applied sciences Article The Soil Chemical Properties Influencing the Oribatid Mite (Acari; Oribatida) Abundance and Diversity in Coal Ash Basin Vicinage Zuzana Feketeová 1,* , Barbara Mangová 2,* and Malvína Ciernikovˇ á 1 1 Department of Soil Science, Faculty of Natural Sciences, Comenius University in Bratislava, Mlynská Dolina, Ilkoviˇcova6, 84215 Bratislava, Slovakia; [email protected] 2 Institute of Zoology, Slovak Academy of Sciences, Dúbravská cesta 9, 84104 Bratislava, Slovakia * Correspondence: [email protected] (Z.F.); [email protected] (B.M.) Abstract: The samples of two technogenic sediments (MOS, coal ash sediment; MOD, a mixture of ash and natural soil) and two natural soils (MOM, meadow; MOF—forest; both Cambisol Dystric) in the vicinage of the coal ash basin were studied. We evaluated risk element concentrations and select-ed chemical and microbiological parameters to determine their influence on the community structure of Oribatida. High concentrations of various toxic elements, alkaline pH, and low hu-midity negatively affected the abundance of oribatid mites. The microbial indicator values showed that the soil microbial community formed in technogenic sediments could effectively use organic carbon. However, considering the wide C/N ratio of the substrates, the process of soil organic matter (SOM) decomposition was slowed down, and thus nutrients were less available for the mites, which could be among the reasons why we did not find any individual of oribatid mite in MOS. In MOD, however, we found representatives of three species, but only Tectocepheus velatus sarekensis established an abundant community at the highly contaminated site. Anthropogenic pressure resulted in the selection of r-strategists, which became Citation: Feketeová, Z.; Mangová, B.; dominant and reduced the whole community’s species’ diversity. Therefore, we consider it a suitable Cierniková,ˇ M. The Soil Chemical indicator of improper human intervention in the ecosystem. Properties Influencing the Oribatid Mite (Acari; Oribatida) Abundance Keywords: oribatid mites; metal pollution; Tectocepheus velatus sarekensis; bioindicators; diversity and Diversity in Coal Ash Basin Vicinage. Appl. Sci. 2021, 11, 3537. https://doi.org/10.3390/ app11083537 1. Introduction Global changes in climate and land use are affecting the biodiversity of soil arthropods Academic Editor: Sung-Deuk Choi in many ecosystems [1]. Oribatid mites represent species-rich and highly abundant soil mites inhabiting the soil’s upper layers [2]. There is increasing interest in their reaction to Received: 28 March 2021 environmental conditions such as metal pollution because of their essential role in detrital Accepted: 13 April 2021 Published: 15 April 2021 food webs. They are actively involved in the soil decomposition process, increasing soil fertility by recycling nutrients out of dead organic matter. The low reproduction and long Publisher’s Note: MDPI stays neutral life-cycle rates suggest that oribatid mites may be sensitive to environmental change [3]. with regard to jurisdictional claims in They are not passive inhabitants of ecosystems; instead, they are strong interactors. Despite published maps and institutional affil- being generally sensitive to disturbance, oribatid mites show a considerable variation in iations. sensitivity among taxonomic groups. The various scientific papers declare that the number of species is declining toward the more significant disturbance, and, at the same time, the composition of their entire community is changing [4,5]. The large number of factors determine the distribution and abundance of soil mites. Among the abiotic factors, soil pH and soil organic matter (SOM) represent the most significant drivers concerning soil Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. pH variation on soil arthropods’ presence [6,7] and arthropods’ role in SOM degradation. This article is an open access article Highly increased levels of metals, wide C/N ratio, and pH values were recognized as the distributed under the terms and structuring forces that influence the distribution, diversity, and richness of oribatid species, conditions of the Creative Commons whereas the soil reaction (pH) influences these differences fundamentally [8]. Changes Attribution (CC BY) license (https:// in the value of the soil reaction mostly come from pollution from industrial production creativecommons.org/licenses/by/ and mining. Its value affects the soil’s chemical, physical, and biological properties and 4.0/). the solubility of organic carbon and increases the bioavailability of various toxic elements, Appl. Sci. 2021, 11, 3537. https://doi.org/10.3390/app11083537 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 3537 2 of 14 mainly