non-eroded and eroded arable plots as well as from an adjacent pristine forest windbreak. Key structural fragments of soil organic matter under different land use types were identified and quantified using ¹³C-NMR spectroscopy. The results showed that the water-physical properties deteriorated in agrochernozems: the number of valuable soil aggregates decreased and the soil bulk density increased, which may limit the growth of crops. The soil organic matter content for the different samples varied in the following direction: arable non-eroded > forest windbreak > arable eroded. It has been found that long-term plowing by conventional methods decreases aliphatic and increases aromatic structures in soils. As a result of the reduced inputs of plant residues, the processes of humification slowed down compared to unplowed soils. To increase soil fertility and carbon sequestration potential, it is necessary to stop degradation processes and implement conservation tillage practices. Full article

The key to soil organic matter management is understanding the transformation processes of fresh organic matter to permanent humus that control soil organic carbon dynamics. This paper presents the results of the effect of long-term soil fertilization with increasing doses of (manure) FYM and ammonium nitrate on the composition of humus compounds not limed of loamy sand. The study showed that both crop rotation and manure fertilization significantly affected organic matter fractions and humus humification index. The soil in crop rotation B (with enriching effect) accumulated significantly larger amounts of soil organic carbon in 0–30 cm layer despite a lower soil pH, but had lower soil organic quality compared to the soil in crop rotation A (with depleting effect). In both rotations, FYM was applied once per 4-year rotation at a dose of 40 t·ha⁻¹ which improved the soil organic carbon content. Depleting crop rotation (A) showed no increase in soil accumulation of humins fractions (H), while the “enriching” crop rotation (B) had such a tendency. Mineral nitrogen fertilization significantly affected the composition of the humic acids fraction. The soil in crop rotation B was characterized by a significantly higher content of fulvic acid (FA) and lower of humins (H) carbon fraction. The large input of organic matter into the soil under lower content pH conditions may hinder its transformation into permanent humus. Changes in the humification index were the consequence of changes in the carbon content of humic and fulvic acids, under the influence of plant selection in crop rotation and manure fertilization. According to the criterion of this parameter, soil in crop rotation A and soil fertilized with manure had lower loss of the more labile FA fraction (lower value of humification index—1.48). Full article

The investigation of the mechanisms organic matter transformation in compost organic fertilizers is an urgent task of modern soil ecology and soil chemistry. The main components of such fertilizers are newly formed, weakly humified labile humic acids (HAs). The objective of the study is to determine the mechanism of converting the newly formed HAs into the forms with increased resistance to microbiological and biochemical influences. Obtained during the plant residues decomposition, HAs were studied in the incubation experiment (0, 30, 90 days). Calcium carbonate and biochar produced by rapid pyrolysis from birch and aspen wood, at 550 °C, were used as the composting mixture compounds. Decomposed plant residues—fresh aboveground mass of clover (Trifolium pratense L.), rye (Secale cereale L.), as well as dry oat straw (Avena sativa L.) were the material used for humification. To obtain Has, 0.1 M NaOH and 0.1 M Na₄P₂O₇ were used. Then, HAs were separated from fulvic acids (FAs) using a 0.5 M H₂SO₄. The amount of labile HAs (HAs1) was estimated by their content in 0.1 M NaOH. The amount of stabilized HAs (HAs2) was calculated by the difference between the HAs content in 0.1 M Na₄P₂O₇ and 0.1 M NaOH. Preparation of HAs for elemental composition and NMR analysis was performed according to the International Humic Substances Society’s recommendations. The possibility of converting newly formed HAs into stable forms (calcium humates), whose share in the HAs composition reaches 40–50%, has been shown. However, the mechanism of HAs transformation under the studied reagent’s influence was different. In the presence of calcium carbonate, it is caused by the physicochemical processes of newly formed HAs rearrangement. However, in the presence of biochar, this is due to the humification processes’ intensification and to the increase in the aromatization degree confirmed by the increase in the optical density, as well as by the increase in carbon and oxygen proportion, and by the decrease in hydrogen proportion in HAs molecules. The understanding of HAs formation and transformation mechanisms at the early humification stages can help to optimize the methods of obtaining organic fertilizers. Full article

