# Trends of Global Scientific Research on Reclaimed Coal Mine Sites between 2015 and 2020
## 1 Introduction  
Coal mining sites present a serious issue for modern society. Open-cast coal mining significantly changes the landscape and utterly destroys soil functions, making these vast sites practically desolate, thus positioning itself as probably the most disputed industry in the world [1, 2, 3, 4]. Besides disturbing the landscape and soil, it also impacts the integrity of the habitat, environmental flows, and ecosystem functions, as well as water and air quality, thus often leading to human health problems [5].  
Many countries have developed legislation about the ways that these sites need to be treated after the excavation process is done [6, 7, 8, 3], and our knowledge expands every year with various research results that we obtain from these localities. Despite the fact that they have been more precisely defined in recent years, the so-called R4 terms (reclamation, rehabilitation, restoration, and remediation) are still a source of confusion for many [5, 9]. Many researches have shown that technical reclamation, an approach widely used in the 20 th century, might have its flaws, and there are other, in many cases, more feasible and efficient solutions, such as natural succession [8, 10]. Technical reclamation is still the most common approach used on post-mining sites, and it is even a mandatory practice in some areas, due to many countries' legislation [9]. The full effects of the reclamation process can be visible only after some time has passed, usually ranging from several decades to the scale of centuries. As stated by [PERSON] et al. [11], continuous monitoring of reclamation sites should be given emphasis in order to devise a feasible and effective policy for degraded land reclamation and restoration.  
In order to observe the research trends on coal mining sites, for the purpose of this research it was decided to split the research into different categories. Although it could roughly be said that research on coal reclaimed mine sites (RMS) is most usually of a physical, chemical, or biological nature, recent technological achievements such as satelliteimagery, remote sensing, the use of Unmanned Aerial Vehicles (UAVs), GIS, modelling, and various new theories and equipment which require calibration or testing often demand a section of their own [1; 12]. Review papers, as well as the implementation of new technologies with data and measurements from older research, can be considered a separate, although not less important, category, since they often provide more comprehensive information and interconnect various scientific branches. Keeping in mind that soil organic matter (SOM) is one of the segments that is most drastically impacted by open-cast coal mining, and its physical, chemical, and biological properties and functions are deeply interrelated, as well as the fact that it is an unavoidable and very abundant component of much modern soil RMS research, a section of its own is called for. Soil organic carbon (SOC) research, as well as the capability of certain soils to sequester it, is becoming more and more abundant in recent years [13; 14; 15; 16; 17; 18; 19; 20]. Since nutrients, most commonly nitrogen, are often investigated together with soil carbon, sometimes it is difficult to divide these as well. Being interested mainly in the direction that research on coal reclamation sites is going towards, the main purpose of this paper is not to compare the effectiveness of one restoration strategy or method over the other, distinguish between the terminology issues mentioned, or compare the vegetation species, age, or effectiveness, but only to point out the general trends and abundance of certain research categories over others. This paper does not provide comprehensive statistical and/or bibliometric analysis, but merely aims to reflect the trends of research performed on these sites over a specified time period, in publications visible in the Web of Science (WoS) online library.