## 4 Conclusions  
Organic amendment of the gob spoil soil by both biochar and the compost led to a significant increase (\(p<0.05\)) in its water-holding capacity leading to the enhancement in the soil quality for the growth of vegetation. No significant increase (\(p>0.05\)) in the OM content of the gob spoil soil was observed due to the organic amendments. However, as the soil itself had a good amount of organic matter (-28%), any enhancement in OM content due to the amendments may not make a big difference in soil fertility. On the other hand, the plant-available N and plant-available P content of the gob spoil soil were improved drastically due to the biochar and compost amendments. Plant-available N content increased from <200 mg N/kg for unamended soil to 400 to 800 mg N/kg in the amended soil. The period of incubation was also a significant factor in the improvement of plant-available N content as it increased with incubation time in most cases. The plant-available P content also increased as a result of organic amendment. The impact of compost was much higher than the biochar in improving plant-available phosphorus levels. The potentially toxic trace metal content in the gob spoil soil was low and hence any concern about organic amendments mobilizing toxic metals was negligible. The only exceptions were Al and Fe, which did not show a significant increase in soluble and exchangeable fractions, which are the plant-available forms of the metals. The organic amendment of the gob spoils soil by either the biochar or the compost potentially improved soil fertility by improving its water-holding capacity and the plant-available nutrient contents. The incubation time was a significant factor in improving the plant-available N content. In the case of plant-available P, compost amendment was more effective than biochar amendment. This study provides the data necessary for future experiments to test plant germination and growth on amended gob pile soils. A greenhouse study is currently in progress to test the efficacy of these amendments in aiding plant growth in gob pile soils. Based on the results from the soil incubation study and the greenhouse study, a field study will be performed to establish a vegetative cover on the gob piles of CCF using the optimal rate of organic amendment. The results from this study will aid in future research on the remediation of mine sites with varying properties at different geographic locations. Exploring the effectiveness of different biochar and compost types could also yield valuable insights into the restoration of mine sites. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app14219723/s1](https://www.mdpi.com/article/10.3390/app14219723/s1)]([https://www.mdpi.com/article/10.3390/app14219723/s1](https://www.mdpi.com/article/10.3390/app14219723/s1)), Table S1: Total metal concentrations for soil incubation samples (mg/kg). Conceptualization, D.S., R.D. and A.R. ; methodology, K.M., A.R., Z.Z. and A.S.; formal analysis, Z.Z., A.S., K.M. and A.R. ; resources, A.R., D.S. and R.D. ; writing--original draft preparation, Z.Z. and A.S.; writing--review and editing, R.D., A.R. and D.S. ; supervision, A.R., D.S. and R.D. ; project administration, A.R., D.S. and R.D. ; funding acquisition, A.R., D.S. and R.D. All authors have read and agreed to the published version of the manuscript. This work was supported by the U.S. Department of the Interior Office of Surface Mining Reclamation and Enforcement (USDol-OSMRE) Grant numbers S21 AC10036-00. Not applicable. All data generated or analyzed during this study are included in this article. The authors acknowledge the help from Navajo Technical University undergraduate students [PERSON] and [PERSON] toward this study. The authors acknowledge Navajo Technical University undergraduate student [PERSON] for preparing the GIS maps included in this manuscript.