The Maillard reaction is a type of nonenzymic browning process, and it is also an abiotic humification process of sugars, amino acids and phenols catalyzed by δ-MnO₂. It is considered to be one of the possible pathways for the formation of humic-like substances (HLS). The change in the ratio of the Maillard precursors inevitably affects the chemical characteristics of HLS, among which the effect of amino acids concentration on the humification pathway and HLS formation has not yet been reported. In view of this, the glucose, glycine and catechol were chosen as tested objects for the present study, and the method of liquid shake-flask culture was adopted. Both catechol and glucose with fixed concentrations were added into a phosphate buffer solution (pH 8.0) containing δ-MnO₂, and only the glycine concentration was adjusted in the sterile culture system. The supernatant solution and dark-brown residue were collected dynamically within 360 h through the centrifugation method. The E₄/E₆ ratio and total organic C (TOC) of the supernatant solution, the C content of the humic-like acid (C_HLA), C_HLA/C_FLA (C content of fulvic-like acid) ratio and FTIR spectra of the dark-brown residue and the E₄/E₆ ratio and atomic ratio of humic-like acid (HLA) extracted from the dark-brown residue were systematically analyzed to reveal the effect of different glycine concentrations on the abiotic humification pathways and the characteristics of related products from the Maillard reaction under abiotic processes. The results showed that (1) Under the influence of the addition of different glycine concentrations, the structure of organic molecules in the supernatant after culture tended to be simplified, and the addition of three lower concentrations (0, 0.03 and 0.06 mol/L) of glycine made the E₄/E₆ ratios increase by 100.4%, 57.7% and 33.0%, respectively, and obtained a simpler structure of organic molecules in the supernatant than that of 0.12 and 0.24 mol/L glycine, which made the E₄/E₆ ratios increase by 5.6% and 18.0%, merely. After culture, the TOC content in the supernatant solution of each treatment decreased to varying degrees, and the addition of Maillard precursors effectively inhibited the loss of TOC in the supernatant solution, especially the addition of glycine at a concentration of 0.06 mol/L, which only reduced the TOC content by 0.1%. (2) The greater the concentration of glycine added, the higher proportion of aromatic C structure existed in the dark-brown residue. O-containing functional groups from the dark-brown residue and δ-MnO₂ were bound to each other through hydrogen bonding, and (3) During the culture process, the C_HLA treated with the addition of Maillard precursors was significantly higher than that of the CK control. Compared with the result at 0 h, the addition of higher concentrations of glycine (0.12 and 0.24 mol/L) were more conducive to the formation of HLA, making the C_HLA increase by 666.2% and 422.7%, which were much more than these results for 256.6%, 282.2% and 360.0% from three concentrations of glycine (0, 0.03 and 0.06 mol/L) at the end of culture. After culture, the structure of HLA molecules treated by the addition of Maillard precursors became more complex, and the overall performance showed that the higher the concentration of added glycine, the more complex the HLA molecules became. Under the coexistence of glucose and catechol, the addition of glycine could promote abiotic condensation and improve the condensation degree of HLA molecules, among which the addition of 0.12 mol/L glycine had the most significant effect. Compared with the CK control, the addition of Maillard precursors could achieve a higher increase in the C_HLA/C_FLA ratio, which was more beneficial to the improvement of humus quality. Full article

The purpose of our research is focused on examination of the transformation regularities of molecular composition of humic acids (HAs) in the hummocky frozen peatlands of the European Arctic as a marker of climatic changes in the Holocene, and assessment of the stabilization of soil organic matter under the conditions of modern climatic warming. Histosols located in the two subzones of the European Arctic served as the research subjects. This territory is actively used for reindeer breeding, which is a vital agricultural branch in the Far North of the Russian Federation. The data obtained reveal the main trends in the formation of HAs from Arctic peatlands under different environmental conditions. Modern peat sediments (top layers) in the middle and late Holocene period formed out of bryophyte residues and contained HAs with long-chain carbohydrate and paraffin structures in their composition. These structures enlarged the dynamic radii of HA molecules, and, thus, caused high average molecular weight values. The more favorable climatic conditions of the early Holocene (the Atlantic optimum) defined the botanical composition of peat, which was dominated by tree and sedge communities with high contents of lignin components and, as a consequence, a larger share of aromatic fragments, characterized by thermo-biodynamic resistance in HAs of horizons in the lower and central profile parts. The molecules of HAs are an archive of paleoclimatic records. The Subboreal and Subatlantic climatic conditions determined the specifics of vegetation precursors and, as a result, the molecular structure of HAs in seasonally thawed layers, with a predominance of long-chain aliphatic fragments. The conversion of HAs from Histosols led to an increase in the proportion of carbon in branched and short-chain paraffinic structures with their subsequent cyclization and aromatization. The results of this process are most clearly manifested in layers formed during the Holocene I and II climatic optima. Higher biologically active temperatures of the seasonally thawed layer of soils at bare spots (without vegetation) determined the accumulation of thermodynamically more stable HA molecules with a high content of aromatic fragments. This contributed to both the stabilization of the SOM and the conservation of peatlands in general. Full article

