## 2 Materials and Methods  
### Experimental Design and Samples  
The soil heavy metal remediation experiment was based on the heavy metal pollution area of the dumping field in the vanadium-titanium iron ore district of Panzhihua City, Sichuan Province, China. The study area has the world's third-largest vanadium-titanium magnetite resource and has been plagued by potential and toxic metal pollution for many years. Solid waste contains many harmful substances such as Zn, Cd, Pb, Hg, As, V, Co, Ti, Cu, Ni, Cr, Mn, and so on. In our experiment, Cd, Cr, and Pb--the moderate and severe pollutants in the soil of this area--were used as stress metals. At present, the Panzhihua mining area is undergoing ecological restoration of the soil environment that has been polluted by mining. We used the local need _Neyraudia reynaudiana_ in the area as our experimental restoration plant. Based on these conditions, this study simulated the heavy metal pollution, ecological remediation plants, and climate in the Panzhihua mining area in order to provide a basis for the remediation and monitoring of heavy metals in soil.  
The medium in each culture pot was a 3:1 ratio of clean river sand to loess soil. The metal-adsorbing _N. reynaudiana_ reeds were planted in these pots. CdCl\({}_{2}\):2.5H\({}_{2}\)O, K\({}_{2}\)Cr\({}_{2}\)O\({}_{7}\), and Pb(NO\({}_{3}\))\({}_{2}\) solutions containing Cd\({}^{2+}\), Cr\({}^{6+}\), and Pb\({}^{2+}\) were selected as heavy metal stress sources. As shown in Table 1, based on the determination of soil heavy-metal content in the Panzhihua vanadium-titanium ore district [40], along with the Risk Control Standard for Soil Contamination (GB 15618-2018), 6 concentration gradients of Cd, Cr, and Pb in the simulated polluted soil samples were examined. Three sets of parallel experiments were set up for each gradient, for a total of 54 potted plants (as listed in Table 2). The pH of the soil was maintained at 6.5 (\(\pm\) 0.1) and continuously ventilated for 24 h.  
The experiment began in a greenhouse in May 2018. After 14 days of slow seedling development, the plants with consistent growth were selected and transplanted into potted soil with heavy metal stress. During the experiment, the greenhouse was kept ventilated, and the potted plants were irrigated regularly (every 2 days). Soil samples were collected at different stages of the growth of _Neyraudia reynaudiana_. The specific experimental process is shown in Figure 1.  
In this study, six stages of data were collected at 30, 60, 90, 120, 150, and 180 days after heavy metal stress. From the pots containing soils with different concentrations of heavy metals, 36 samples of surface soil under Cd, Cr, and Pb stress were collected, totaling 108 samples. The soil samples were placed in sealed plastic bags and labeled. In order to eliminate the interference of soil moisture on the spectrum, all soil samples were naturally air-dried at room temperature after removing small stones and plant roots. The soil was ground in an agate bowl and passed through a 100-mesh nylon sieve (sieve size: 0.150 mm) for laboratory analysis. The soil samples were divided in half and sent to the laboratory for hyperspectral measurements and soil property analysis.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Element** & **Content in the Panzhihua Mining Area** & **Risk Screening Value** & **Risk Intervention Value** \\ \hline Cd (mg/kg) & 0.10–0.40 & 0.3 & 2.0 \\ Cr (mg/kg) & 19.00–495.00 & 150 & 850 \\ Pb (mg/kg) & 1.60–15.60 & 90 & 500 \\ \hline \end{tabular}
\end{table}
Table 1: Reference criteria for the experimental design (soil pH = 6.5).  
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**Element** & **1** & **2** & **3** & **4** & **5** & **6** \\ \hline Cd (mg/kg) & 0 & 0.2 & 0.4 & 0.6 & 0.8 & 1.0 \\ Cr (mg/kg) & 0 & 200 & 400 & 600 & 800 & 1000 \\ Pb (mg/kg) & 0 & 200 & 400 & 600 & 800 & 1000 \\ \hline \end{tabular}
\end{table}
Table 2: Experimental soil concentration gradient settings.