The term humic substances (HS) refers to a group of heterogeneous, dark-colored polydisperse substances found in soils, peats, natural waters, and sediments. It is hypothesized that HSs are formed as a result of degradation and transformation of biomolecules of organic residues and free radical condensation reactions (a process called humification). Humic substances are classified as a special category of natural compounds that are dissimilar to the biomolecules of plant and microbial tissues, and are resistant to biodegradation. On the basis of their solubility in alkalis, HSs are divided into humic acids (HAs, soluble, precipitated at pH < 2), fulvic acids (FAs, soluble at all pH values), and humin (insoluble residue). The review provides a critical analysis of the HS terminology and nomenclature, the method of extraction of HS from natural objects, and the hypotheses describing their formation. The ambiguity and duality of the concept of HSs (specific compounds and the sum of operational fractions) are demonstrated, as well as an arbitrary character of the division of organic matter (OM) into the dark-colored compounds of poorly defined structure (HSs) and the substances with a known structure (non-HSs). The interpretation of HSs as mass products of a secondary synthesis requires revision. The possibility of extracellular free radical reactions in soils leaves no doubt; however, a quantitative contribution of the corresponding products to the natural OM has not been established yet. The traditional alkaline extraction should be considered as a method for isolation of hydrophilic polar substances, while the precipitation with acid as their concentration for further analysis. The historical names of humic fractions (HAs, FAs, and humin) should be retained as well-established generic terms and names of the preparations obtained in a certain way without attaching unique features and specificity to all components of these fractions. The C_HA/C_FA (or C_HA/C_org) ratios are simple and convenient indicators of the types of humus reflecting bioclimatic conditions of its formation.

Phenoloxidase activity was found in lichenized ascomycetes belonging to different taxonomic groups. Most of the epigeic and epilithic lichens of the order Peltigeraleswere found to possess both...

The quantity and the qualitative composition (for some species) of phenolic compounds (PC) washed out of the intact thalli of lichens of the orders Peltigerales (the genera Peltigera, Solorina, and Nephroma) and Lecanorales (the genera Cladonia, Alectoria, and Cetraria) were studied. It was shown that the quantity of leachable PCs in Peltigerales was on average 2–3 times higher than in Lecanorales. At the same time, the extractability of PC from intact thalli by water was higher in Lecanorales than in Peltigerales: 48–88% and 34–70%, respectively, of the PC content in ethanol extracts from crushed thalli (i.e., of the total content of soluble PC). Water-soluble PC in the lichens Peltigera aphthosa, Solorina crocea, Cetraria islandica, Flavocetraria nivalis, Cladonia uncialis, and Cladonia arbuscula were represented by 7–12 phenolic compounds with similar qualitative composition in the species of the same order. The most part of water soluble PC were phenylpropanoids. All of the studied species showed the presence of p-hydroxybenzoic acid derivatives; vanillic and protocatechuic acid derivatives were found in Cetraria and Cladonia species, respectively.

Humic acids (HAs) make up to 30–50% of the soil organic matter, which is the main reservoir of organic carbon in the biosphere. The common isolation protocol for HAs implies alkaline...

Soil organic matter (SOM) turnover by ligninolytic fungi is a large‐scale process that controls organic carbon geochemistry in terrestrial ecosystems. However, the role of certain oxidative enzymes (e.g., laccase) in humus degradation remains unclear, as well as the molecular structure and recalcitrance of SOM components. In order to address these questions, the degradation of forest soil humic acid (HA) in the presence of laccase and a versatile peroxidase (VP) has been studied in the liquid culture of Lentinus tigrinus. Contrary to the evolving views on humus structure, we have found that alkali‐extractable and acid‐insoluble constituents of SOM (HA) contain true macromolecular components, stable in the presence of 0.1% sodium dodecyl sulfate but degradable/resynthesizable by oxidative enzymes acting on covalent linkages. The HA degradation in the presence of laccase (high N medium) occurs at slower initial rate than in the presence of VP (low N medium). However, each of the enzymes caused about 60% color loss and almost complete degradation of HA into smaller molecules (gel‐filtration data) within 2 weeks of cultivation. Depolymerization of HA in the culture liquid in the presence of laccase was accompanied by polymerization of degradation products on mycelium. Our results show that (1) humus macromolecules are not stable to oxidative enzymes once desorbed from the mineral phase, (2) laccase of Lentinus tigrinus is comparable by its degradation potential to VP, and (3) interfacial secondary synthesis reactions occur during humus decay in the presence of laccase. Our results highlight the important role of laccases in SOM sequestration in soils.