The possibility of using brown algae in agriculture as an alternative source of nutrients is currentlyunder study and discussion. Our study aimed to evaluate the effect of F. vesiculosus on the agrochemical properties of four soil types: Retisol loamy sand soil, Retisolloam, Retisol clay, and Histosol. The F. vesiculosus waste was added to soil samples at a rate of 0, 0.5, 1.0, 2.0, 5.0, and 10 wt%. The brown algaewaste application significantly decreased soil acidity in the substrates of all soil types, with the larger increasesfor Retisol loamy sand and Retisol clay than for Retisol loam and Histosol. The application of F. vesiculosus waste products increased the C content in all soil types except Histosol. The N and P content in soil substrates were not significantly affected by algaewaste application regardless of soil type. This study showed that the effect of F. vesiculosus waste application varies depending on the soil type, with the strongest impact on Retisol clay and the lowest on Histosol. Full article

The boreal forests of Russia are one of the largest forest areas on the planet. As a result of climate change, the rate of carbon sequestration and stabilization of organic matter are important indicators of environmental conservation. To understand mechanisms of stabilization and the structure of soil organic carbon, the molecular-weight (MW) distribution of humic acids (HAs) in soils of the central deciduous-forest zone of boreal forests (Chernevaya taiga, transitional ecotone forest, coniferous forest) was studied. Analysis of the MW distribution of HAs was conducted using size-exclusion chromatography. This approach allowed us to obtain the distribution of high-, medium-, and low-molecular fractions of HAs. It has been revealed that the content of the low-MW fraction prevails over the content of the medium- and high-MW fractions of HAs, which indicates the process of stabilization in soil organic matter. The accumulation of low-molecular-weight fraction occurs with the aromaticity increase in HAs, which indicates that HAs with a relatively high proportion of aromatic fragments have smaller hydrodynamic radius and a lower MW. It has been statistically substantiated that the low-molecular-weight fraction correlates with the content of aromatic compounds and carboxyl structural fragments of HAs, which indicates the resistance of the soil organic matter of Chernevaya taiga to biodegradation. Full article

The influence of different fertilizers (mineral/organic) on the quantity and quality of soil organic matter was monitored in long-term stationary experiments (27 years) with silage maize monoculture production on Luvisol. The main aim of this study was to investigate the relationship between easily extractable glomalin (EEG), total glomalin (TG), and parameters commonly used for the determination of soil organic matter quality, i.e., the content of humic acids (C_HA), fulvic acids (C_FA), and potential wettability index (PWI). A significant correlation was found between EEG content and C_SOM content, humic acid content (C_HA), humic acid/fulvic acid ratio (C_HA/C_FA), PWI, and index of aromaticity (IAR). Furthermore, the contents of EEG and TG correlated with soil organic carbon (C_SOM). Periodical application of sewage sludge and cattle slurry increased the content of glomalin in soils. From the results, it is obvious that data about glomalin content can be used to study soil organic matter quality. A more sensitive method (a method that reacts more to changes in components of soil fertility) seems to be the determination of EEG rather than TG. The factors supporting use of EEG extraction in agronomic practice are mainly the substantially shorter time of analysis than TG, C_HA, and C_FA determination and lower chemical consumption. Furthermore, the PWI method is even suitable for studying soil organic matter quality. On the other hand, the humus quality ratio (E4/E6) does not provide relevant information about soil organic matter quality. Full article

Increasing agricultural productivity and, in particular, the productivity of livestock is one of the primary tasks in the present stage of development of society. This involves rational feeding and the use of biologically active substances, including humic preparations that activate the digestive and metabolic processes in the animal, promoting the transformation of feed nutrients in assimilable form, raising daily milk production and weight gain, and can play an important role in solving this urgent problem. The applications of humic substances in animal husbandry are diverse, but their use as feed additives is not developed sufficiently, and in the EU countries it is not sanctioned. Researchers in different countries have shown that humic substances in animals operate on the cellular and subcellular level, as they do in plants. Low molecular weight fractions HS enter the cell and are involved in metabolic processes, contributing to the optimization of inorganic ion passage through the intestinal wall, thereby promoting the absorption of minerals necessary for normal functioning of the organism. That is how the stimulating influence of humic substances on separate systems and on the organism as a whole is manifested. Humic substances formed from various natural materials are currently being tested in various branches of animal husbandry (cattle, pig breeding, poultry farming, fish farming, fur farming), and all the information received presents convincing evidence of the high efficiency of the humates. Such materials as peat and brown coal, vegetable waste, and vermicompost can act as a source of humic substances. However, in each case, more research is needed, specifying the dosage and schedule of their use. Full article

