# Trends of Global Scientific Research on Reclaimed Coal Mine Sites between 2015 and 2020
## 4 Discussion  
Ever since the Intergovernmental Panel on Climate Change (IPCC) was held in 1988, the sequestration of carbon in terrestrial and non-terrestrial ecosystems has been one of the strategies for mitigating the negative effects of climate change, and has been recognized as an effective and viable method for the reduction of greenhouse gases under the Kyoto Protocol [156]. Since then, many projects have been developed to fund research and strategies related to carbon sequestration. Reclaimed mine lands offer significant potential for carbon sequestration in terrestrial ecosystems, according to some sources, up to the rate of one ton per hectare per year [157; 158]. If the reclaimed land is converted to forests, it is stated that these values can reach up to roughly 2.5 tons per hectare per year, due to the ability of forests to store more carbon in their vegetative parts compared to other land use types, although the soil carbon levels among them can be similar [158]. Since organic carbon and organic matter levels on coal mining spoil heaps are usually severely depleted, these sites present big natural laboratories for monitoring the changes in carbon levels. It was also noticed that reclamation sites have shown great potential as soil organic carbon sinks [159; 160] and that the pre-mining levels of soil organic carbon can be achieved after only 20 years [161]. These are some of the reasons that explain the results of the C, N, and SOM category and its trends. Keeping in mind the fact that China, the USA, and India combined, according to the BP Statistical Review of World Energy [162], accounted for more than two thirds of the global increase in energy demand in 2018, and that, along with the Russian Federation and Australia, have the greatest coal reserves in the world, it is no wonder that these three countries have had such a share of research performed on reclaimed mine sites. Around the world, RMS research topics can vary due to many reasons, depending not only on current scientific interests, but also, on a broader scale, on geographical, geological, socio-economic, financial, cultural, and other factors. In some European countries and the USA, for example, where some reclamation sites and strategies are close to a century old, the tendencies are to further improve and research the good techniques used, and suppress the ones that might, in the long run, cause certain risks. A good example of this would be the decline in choosing non-native afforestation species, like black locust (_Robinia pseudoacacia_) or red oak (_Quercus rubra_) in many developed European countries [9; 10; 163], due to their potential invasiveness, and despite their many benefits (in case of _R. pseudoacacia_, anti-erosion potential, nitrogen fixation, honey production, tolerance to various environmental factors, etc. ), whereas, in many Asian, and even some developing European countries, it is still a common practice due to their mentioned benefits, low mortality rate, and, above all, low cost. The same applies for creating monoculture forest stands instead of mixed forests on RMS. On the other hand, in certain areas, where it is difficult to establish vegetation for various reasons (e.g., parent or overburden material which can be eco \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Country** & **No. of Papers** & **2015** & **2016** & **2017** & **2018** & **2019** & **2020** \\ \hline Indonesia & 3 & 0 & 0 & 0 & 1 & 2 & 0 \\ South Africa & 2 & 0 & 0 & 0 & 1 & 0 & 1 \\ Spain & 2 & 1 & 0 & 0 & 0 & 0 & 1 \\ Brazil & 2 & 0 & 2 & 0 & 0 & 0 & 0 \\ Chile & 1 & 0 & 0 & 0 & 1 & 0 & 0 \\ Canada & 1 & 1 & 0 & 0 & 0 & 0 & 0 \\ \hline \end{tabular}
\end{table} 
Table 3: _Cont._toxic, compacted, or overly clayey, severe climatic conditions or erosion, the presence of water, etc. ), this can direct the way that measures are taken and the course of action and subsequent research practices that are prioritized.