## 3 Results
### Effects of Organic Amendments on Soil Organic Matter and Plant-Available Nutrients  
The average OM in gob spoil soil ranged from 25% to 30% during the 90-day incubation period (Figure 3B). The average OM percentage in soil did not show a significant (\(p>0.5\)) increase in OM content after adding the amendments. \begin{table}
\begin{tabular}{l c c c c} \hline \hline \multicolumn{1}{c}{**Sample Type**} & \begin{tabular}{c} **Water-Holding** \\ **Capacity** \\ \end{tabular} & **Total Phosphorus** & \
\begin{tabular}{c} **Plant-Available** \\ **Phosphorus** \\ \end{tabular} &
\begin{tabular}{c} **Plant-Available** \\ **Nitrogen** \\ \end{tabular} \\ \hline Soil + 7.5\% biochar & 70.33 \(\pm\) 6.92 a & 216.12 \(\pm\) 23.98 e & 168.26 \(\pm\) 21.70 d & 598.36 \(\pm\) 87.94 a \\ Soil + 10\% biochar & 71.02 \(\pm\) 7.48 a & 257.61 \(\pm\) 15.88 bcd & 188.07 \(\pm\) 24.18 bc & 665.57 \(\pm\) 68.67 a \\ Soil + 5\% mix & 58.01 \(\pm\) 18.46 b & 238.56 \(\pm\) 21.59 de & 184.10 \(\pm\) 12.16 cd & 577.57 \(\pm\) 72.07 a \\ Soil + 7.5\% mix & 70.77 \(\pm\) 14.42 a & 252.70 \(\pm\) 24.11 bcde & 249.42 \(\pm\) 16.48 bcd & 636.90 \(\pm\) 46.93 a \\ Soil + 10\% mix & 70.95 \(\pm\) 7.60 a & 286.42 \(\pm\) 22.95 ab & 255.93 \(\pm\) 10.51 ab & 695.00 \(\pm\) 25.56 a \\ Soil + 5\% compost & 58.33 \(\pm\) 14.12 ab & 242.87 \(\pm\) 11.93 cde & 220.77 \(\pm\) 21.75 bcd & 556.27 \(\pm\) 43.52 a \\ Soil + 7.5\% compost & 66.69 \(\pm\) 6.80 ab & 279.81 \(\pm\) 22.88 abc & 272.24 \(\pm\) 7.04 ab & 627.70 \(\pm\) 7.55 a \\ Soil + 10\% compost & 70.48 \(\pm\) 7.23 a & 318.49 \(\pm\) 2.01 a & 294.66 \(\pm\) 8.97 a & 667.34 \(\pm\) 52.21 a \\ \hline \hline \end{tabular} Note: Different letters in the same column correspond to statistically significant differences (\(p<0.05\)).  
\end{table} 
Table 2: _Cont._  
Figure 3: The impact of organic amendments on (**A**) soil water-holding capacity and (**B**) soil organic matter. Data are shown as mean (n = 3) \(\pm\) standard deviation. The availability of nutrients is an important indicator of soil quality. Nutrient depletion leads to soil chemical degradation as well as decreased vegetation cover [35]. The concentration of plant-available nitrogen in the gob spoil soil was below 200 mg N/kg during incubation (Figure 4A). This concentration significantly (\(p<0.05\)) increased to approximately 500 mg N/kg after adding biochar at a 5% rate, which further increased to over 600 mg N/kg at higher biochar amendment rates at 7.5% and 10%. The increasing trend in plant-available nitrogen along with increased biochar amendment rates was similar to that of using compost as an amendment. By day 90, the average concentrations of plant-available nitrogen were 556, 628, and 667 mg N/kg for compost amendment rates at 5%, 7.5%, and 10%, respectively (Figure 4A). Both biochar and compost increased nutrient availability for plants, which was consistent with previous literature [33]. For the mixed amendment treatment, the concentrations of plant-available nitrogen by the end of the soil incubation study were 578, 637, and 695 mg N/kg at amendment rates of 5%, 7.5%, and 10%, respectively, which were similar to using biochar or compost alone as soil amendments (Table 2). Figure 4: The impact of organic amendments on (**A**) plant-available nitrogen and (**B**) plant-available phosphorous. Data are shown as mean (n = 3) \(\pm\) standard deviation. As shown in Figure 4B, the addition of soil amendments significantly (\(p<0.05\)) boosted the plant-available P in gob spoil soil (Table 2). The average plant-available P in gob spoil soil was 122 mg P/kg without any soil amendment. This concentration increased by approximately 40, 50, and 70 mg P/kg after adding biochar at amendment rates of 5%, 7.5%, and 10%, respectively. In comparison, more plant-available P was provided by the compost. After 90 days, the concentration increased by approximately 120, 140, and 160 mg P/kg at 5%, 7.5%, and 10% amendment rates, respectively.