Humic substances of soils consist of various organic compounds, including polycyclic aromatic hydrocarbons (PAHs). Soil as a fairly stable medium allows the correct use of polyarenes as markers of the humus formation process. Monitoring of the accumulation of PAHs as resistant organic toxicants is also necessary due to their carcinogenic and mutagenic properties. Natural plant resources serve as the feed base of northern reindeer husbandry. In this study, high-performance liquid chromatography in a gradient mode and gas chromatography-mass spectrometry methods were used to estimate the content of PAHs in mountain tundra and meadows of the Polar Urals (Russia). The accumulation of polyarenes in soils on carbonate rocks of the Bolshoi Paipudynsky ridge occurs mainly in the process of soil formation and largely depends on factors such as productivity of plant communities, the composition of standing biomass, the site’s position in relief, the granulometric composition of soils, cryogenesis process and pyrogenesis. According to the set of polyarenes, their number, and ratio, the studied objects were classified into separate groups by discriminant analysis. The most typical representatives of pedogenic origin are naphthalene and phenanthrene. The accumulation features of dibenz[a,h]anthracene and benz[b]-, benz[k]fluoranthene, benz[ghi]perylene, and benz[a]perylene are shown. In mountain tundra landscapes, the characteristics of PAHs can be used to diagnose the intensity and direction of soil formation processes in general and humification in particular. Full article

Farmland is one of the most important and active components of the soil carbon pool. Exploring the controlling factors of farmland soil organic carbon density (SOCD) and its sequestration rate (SOCDSR) is vital for improving carbon sequestration and addressing climate change. Present studies provide considerable attention to the impacts of natural factors and agricultural management on SOCD and SOCDSR. However, few of them focus on the interaction effects of environmental variables on SOCD and SOCDSR. Therefore, using 64 samples collected from 19 agricultural stations in China, this study explored the effects of natural factors, human activities, and their interactions on farmland SOCD and SOCDSR by using geographical detector methods. Results of geographical detectors showed that SOCD was associated with natural factors, including groundwater depth, soil type, clay content, mean annual temperature (MAT), and mean annual precipitation. SOCDSR was related to natural factors and agricultural management, including MAT, groundwater depth, fertilization, and their interactions. Interaction effects existed in all environmental variable pairs, and the explanatory power of interaction effects was often greater than that of the sum of two single variables. Specifically, the interaction effect of soil type and MAT explained 74.8% of the variation in SOCD, and further investigation revealed that SOCD was highest in Luvisols and was under a low MAT (<6 °C). The interaction effect of groundwater depth and fertilization explained 40.4% of the variation in SOCDSR, and fertilization was conducive to SOCD increase at a high groundwater depth (<3 m). These findings suggest that low soil temperature, high soil moisture, and fertilization are conducive to soil carbon accumulation. These findings also highlight the importance of agricultural management and interaction effects in explaining SOCD and SOCDSR, which promote our knowledge to better understand the variation of SOCD and its dynamics. Full article

Humic acids (HAs) regulate soil chemical reactivity and improve many soil functions. The amendment of soil with organic materials increases soil organic matter (SOM) content and promotes the formation of HAs. However, the effect of the type, frequency and duration of amendment, and pedoclimatic conditions on SOM transformation and HA structural changes remains unclear. Herein, four experimental field sites (S1–4) with short-to-long-term organic fertilisation schemes were used to assess the effects of such factors, i.e., S1: loamy sand amended once with farmyard manure (FYM), brown coal waste (BCW), and biochar (BIO) for 0.5 and 1.5 years; S2: silt loam amended once with BIO for 8 years; S3: loamy sand amended every 5 years with FYM for 94 years; and S4: clayey silt amended every 2 years with FYM for 116 years. All HAs were extracted and analysed for structural differences by elemental analysis (EA), attenuated total reflectance–Fourier transform infrared spectroscopy (ATR-FTIR), solid-state cross polarisation magic angle spinning nuclear magnetic resonance spectroscopy (CP/MAS ¹³C-NMR), and differential scanning calorimetry (DSC). Results from EA, FTIR, and NMR showed that the long-term samples from S3 (treatments, T9–T10) and S4 (T11–T12) had the greatest aromatic characteristics, which increased with FYM amendment (T10 and T12). These agreed with DSC data, which indicated lower aliphatic contents compared with other samples. Samples from S2 (T7–T8), with receded amendment effects, had less aromatic and greater aliphatic characteristics compared with the short-term samples, S1 (T1–T6). In S1, structural changes were limited, but aromaticity increased with BIO (T3 and T6) compared with corresponding FYM (T1 and T4) and BCW (T2 and T5) amendments due to inherently high aromatic groups in the former. Overall, the results showed that the site (due to differences in pedoclimatic conditions), field age of OM, and amendment frequency were the main factors that influenced HA structure, and hence SOM transformation. Regular, long-term organic amendment increases the aromatic characteristics of HAs, which can improve soil functionality, but short-term structural improvements are achievable only when amending material is rich in aromatic compounds. Full article

