Morphological and chemical properties of soils within geological complexes affected by sulfuric acid in forest-steppe of the Central Russian Upland (Russia)

In the Central Russian Upland, black soils become contaminated by toxic substances containing sulfuric acid, which leak from non-recultivated stockpiles of coal mines. Sites for studying properties of soils that represented main geocomplexes of this region were selected using GIS analysis. Background Haplic Chernozems occupy interfluves and slopes; Phaeozems are located at dry gullies. Stockpiles serve parent materials for Spolic Technosols (Arenic/Loamic, Dystric, Sulfidic, Phytotoxic) characterized by high contents of sulfates and total organic carbon (TOC). Burned stockpiles were occupied by less toxic Spolic Technosols (Loamic, Eutric, Phytotoxic), which had a neutral reaction in topsoils and relatively low contents of sulfates and TOC. Remediation with different substrates and phytoremediation resulted in the creation of Technosols (Dystric, Loamic, Molic, Organotransportic). Deluvial/proluvial deposits were occupied by soils with lithic discontinuities that were identified as Regosols over Phaeozems. Near to stockpiles, there were Dystric Colluvic Stagnic Regosols (Arenic/Loamic, Lamellic, Loaminovic, Sulfidic, Phytotoxic), which had strong and moderate acidity and the high content of sulfates that was lower than that in Spolic Technosols (Dystric). Phaeozems (Loamic, Loaminovic, Phytotoxic) within impact zones of stockpiles had lithic discontinuities due to the surface redeposition of materials washed off the tips, decreased acidity and increased contents of sulfates.


Introduction
Serious environmental problems within coal mining areas are associated with land-use conflicts [1,2]. In the Tula Region of Russia such conflicts are predetermined by the following reasons: 1. an absence or an incompleteness of land remediation measures following unplanned closures of mines within the Moscow Lignite Basin; 2. a high population density and 3. re-ploughing of agricultural lands, which were abandoned in the 1990s. For the last 25 years, the Tula Region has been regarded as a natural experimental site, studies of which allow for obtaining data on directions and rates of degradation processes within natural complexes, the development of new soils and the effectiveness of land remediation measures [2][3][4].
Spoil tips and stockpiles contain toxic substances that leak into surrounding landscapes due to combined effects of climatic conditions and an imperfect management of local coal mines, in particular, waste storage practices [5]. Materials of mine spoils containing pyrite and lignite (brown coal) and stored 2 on the land surface are mechanically transported and transformed by natural processes. The natural colonization of mine spoils by ruderal vegetation leads to primary soil formation on these technogenic parent materials [6,7], while highly fertile Chernozems and Phaeozems within impact zones of spoil tips and fans undergo irreversible degradation due to the impact of sulfuric acid leakages from stored wastes [2].
The aim of the present study was to conduct a combined analysis of morphological and chemical properties of soils that developed as a result of a spontaneous closure of coal mines within the Tula Region.

Materials and methods
Changes in the soil-vegetation cover under either decreasing or increasing technogenic impacts, which are directly or indirectly connected with coal mining, reflect the current development of soils within the forest-steppe zone of the Central Russian Upland.

Study area
The study was conducted within an area of 10 thousand hectares in the Kireevskii and Uzlovskii districts of the Tula region, where 13 coal mines operated until the mid-1990s and left behind spoil tips and stockpiles that have heights of 30-70 m and volumes of 1-3 million m 3 each ( Figure 1). According to [2,8], areas of mine spoils are occupied by four main soil types including Technosols, Chernozems, Phaeozems and Regosols over Phaeozems. These soil types were characterized by us in seven different sites (Table 1), which were associated with the former coal mines and characterized by different phases of soil development [8].

