## 4 Discussion  
Heavy metal pollution in soil has become a major problem in the ecological restoration of contaminated areas. The use of plants to adsorb heavy metals in soil is one of the green restoration techniques that is inexpensive and causes little damage to soil. Previous studies have revealed that the plant _Neyraudia reynaudiana_ has a strong sensitivity to Pb, and its adsorption capacity is relatively high, while its absorption rate of Cd is also significant, and it exhibits certain remediation potential. Figure 10: Individual points of heavy metal concentrations in soils predicted by MEA-BPNN model compared with measured values. (**a**) Individual points of Cd concentrations; (**b**) Individual points of Cr concentrations; (**c**) Individual points of Pb concentrations. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Elements**} & \multirow{2}{*}{**Models**} & \multirow{2}{*}{**Transformation**} & \multicolumn{2}{c}{**Training Set**} & \multicolumn{2}{c}{**Test Set**} \\ \cline{3-8}  & & & \(\mathbf{R^{2}}_{\text{CV}}\) & \(\mathbf{RMSE_{CV}}\) & \(\mathbf{R^{2}}\) & **RMSE** & **RPD** \\ \hline \multirow{3}{*}{Cd} & PLSR & OR & 0.685 & 3.779 & 0.759 & 2.484 & 1.654 \\  & BPNN & RL & 0.560 & 6.245 & 0.728 & 0.994 & 1.950 \\  & MEA-BPNN & SD & 0.767 & 1.462 & 0.873 & 4.316 & 2.114 \\ \hline \multirow{3}{*}{Cr} & PLSR & FD & 0.539 & 116.997 & 0.664 & 141.516 & 1.730 \\  & BPNN & SD & 0.727 & 295.678 & 0.737 & 120.985 & 1.944 \\  & MEA-BPNN & OR & 0.679 & 103.845 & 0.884 & 33.415 & 3.000 \\ \hline \multirow{3}{*}{Pb} & PLSR & FD & 0.502 & 102.271 & 0.852 & 60.585 & 2.193 \\  & BPNN & FD & 0.518 & 102.081 & 0.675 & 69.222 & 1.792 \\ \cline{1-1}  & MEA-BPNN & SD & 0.711 & 67.169 & 0.857 & 50.926 & 2.560 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Results of heavy metal estimations for three models. Figure 9: Individual points of heavy metal concentrations in soils predicted by BPNN model compared with measured values. (**a**) Individual points of Cd concentrations; (**b**) Individual points of Cr concentrations; (**c**) Individual points of Pb concentrations. The change of Cd found in this study was not large, however, which may be related to the concentration range of Cd in the soil. The overall Cd concentration was lower than that of the other two metals, so the change was not obvious. In this study, it was also discovered that the concentration of the heavy metal Cr in the soil also decreased significantly during the growth of _Neyraudia reynaudiana_, indicating that this plant has the potential to absorb Cr. After 120 days of restoration (in November 2018), however, the heavy metal content in the soil increased slightly. Due to the decrease of temperature at that time of year in Xuzhou, Jiangsu Province, _Neyraudia reynaudiana_ could not grow normally. Therefore, the activities of antioxidant enzymes and the ability of cells to absorb complex heavy metal ions in _Neyraudia reynaudiana_ were weakened, and then some heavy metal ions were transferred to the soil. However, at the end of the experiment, the heavy metal contents in the soil were still lower than the screening values of soil pollution risk for agricultural land. Due to the direct spectral response of C-H, N-H, O-H, and C-O covalent bonds in the VNIR spectrum, VNIR spectroscopy has proven to be a feasible method for detecting total soil properties such as SOM, TOC and clay [15]. However, VNIRS cannot directly estimate heavy metal concentrations. The only way VNIRS detects heavy metals is through their association with proxy (organic, iron, and clay minerals). Referring to previous studies, due to the strong adsorption capacity of SOM and iron oxides, heavy metal content is highly correlated with SOM and iron oxide content [17; 20]. The overlap of important wavelengths of soil properties also indicated that Cd and Pb were closely related to SOM, while Cr was closely related to Fe.