For many years and all over the world, agricultural production has been observed to slow down on low-quality soils in territories featuring difficult topography and poor spatial structure (land fragmentation, excessive elongation of plots, plots without direct access to public roads, and land scattering). This paper proposes a new, self-designed algorithm for delimiting rural areas that allows the clustering of villages featuring low soil productivity, based on three factors used for determining the overall value of the area of land (Wcag), i.e., the overall area of the village (ha), share of specific type of land in the overall area of the village (%), and mean score for specific soil type (pts.), which allows the villages to be grouped according to classes of land occurring in the examined district. The results of the surveys provide a basis for further detailed studies into efficient management of areas featuring low soil classes during land consolidation works. Further surveys will involve a detailed analysis of the identified clusters of villages to ensure that their potential is used to the optimum extent. As a consequence, these areas will potentially become more competitive and operations conducted there will be beneficial to the local inhabitants and contribute to improving their living standard. Full article

The most important component of agricultural system are soils as the basis for the growth of plants, accumulation of water, plant nutrients and organic matter. The main task of our research was to ascertain changes in soil organic carbon (SOC) and mobile humified carbon fractions in digestate-treated soils. We have performed three field experiments using the same design on two soil types in 2019–2020. We studied the fertilization effects of different phases of digestate on Retisol and Fluvisol. Fertilization treatments: control; separated liquid digestate 85 kg ha⁻¹ N; and 170 kg ha⁻¹ 170 N; separated solid digestate 85 kg ha⁻¹ N; and 170 kg ha⁻¹ N. We have found a greater positive effect on the increase in SOC because of the use of the maximum recommended fertilization rate of the solid digestate. The content of mobile humic substances (MHS) tended to increase in grassland and crop rotation field in digestate-treated soil. In our experiment, maximum concentration of SOC was found in 0–10 cm soil layer, while in the deeper layers the amount of SOC, MHS and mobile humic acids proportionally decreased. We concluded, that long-term factors as soil type and land use strongly affected the humification level expressed as HD (%) in the soil and the highest HD was determined in the grassland soil in Fluvisol. Full article

Agricultural soil use does not only affect the amount of soil organic matter, but also the molecular composition of humic (HAs) and fulvic acids (FAs). Changed hydrothermal conditions and composition of the incoming plant residues are reflected in the rate of humification and its products. The objective of this study was to compare the molecular composition of HAs and FAs isolated from Eutric Albic Stagnic Histic Retisol (Loamic), two Eutric Albic Retisols (Loamic)—mature and arable. Plots of mature Retisols are located at a middle taiga (Komi Republic, Russia) in a bilberry-green-moss birch-spruce forest. The plot of Retisols arable is located in the fields of the Syktyvkar state farm, which is 3.3 km northeast of mature Retisol plots. The development period is about 40 years, it is sowed with a pea-oat mixture. The results obtained indicate that soil reclamation essentially increases the proportion of aromatic components and decreases the content of carboxyl and ester groups in the humic substance (HS) structure. An increased extent of hydromorphism of Retisols leads to the enrichment of HS with aliphatic fragments. Full article

Soil samples from T (0~20 cm) and S (20~40 cm) layers of four saline–alkali rice fields (R5, R15, R20, and R35) with different reclamation years were selected to study the distribution of soil aggregates and the contents of readily oxidizable organic carbon (ROC), dissolved organic carbon (DOC), microbial biomass carbon (MBC), potentially mineralizable carbon (PMC), and soil organic carbon (SOC). The effects of large macroaggregate (>2 mm, LMA), small macroaggregate (0.25 to 2 mm, SMA), and microaggregate (<0.25 mm, MA) particle size, soil layer, and soil physicochemical properties on SOC fractions were also analyzed. The results showed that the LMA size in saline–alkali paddy fields were easily decomposed and was unstable due to the influence of the external environment. With the increase in reclamation years, the proportion of LMA in the S layer decreased gradually. The ROC, DOC, MBC and TOC contents of aggregates in the T and S layers gradually increased with the increase in reclamation years, and SOC fractions contents of aggregates in different grain sizes were SMA > LMA > MA. An effective way to increase carbon sink and improve the ecological environment in western Jilin Province is to change the soil environment by planting rice in saline–alkali land. Full article

Five sample-preparation techniques were compared to quantify 31 elements in coal humic substances (HS) by ICP–AES from the viewpoints of complete isolation and speciation of elements. They include, for bulk composition, preparation of an aqueous colloidal HS solution followed by direct injection of the sample without decomposition and ashing followed by metaborate fusion; for element speciation, preparation of an aqueous colloidal HS solution followed by centrifugation and direct analysis without decomposition for water-soluble species; treatment with boiling nitric acid; and microwave-assisted treatment with nitric acid at 250 °C for acid-isolated species. The results of analysis significantly depend on the selected method of sample preparation due to specific features of HS, the simultaneous presence of many inorganic components in wide concentration ranges, and a significant fraction of the organic matrix; therefore, the total mineral composition of HS, both macro- and microcomponents, requires a combination of decomposition methods. Full article