Methods
Soil descriptions were performed in accordance with [9]. Soils and horizons were named using [10]. Soil color at field moisture was determined using the Munsell color system. Samples were taken from each genetic horizon of 9 soil profiles, which made 81 samples in total. In all soil samples, pH values were measured in suspension (with a 1 : 2.5 soil : water ratio) using an Expert-рН meter (Russia) at the Faculty of Geography of the Lomonosov Moscow State University and the total organic carbon (TOC) contents were determined using the titrimetric method with phenylanthranilic acid. In a 1 : 2.5 soil : water ratio extract, carbonates and bicarbonates were determined by potentiometric titration with 0.02 M H2SO4 to pH 8.4 and 4.4, respectively. Sulphates was precipitated as BaSO4 and determined turbidimetrically [11].
The data obtained were statistically processed. Standard parameters of descriptive statistics (mean and standard deviation) were determined and mean values were compared using the Manne Whitney U test.

Soils of background geocomplexes
Background geocomplexes, which are free from prolonged influences of the runoff contaminated by sulfuric acid, are occupied by Haplic Chernozems (Aric, Loamic) and Chernic Phaeozems (Loamic) formed on calcareous loess-like loams. The studied Haplic Chernozems (Aric, Loamic) were characterized by pH values increasing with depth, from 4.9±0.5 within the A horizon to 5.1±1.0 within the С horizon. The vertical distribution of CaCO3, concentrations was characterized by a minimum within the A horizon and a maximum within the C horizon, which was predetermined by the pH distribution pattern. Contents of TOC decreased with depth from 3.1±0.8% within the A horizon to 0.3±0.1% within the C horizon. Sulfates (SО4 2ions) were present throughout the profile, with a maximum in the parent material. The studied Chernic Phaeozems (Loamic) were characterized by insignificant variations in pH values, generally low contents of carbonates with a maximum in topsoil, TOC concentrations decreasing with depth and significant contents of SО4 2with a maximum in topsoil.

Soils of fans
Deluvial-proluvial fans around spoil tips and stockpiles were characterized by the formation of Dystric Colluvic Stagnic Regosols (Loamic, Lamellic, Loaminovic, Sulfidic, Phytotoxic) with lithic discontinuity, i.e., their upper layers are derived from redeposited materials of spoil tips and lower layers from chemically transformed Phaeozems or Chernozems affected by sulfuric acid. The thickness of redeposited stratified materials decreased from 130 cm at the foot of a pile to first centimeters further away. O-horizons were found in soil profiles developed within vegetated areas of the fans. Topsoil of the studied Dystric Colluvic Stagnic Regosols (Loamic, Lamellic, Loaminovic, Sulfidic, Phytotoxic) was moderately acidic (рН of 3.4-5.1) and impoverished in sulfates (3.9 mg/100 g) in comparison to that of Spolic Technosols (Arenic/Loamic, Dystric, Sulfidic, Phytotoxic) on non-recultivated spoil tips and stockpiles (p-value < 0.00001). Humus horizons of buried Chernozems and Phaeozems, as compared to their background equivalents, had a stronger acidity with an insignificantly decreased carbonate content, but significantly increased contents of sulfates (multiplied by 4.6; p-value = 0.024) and TOC (p-value = 0.001). Subsoil of buried Chernozems and Phaeozems was weakly acidic (nearly neutral) and more calcareous than their topsoil (Table 3).
Toxic compounds were present in soils of deluvial fans directly adjacent to spoil tips (Dystric Colluvic Stagnic Regosols (Loamic, Lamellic, Loaminovic, Sulfidic, Phytotoxic)), but absent from soils of proluvial fans developed under more hydromorphic conditions. As compared to soils of deluvial fans, soils of proluvial fans were weakly acidic and non-calcareous and had significantly (p-value 0.038) higher contents of sulfates (up to 21 mg/100 g) within the upper part of profile, above lithic discontinuity. Lower down, the A horizon of the buried soil had significantly higher pH (p-value = 0.05) and carbonate content (multiplied by 25, p-value = 0.01) and lower TOC content (decreased by 6.5 times; p-value = 0.0012). Subsoil of the buried soil was characterized by a further increase in pH values along with a decrease in TOC and negligible quantities of sulfates and carbonates. Cr,180-210 10YR5/6 CL/L massive a Named according to [10]. b Texture classes: CL -Clay loam, L -Loam, Ssand. c Structure: Mmassive, SBCfine subangular blocky-crumb, SBsubangular blocky, SABsubangular-angular blocky, P coarse blocky-prismatic. d Roots: ffine, mmedium, ccoarse, 1single, 2few, 3many.  Subsidence zones, both related and unrelated to spoil tips, were occupied by Chernic Phaeozems (Loaminovic), which differed from their background equivalents by stronger development of hydromorphic features. In addition, soils of subsidence zones related to spoil tips were characterized by transformation of calcareous pedofeatures and/or their replacement by gypsum pedofeatures under the impact of solutions containing sulfuric acid. Surface deposits of technogenic materials had nearly neutral pH values, high contents of TOC (9.3±2.4%) and low contents of carbonates and sulfates. Buried topsoil was distinguished by twice as low TOC contents (p-value = 0.04), with values of other parameters similar to those in the overlying technogenic material. Buried subsoil was distinguished by an absence of carbonates and higher concentrations of sulfates due to influences of acidic solutions.

