Content and structure of carbon stocks in coal-containing soils of technogenic landscapes: a case study from the Gorlovskoe anthracite deposit

The estimation of organic matter stocks in technogenic soils is a topic of active discussion. The significance of studying the carbon balance in natural landscapes is underscored by large-scale projects; however, there is an increasing acknowledgment of the necessity to incorporate technogenically altered areas into climate research. This need arises not only from their heightened emissions of greenhouse gases but also from their potential for long-term carbon sequestration through the implementation of advanced reclamation technologies. The aim of this study is to evaluate the carbon content, total stocks, and stock structure in coal-enriched soils found in the spoil heaps of coal mining enterprises. The objects of the study were the technogenic soils from the external transport spoil heaps of the Gorlovka coal deposit in Novosibirsk Oblast. The research was conducted in 2023 across ten representative sites of dumps with varying ages, heterogeneous relief conditions, rock types, vegetation, and soil characteristics (see Table 1). Soil samples were collected from layers at depths of 0-5, 5-10, and 10-20 cm. A single sample was obtained from the 0-20 cm layer of initial embryozems at dump sites less than four years old. In the laboratory, the samples were air-dried at room temperature. Subsequently, all samples were sieved through standard sieves for fractions of 1-3, 3-5, 5-10, and > 10 mm. Coal particles and large roots (remaining on sieves > 1 mm) were manually selected from separated sieved samples of each fraction and weighed. The total carbon content, the carbon content in coal particles, and the carbon content of large roots were determined using an automatic Perkin CHN analyzer, the 2400 Series II. The inorganic carbon content was measured using the man-ometric method for the fine fraction and the gravimetric method for the stony fraction. The proportions of organic carbon pools were established using oxidative fractionation. One soil sample, after the manual removal of small roots, was subjected to oxidation using potassium bichromate, following Tyurin's method. The resulting data were attributed to the pool of pedogenic carbon. A second portion of the same sample was oxidized in a muffle furnace at a temperature exceeding 525°C, employing the dry combustion method. The organic matter that remained unoxidized at this temperature contains lithogenic carbon, represented by inclusions of unoxidized anthracite. At this temperature, a portion of carbonaceous particles, along with products of their transformation, small roots, and other organic matter of biogenic origin, is combusted. Consequently, the values obtained for losses during calcination, accounting for losses of hygroscopic moisture, were multiplied by a coefficient of 0.58. After deducting pe-dogenic carbon, these values were accepted as the carbon content of non-specific organic matter. The total carbon content is calculated as the sum of the various pools (see Fig. 1). Carbon stocks by pools were estimated while considering soil density. The studied soils are characterized by high rockiness and density (see Table 2), which significantly influences the content of carbon pools and the structure of their stocks. The total carbon content ranges from 0.6% to 27.1% (see Table 3). Similar to other properties, this carbon content is not evenly distributed across different sites or within the vertical profile of the soils. In sites with organo-accumulative embryozems on aged dumps, the minimum total carbon content is observed under herbaceous communities (up to 10%), while it increases to 27.1% in the 0-5 cm layer beneath forest plant communities. A substantial portion of the total carbon content in embryo-zems on coal-containing substrates is derived from coal particles (see Table 3). The highest concentrations are found in initial embryozems (site 2IEy) and organo-accumulative embryozems (sites 7-8OAEf) that have developed under forest plant communities. During field observations, it was noted that the fine fraction exhibited no significant reaction to the application of 10% HCl. However, accumulations of secondary carbonates, in the form of cutans and clusters of crystals, were observed on the surfaces of stony inclusions, both on the soil surface and within the profile (see Fig. 2). Additionally, it was determined that the contribution of pedogenic carbon to the total carbon content of embryozems is minimal, ranging from 0.01% to 1.0% (see Table 3). The maximum concentration of pedogenic carbon is found in sites with hilly microrelief in organo-accumulative embryozems enriched with coal inclusions. This phenomenon can be attributed to several factors, including the presence of numerous weathered carbonaceous inclusions and the accumulation of organic matter in the depressions between hillocks. The carbon content of non-specific organic matter ranges from 0.12% to 6.34%. The distribution of this organic matter in soils, both spatially and vertically, is highly heterogeneous and lacks distinct regularities. The predominance of non-specific organic matter carbon content over pedogenic carbon has been observed, primarily due to the low content of the physical clay fraction in embryo-zems (see Table 3) and the dominance of coarse organic matter accumulation processes (such as plant residues) over humus accumulation. This combination is not conducive to the establishment of organomineral interactions and the formation of proper humus in soils of technogenic landscapes. In general, the soils in designated areas of stony rock dumping exhibit a progression from initial to soddy embryozem, characterized by an increase in the content of pedogenic carbon and non-specific organic matter. This trend is also evident in the pool of carbon from large roots, although its quantity in the studied soils is relatively small (see Table 3). The research has shown that the total carbon stocks in the examined embryozems range from 87.5 to 473.9 t/ha (see Fig. 3). The largest carbon stocks are found in initial embryozem (1-4IE), while the minimum stocks are present in soddy embryozem (9-10SE) and organo-accumulative embryozems under herbaceous vegetation (5OAEh). The carbon from coal particles in the stony fraction of the soils predominates in the carbon stock structure of most of the studied soils, accounting for up to 96.7% of the stocks in initial embryozems. The accumulation of lithogenic carbon in horizontal sites (3IEo and 9SEh) is associated with the lack of removal of fine coal particles due to slope processes. All of this occurs against the backdrop of uneven coal content in soils, which is influenced by the characteristics of non-standard anthracite in both coal and host seams, as well as the specifics of dump formation. The carbon stocks of organic matter that are less resistant to oxidation - such as pedogenic carbon, carbon from nonspecific organic matter, and large roots - tend to increase as a proportion of total stocks in the evolutionary series of soils, ranging from initial to soddy embryozems (see Fig. 3). The carbon stocks associated with large roots are characterized as minimal. This is attributed to the high stoniness of the soils and the density of stony detachments (see Table 2), which results in a relatively small mass of roots compared to the large mass of stones (see Table 3). In the embryozems of anthropogenic landscapes, alongside the sequestration of organic carbon, carbon fixation also occurs in the form of carbonate deposits on stone surfaces. The stocks of carbonate carbon are characterized by minimal values (see Table 3), ranging from 0.2% to 0.5% of total stocks (up to 2 t/ha). A common feature of all investigated soils in anthracite deposit dumps is that the share of carbon determined by the traditional method of bichromate oxidation (pedogenic carbon) does not exceed 7% of the total carbon stocks. This factor should be considered when assessing the carbon footprint of coal mining. To minimize the negative consequences associated with the transformation of coals in coal mining waste dumps and to enhance the carbon sequestration potential of technogenic soils, the surfaces of dumps should be composed of loose sedimentary rocks. The article contains 3 Figures, 3 Tables, 48 References. The Authors declare no conflict of interest.

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