The interaction of organic matter with mineral components of the solid phase of soils is the most important process that regulates the cycle and balance of carbon in the biosphere. The adsorption of humic acids on minerals is accompanied by their fractionation in size, composition, and amphiphilicity, thus decreasing their heterogeneity. Despite a strong interest in studying the regularities and mechanisms of the interaction between natural organic matter and layered aluminosilicates, it is necessary to take into account the natural diversity of soil organic matter, adsorption conditions, and mineral composition. This study was designed to investigate the adsorption regularities of fulvic acid (FA) and water-extractable organic matter (WEOM) isolated from horizon H of peaty-podzolic-gleyic soil on kaolinite and muscovite. Sorbates and sorbents were examined by the following methods: high-pressure size exclusion chromatography (HPSEC), high-performance liquid chromatography (HPLC), and potentiometric titration. The specific surface areas of the sorbents were determined by the sorption of N₂ molecules. We found that hydrophobic components of FA and WEOM are mainly adsorbed on mineral surfaces. The adsorption of FA and WEOM on kaolinite and muscovite is followed by decreased hydrophobicity of organic matter and decreased heterogeneity of its amphiphilic properties in an equilibrium solution. At pH levels around 6, sorption of organic matter from FA solution containing 19% and 81% hydrophilic and hydrophobic components, respectively, onto kaolinite and muscovite occurs mainly due to hydrophobic components. Hydrophobic interactions on siloxane surfaces are the main mechanism to fix FA on both minerals. Kaolinite adsorbs slightly more organic carbon per unit area than muscovite. The adsorption of WEOM from a solution with 41% hydrophilic and 59% hydrophobic components results not only from hydrophobic and hydrophilic components but also from hydrophobic and electrostatic interactions and depends on pH. The most hydrophobic fractions of organic matter are adsorbed from the hydrophobic components on the surface of both minerals. Under conditions of the performed experiments at pH < 5, more WEOM is adsorbed on muscovite than on kaolinite. Full article

Soil cover of the Earth is faced with intensive polychemical contamination. The pathways of the key pollutants are not investigated enough. Thus, the occasional transportation of soils to remote regions could serve as an informative tool for the elaboration of threshold levels of hazardous materials concentration. One of the most striking examples of such transboundary impact was the transfer of soils and grounds to the Antarctic stations Russkaya and Leningradskaya (before the implementation of the Madrid Protocol in 1991). Thus, the complex investigation of qualitative and quantitative composition of polycyclic aromatic hydrocarbons (PAHs) in soils of various genesis (transported from Eurasia and pristine) of Antarctic have been conducted by the method of high-performance liquid chromatography (HPLC) in a gradient elution mode. The variability of PAHs content was evaluated across landscapes: pristine (the Hudson Mountains and the Haswell Archipelago), contaminated soils (stations Myrniy, Druzhnaya 4 and Bellinshausen) and unique samples of former agrosoils transported by fly in-fly of polar staff from St. Petersburg to Antarctic for local polar greenhouses in the Soviet times, when it was not strictly prohibited. The selected objects of study allow us to not only estimate the degree of contamination of Antarctic soils and grounds, but to also make it possible to assess the rate of PAH degradation in Antarctic conditions. Both high molecular and low molecular PAHs are accumulated following intense anthropogenic activity (fossil organic fuel combustion). The PAHs pool is dominated by low molecular weight representatives (naphthalene, phenanthrene, fluoranthene, pyrene). In most cases, the highest concentrations of benz(a)pyrene does not exceed the Russian Threshold Standard rate, which is the strictest one in the world. The statistical analysis of raw data allowed us to conclude that the contamination of pristine soils of Antarctica across variable landscapes is at the very initial stage. However, we recorded extremely high levels of PAHs in the transported former agrosoils. We can assume that our data could be used as background levels for the elaboration of threshold concentrations of the PAHs for such an internationally managed region as Antarctica. Full article

Biochemical stability of soil humus is an important factor affecting soil quality. Fungi are among the most efficient decomposers of humic matter due to presence of oxidative enzymes, including phenoloxidase laccase. Production of laccase by zygomycetes, a group of cellulolytic fungi widespread in soil, is poorly studied. The potential role of laccase from zygomycetes in humus turnover is unknown. Here, we show for the first time that laccase of zygomycetous fungus Mortierella elasson can effectively depolymerize humic acids in vitro. The fungus produced laccase extracellularly in a liquid culture medium. Unlike in case of laccases in ligninolytic basidiomycetes, attempts to increase enzyme activity using inductors, changes in the source of nitrogen and carbon failed to lead to any increase in laccase production. Laccase was purified using ion exchange chromatography and gel filtration. The molecular weight of the laccase was 51.75 kDa. The laccase catalyzed the oxidation of ABTS and K₄[Fe(CN)₆], phenolic compounds, but not tyrosine. The laccase activity was inhibited by NaN₃ and NaF. The pH optimum of the laccase activity was 3.0 for ABTS and 5.0 for 2,6-dimethoxy phenol. The enzyme had moderate thermal stability and was rapidly inactivated at 70 °C. Purified laccase depolymerized humic acids from retisol, compost and peat more effectively than culture liquid containing laccase. The results of the study extend our knowledge of the role of laccases from different producers in the transformation of natural organic matter. Full article