Discussion
Forest-steppe soils within impact areas of coal mines of the Moscow Lignite Basin develop under a continuous influence of sulfuric acid solutions discharged from spoil tips and stockpiles [2,4,8], which leads to changes in their morphological and physical properties.
In particular, soil pH decreases and contents of TOC, sulfates increase with increasing influences of sulfuric acid solutions, according to previous research on coal mine areas located in different geographical regions [12][13][14][15]. However, there are different properties in previously unstudied soils that develop on pyrogenically-transformed spoil tips and stockpiles with incinerated coal residues, completely oxidized sulfides and alkaline medium.
TOC contents were very high in Technosols (Dystric), Dystric Colluvic Stagnic Regosols and Chernic Phaeozems (Loaminovic, Phytotoxic) due to the presence of coal particles. Spolic Technosols (Loamic, Eutric, Phytotoxic) had lowest TOC contents because the coal residues had burned off. Humus horizons of buried soils had highest TOC contents due to additions of coal dust.
The pH of A-horizons had lowest values in Dystric Colluvic Stagnic Regosols (Loamic, Lamellic, Loaminovic, Sulfidic, Phytotoxic) and maximal values in Chernozems. Buried A-horizons of Chernozems and Phaeozems and their surface equivalents had similar pH values, which can be explained by a high buffer capacity of these soils [16][17][18][19].
Sulfates in the studied soils had autochthonous and allochthonous origins. Autochthonous sulfates had contents of up to 0.3 mg/100 g in native soils and were also present in technogenic soils on spoil tips, where they were formed during a microbiological oxidation of sulfide minerals [20-23]. The content of SO4 2in the buried A horizon of Dystric Colluvic Stagnic Regosols (Loamic, Lamellic, Loaminovic) was 28 times higher (p-value < 0.001) than that in the A horizon of background Haplic Chernozems. Contents of SO4 2in mineral horizons and parent materials of Dystric Colluvic Stagnic Regosols (Loamic, Lamellic, Loaminovic, Sulfidic, Phytotoxic) and Haplic Chernozems had insignificant differences.
Contents of carbonates were comparable in humus horizons of buried and background Chernozems. In their mineral horizons, carbonate contents significantly increased with increasing distance from the sources of acidic solutions, i.e., spoil tips and stockpiles of coal mines.

Conclusion
The development of soils of industrial coal mining landscapes of the Moscow Lignite Basin was affected by technic substances including acidic compounds. The character and intensity of such technogenic impact predetermined morphological features and chemical properties of soils.
Spolic Technosols (Arenic/Loamic, Phytotoxic) formed on stockpiles and spoil tips consisted of unsorted materials that contained fragments of coal and enclosing rocks. Toxicity of such materials decreases after burning, which resulted in sulfide oxidation.
Deluvial and proluvial fans within impact zones of spoil tips and stockpiles were characterized by the formation of Dystric Colluvic Stagnic Regosols (Arenic/Loamic, Lamellic, Loaminovic) with lithic Chernic Phaeozems (Loamic, Loaminovic, Phytotoxic) within impact zones of spoil tips and stockpiles were also characterized by a presence of lithic discontinuity (under a technogenic layer of less than 40 cm in thickness), low acidity and high contents of sulfates. Contents of TOC and sulfates increased in buried soil layers, with insignificant changes in acidity.