metals. Soils around heating power plants are often enriched in metals due to ash blowout of refuse heaps, and toxic metals tend to accumulate in high concentrations in the topsoil near point sources [9]. Oribatid mites proved to have a high capacity to colonize postindustrial dumps [10]. With an increasing metal load of the soil, sensitive species may be replaced by resistant ones without changing the total number of mites. One of the critical reasons for reducing the abundance and species’ richness of Oribatida at highly contaminated sites may be explained by toxic effects through the uptake of toxic metal-loaded microorganisms, espe- cially fungi [11]. In general, oribatids and their communities are relatively tolerant of toxic metal exposure [12,13]. Many studies have measured their responses to environmental pollution at the community level [14,15]. The studies proved that heavy pollution exposure decreases both abundance and species’ diversity, but species vary in their sensitivity to pollution, and some species may even benefit from moderate pollution levels [14,16]. Orib- atida includes some species with resistance to metals to maintain their populations in even highly contaminated soils [15–17]. The findings mentioned above suggest that the oribatid mites have a great potential to be used as bioindicators of changing soil environment. Therefore, it is crucial to understand how the mite community responds to ongoing soil ecosystems’ changes. Soil microarthropods play a vital role in the nutrient cycle due to the litter’s fragmentation, increasing the microbial action area and stimulating microorganisms’ growth [18]. They accelerate the circulation of elements within ecosystems, either directly or indirectly, by stimulating soil microbes and contributing to soil fertility maintenance [19]. Their essential role in litter decay as detritivores suggests that any change in their community structure in response to global environmental change is likely to have further consequences for ecosystem functioning. The hypotheses were as follows: (1) Anthropogenic pressure results in the selection of r-strategists, which will become dominant and reduce the whole community’s species’ diversity. (2) Oribatids can be considered appropriate indicators of improper human intervention in the soil ecosystem, reflecting extremely low abundance compared to their abundance in undisturbed habitats. This study’s main goal was to characterize the oribatid mite community, which inhabits the coal ash basin’s extreme environment and its close surrounding. Knowing the biological parameters and the properties of soil and contaminants acting in situ is crucial for proper ecological risk assessment. Concerning coal ash disposal sites, representative data about toxicity on oribatid mites are lacking. Therefore, the aim was to detect the influence of environmental load caused by coal processing to the Oribatida community as a change in species’ occurrence. Based on the hypotheses we made, we targeted identifying species with negative responses under stress and those prospering in deteriorating living conditions. 2. Materials and Methods 2.1. Study Area This investigation aimed to study the species’ composition and diversity of Oribatid mites and microbiological activity in a power plant ash pond, the dam, and surrounding meadow and forest soil located in Zvolen—Môt’ová, Slovakia (Central Europe), and the relationship between biological parameters of such soils and their chemical properties (Figure1). The ash pond’s dam is 10 m high, and the whole area reaches approximately 7 hectares. The sedimentation of ash sediment with permanent storage in the basin was carried out from 1991 to 2005. This storage represents more than 320,000 m3 of stored ash. At present, it is operated in a cyclical manner—the period of flotation and ash export alternates [20]. Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 14 (Figure 1). The ash pond’s dam is 10 m high, and the whole area reaches approximately 7 hectares. The sedimentation of ash sediment with permanent storage in the basin was car- Appl. Sci. 2021, 11, 3537 ried out from 1991 to 2005. This storage represents more than 320,000 m³ of stored ash. At 3 of 14 present, it is operated in a cyclical manner—the period of flotation and ash export alter- nates [20]. FigureFigure 1. 1.Study Study area area inin the map of of Slovakia Slovakia.. IndividualIndividual sites sites at at which which samples were were taken taken represent represent four four biotopes biotopes with withgradually gradually decreasingdecreasing exposure exposure to to the the contaminant.contaminant. They They include include (1) (1)ash ash pond pond sediment—ash sediment—ash (MOS), (MOS), (2) forming Technosol (MOD)—a