The article presents the results of a 3-year experiment to study the soil structure with the combined use of insecticides and a humic preparation. Crops were peas (Pisum sativum) and chickpeas (Cicer arietinum). The experiment was carried out in 2019–2021 at the experimental fields of the Federal Rostov Agrarian Scientific Center, located in the Rostov region in the south of the European part of Russia (47°21′ N, 39°52′ E). Soil was Calcic Chernozem Loamic. The studies were carried out both without fertilization and with the use of mineral fertilizers (N40P40K40). Sowings of leguminous crops were treated with a mixture of insecticides and a humic preparation in the phase of bean formation. The humic preparation BIO-Don10 was obtained according to the authors’ technology by alkaline extraction from vermicompost—a product of manure processing by compost worms of the Eisenia foetida species. The preparation contains salts of humic acids and fulvic acids in the amount of 1.9 g/L; the concentration in the mixture with the insecticide was 0.002%. An increase in the structure and water resistance coefficients was observed. A decrease in the block fraction and an increase in the total number of agronomically valuable aggregates were substantiated. It was found that in the cultivation of legumes, the application of mineral fertilizers had a negative effect on the structure of the soil, while, when using pesticides, the structure was preserved better by the end of the growing season. The revealed patterns can be associated with the activity of microorganisms participating in the hydrolysis of organic compounds that stabilize soil aggregates. Full article

Redox moieties, which are present in the molecular backbone of humic substances (HS), govern their antioxidant properties. We hypothesized that a directed modification of the humic backbone via incorporation of redox moieties with known redox properties might provide an efficient tool for tuning up antioxidant properties of HS. In this work, hydroquinonoid and hydronaphthoquinonoid centres were used, which possess very different redox characteristics. They were incorporated into the structure of coal (leonardite) humic acids CHA) and peat fulvic acids (PFA). For this goal, an oxidative copolymerization of phenols was used. The latter was induced via oxidation of hydroquinones and hydroxynapjtaquinones with a use of Fenton’s reagent. The structure of the obtained products was characterized using NMR and FTIR spectroscopy. H/D labelling coupled to FT ICR mass spectrometry analysis was applied for identification of the reaction products as a tool for surmising on reaction mechanism. It was shown that covalent -C-C- bond were formed between the incorporated redox centers and aromatic core of HS. The parent humic acids and their naphthoquinonoid derivatives have demonstrated high accepting capacity. At the same time, fulvic acids and their hydroquinonoid derivatives have possessed both high donor and high antioxidant capacities. The kinetic studies have demonstrated that both humic acids and their derivatives showed much slower kinetics of antioxidant reactions as compared to fulvic acids. The obtained results show, firstly, substantial difference in redox and antioxidant properties of the humic and fulvic acids, and, secondly, they can serve as an experimental evidence that directed chemical modification of humic substances can be used to tune and control antioxidant properties of natural HS. Full article

Russia holds the largest store of carbon in soils, forests and permafrost grounds. Carbon, stored in a stabilized form, plays an important role in the balance of the global biogeochemical cycle and greenhouse gases. Thus, recalcitrance of soil organic matter to mineralization results in a decrease in current emissions of carbon dioxide into the atmosphere. At the same time, stabilization of organic matter in the form of humus due to organo–mineral interactions leads to the sequestration of carbon from the atmosphere into soils and biosediments. Thus, global carbon balance is essentially determined by soil cover state and stability. Currently, Russia is faced with a set of problems regarding carbon offsets and the carbon economy. One of the methods used to evaluate carbon stocks in ecosystems and verify offsets rates is carbon polygons, which are currently being organized, or are under organization, in various regions of Russia. This discussion addresses the current issues surrounding the methods and methodology of carbon polygons and their pedological organization and function. Full article

