Abstract
Soil organic matter (SOM) includes many classes of labile compounds available for microbial decomposition or, conversely, stable compounds protected from biodegradation by biological, chemical, and physical stabilization. It is believed that the more thermal energy is spent on the destruction of soil organic matter, the more stable and more resistant for biodegradation it is. We compared the thermal and biological stabilities of organic matter in eleven soil types from deciduous forest, forest-steppe, steppe, and semidesert bioclimatic areas of the European Russia. According to the activation energy (Ea), the highest SOM thermal stability was typical of the ordinary chernozem and meadow vertic soil. The lowest SOM thermal stability was found for gray forest soil; other soil types were characterized by an intermediate resistance towards thermal oxidation. The thermally labile pool (<390–400°C) of organic matter in soils was on the average 41% (32–60%) of the total SOM, while the thermally stable pool (>390–400°C) was on the average 59% (40–68%). The SOM biological stability estimated by the ratio of potentially mineralizable organic matter to that resistant to mineralization (biological stability index) decreased in the following order: ordinary chernozem (Haplic Chernozem (Loamic, Pachic)) > meadow vertic soil (Pellic Vertisol (Gleyic, Humic)) > gray forest soil (Luvic Greyzemic Phaeozem (Loamic)) = meadow chestnut soil (Gleyic Kastanozem (Chromic)) > meadow solonetz (Endosalic Gleyic Solonetz (Loamic, Cutanic)) > alluvial meadow soil (Eutric Fluvisol (Humic, Oxyaquic)). The potentially mineralizable SOM pool in the studied soils was 6–27-fold lower as compared with the pool of thermally labile SOM, and the parameters that characterize SOM thermal stability did not correlate with the biological stability index. Thus, SOM thermal lability is not identical to its biodegradability.
Highlights
Soil is a complex self-regulating multicomponent system represented by solid, liquid, gas, and living phases
The goal of this work was to compare the sizes of thermolabile, thermostable, and biologically active soil organic matter (SOM) pools isolated by derivatographic and biokinetic methods for the soils differing in their particle size distribution and humus-forming conditions and to clarify the degree to which the SOM thermostability characterizes its biological stability
The lowest ratio of mineral mass to organic matter is observed in the ordinary chernozem and meadow vertic soil (5 to 8) and the highest ratio, in the arable gray forest soil and the lower layers of humus horizon in the meadow solonetz and alluvial meadow soil (20 to 29)
Summary
Soil is a complex self-regulating multicomponent system represented by solid, liquid, gas, and living phases. Particulate matter representing the remains of biota, nonhumic biomolecules, and ensembles of humic substances stochastically adsorbed in a conglomerate of mineral particles constitute the solid part of soil organic matter (SOM) [5]. The biota remains and the products of microbial metabolism forming SOM have different initial and secondary chemical stabilities [9, 20, 24]. The initial stability of organic matter is determined by the properties of its constituent compounds that which differ in the ratio of elements, forms of molecules, and composition of functional groups. The secondary stability of organic matter emerges from its biological and chemical transformation in soil as a result of an increase in the share of lignin and polyphenols in the decomposed residues, synthesis of microbial metabolites of melanin and glomalin type, formation of humic substances, and charring [9]. It is traditionally believed that the higher the share of stable organic compounds in SOM, the higher is its stability [3, 9, 14, 29, 30, 46]
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