The soils of Chernevaya taiga (tallgrass fir-aspen hemiboreal rainforest) have high fertility in comparison with oligotrophic analogs formed in boreal taiga. We have studied humic acids isolated from the soils of Chernevaya and oligotrophic taiga in the Novosibirsk, Tomsk, Kemerovo and the Altai regions of Russia and for the first time the structural and molecular composition of humic acids was determined using ¹³C CP/MAS and ¹H-¹³C HETCOR NMR spectroscopy. According to data obtained in this study, up to 48% of aromatic compounds accumulate in the soils of Chernevaya taiga, which is higher than in the oligotrophic taiga and comparable with this rate of steppe Chernozems. In the course of active processes of transformation of organic matter, a significant number of aromatic fragments accumulates in the middle horizons of soil profiles. Using ¹³C CP/MAS spectroscopy, it was possible to identify the main structural fragments (aliphatic and aromatic) that formed in humic acids of the Chernevaya taiga. The HETCOR experiment made it possible to accurately determine the boundaries of chemical shifts of the main groups of structural fragments of humic acids. Our results demonstrate that the stabilization of organic compounds occurs in the soil of the Chernevaya taiga, which leads to the resistance of organic matter to biodegradation that is not typical for benchmark soils of boreal environments. Full article

This study aims to compare photoacoustic (FTIR–PAS), diffuse reflectance (DRIFT), and attenuated total reflection (ATR) FTIR modalities in the wide wavenumber range from NIR (7500 cm⁻¹) to FIR (150 cm⁻¹) for the same silicate soil samples under the same conditions. The possibilities of non-destructive rapid qualitative analysis of soils by these modalities without comprehensive data treatment were compared. The assignment of more than 100 bands for the chernozem and sod-podzolic as common types of silicate types of soil was made. The following groups of bands of organic matter and inorganic matrix were reliably found in spectra of all or at least two modalities: 3690–3680 cm⁻¹ (hydrogen-bonded SiO–H…H₂O stretch, not ATR), 2930–2910 cm⁻¹ and 2860–2850 cm⁻¹ (methylene stretch), 1390–1380 cm⁻¹, (symmetric stretch carboxylate, DRIFT and FTIR–PAS); 2000–1990 cm⁻¹, 1885 cm⁻¹, and 1790–1783 cm⁻¹ (SiO₂ overtones, DRIFT and FTIR–PAS), 1163–1153 cm⁻¹, SiO₂ lattice (not FTIR–PAS), 1037 cm⁻¹ (Si–O or Al–O stretch), 796 cm⁻¹ (lattice symmetrical Si–O–Si stretch); 697 cm⁻¹, SiO₂; and 256 cm⁻¹ (not FTIR–PAS). Amide I, II, and III bands appear in DRIFT and FTIR–PAS spectra while not in ATR. Except for methylene and carboxylate groups, CH vibrations (3100–2900 cm⁻¹) are not seen in ATR. Bands at 1640–1630 cm⁻¹, 1620–1610 cm⁻¹, 1600–1598 cm⁻¹ (primary water bands and probably carboxylate) appear in the spectra of all three modalities but are unresolved and require data treatment. It is preferable to use all three modalities to characterize both soil organic matter and mineral composition. DRIFT provides the maximum number of bands in all three modalities and should be selected as a primary technique in the NIR and 4000–2000 cm⁻¹ regions for hydrogen-bonding bands, CH_X groups, and the silicate matrix. ATR–FTIR complements DRIFT and provides a good sensitivity for soil water and the matrix in 2000–400 cm⁻¹. FTIR–PAS in 4000–1500 cm⁻¹ reveals more bands than DRIFT and shows the highest sensitivity for absorption bands that do not appear in DRIFT or ATR-IR spectra. Thus, FTIR–PAS is expedient for supporting either DRIFT or ATR–FTIR. This modality comparison can be a basis for methodological support of IR spectroscopy of soils and similar organomineral complexes. Full article

The capabilities of temperature-monitored IR spectroscopy for studying the organic matter and mineral composition of humic substances (HS) were tested. Temperature dependences of the mid-IR spectra of humic substances heated in the air in the range 25–215 °C (298–488 K, with a step of 2.5 °C)—for three commercially available samples isolated from brown coal (leonardite)—were performed. The characteristic bands were identified, and their changes in band maxima positions and intensities were compared. From the viewpoint of interpretation of HS components, the spectra were divided into regions of quartz lattice region (800–260 cm⁻¹), quartz overtone region (1270–800 cm⁻¹), humic substance organic matter region (1780–1270 cm⁻¹), quartz combination region (2800–1780 cm⁻¹), CH-speciation region (3100–2800 cm⁻¹), and hydrogen-speciation region (4000–3100 cm⁻¹) thus selected to contain the dominating type of bands. For the first time, a reversible change in the frequencies of the band maxima in IR spectra upon heating was observed, which can be interpreted as forming structures with a particular order in the studied humic substances in the dry state. For a single sample, both the band-shift scale and the functional dependence of the various bands on temperature differ significantly. The approach differentiates crystalline quartz bands, amorphous silica, and HSOM/surface groups experiencing a different temperature behavior of the band maxima and their intensities. Band-maximum temperature dependence can be considered more stable to changes in experimental conditions than band maxima at a single temperature, thus providing a more detailed HS structure analysis without HS decomposition or destruction. Full article