## 5 Discussions
### Lithological Influences on the Enrichments and Fractionations of REEs in the Soils
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Flavonoids** & **Habitats** & **Average** & **Ranges** & **Skewness** & \(p\)**Value** \\
**Compounds** & **Habitats** & **(mg\(\cdot\)g\({}^{-1}\))** & **(mg\(\cdot\)g\({}^{-1}\))** & **Skewness** & \(p\)**Value** \\ \hline \multirow{3}{*}{Wogonoside} & NGHs (\(n=11\)) & 5.24 & 1.96-12.2 & 1.79 & 0.099 \\  & APHs (\(n=10\)) & 1.81 & 0.080–3.02 & \(-\)0.588 & 0.396 \\  & BCHs (\(n=8\)) & 2.69 & 1.03–4.56 & 0.031 & 0.585 \\ \hline \multirow{3}{*}{Baicalein} & NGHs (\(n=11\)) & 2.43 & 0.820–4.68 & 0.765 & 0.605 \\  & APHs (\(n=10\)) & 1.15 & 0.550–2.09 & 1.42 & 0.356 \\  & BCHs (\(n=8\)) & 1.09 & 0.330–2.43 & 1.22 & 0.357 \\ \hline \multirow{3}{*}{wogonin} & NGHs (\(n=11\)) & 0.966 & 0.310–1.75 & 0.476 & 0.508 \\  & APHs (\(n=10\)) & 0.468 & 0.200–0.810 & 0.862 & 0.598 \\  & BCHs (\(n=8\)) & 0.478 & 0.090–1.19 & 1.38 & 0.301 \\ \hline \multirow{3}{*}{Oroxylin A} & NGHs (\(n=11\)) & 0.386 & 0.200–0.560 & \(-\)0.036 & 0.61 \\  & APHs (\(n=10\)) & 0.23 & 0.060–0.440 & 0.378 & 0.561 \\ \cline{1-1}  & BCHs (\(n=8\)) & 0.184 & 0.030–0.570 & 1.92 & 0.031 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: _Cont._topsoil contained higher REE concentrations since organic matter has a fixed effect on REEs [43; 44; 45]. The clays and iron (hydro) oxide that formed during chemical weathering in hornblende-genesis locations were notably different, resulting in apparent different fractionations within REEs [46; 47; 48; 49; 50], since the mixed speciation of the REEs in clays and Fe-Mn oxyhydroxides is typical for REEs in soils [38; 51; 52; 53]. Under natural pH, clay minerals with a negative layer charge are effective in the adsorption of REEs through ion exchange, surface complexation, electrostatic attraction and migration into clay structures [54]. The adsorption capacity is largely determined by the surface structure, surface charge of clay minerals and composition [55; 56]. In our study area, under alkaline conditions (pH \(>\) 7) with abundant Fe\({}^{2+}\), Mg\({}^{2+}\), and K\({}^{+}\), clay minerals were dominated by smectite, illite or chlorite, especially in profile 2 [46; 50; 57; 58]. Smectite and illite may intrinsically retain HREE more efficiently than LREE [46; 58]. As shown in Figure 7a, LREE/HREE had a remarkable positive relationship with SiO\({}_{2}\)/(Al\({}_{2}\)O\({}_{3}\)+Fe\({}_{2}\)O\({}_{3}\)) in profile 2 according to Pearson correlation analysis. As weathering proceeded, clay minerals were gradually generated from aluminosilicate minerals with the enrichment of Al\({}_{2}\)O\({}_{3}\) and depletion of SiO\({}_{2}\). In addition, enhanced oxidation contributed to the enrichment of Fe\({}_{2}\)O\({}_{3}\). Therefore, the decrease of the SiO\({}_{2}\)/(Al\({}_{2}\)O\({}_{3}\)+Fe\({}_{2}\)O\({}_{3}\)) ratio showed the enrichment of clay mineral (smectite and illite) and iron (hydro) oxide, favoring the preferential adsorption of HREE and fractionation within REEs [59]. However, according to CIA, the chemical weathering in profile 1 (CIA = 65.0-71.1) was generally stronger than that in profile 2 (CIA = 51.1-67.2), and a small amount of kaolinite and boehmite with higher maturity was formed from illite/smectite [60]. Therefore, LREEs were preferentially adsorbed by kaolinite in profile 1. In addition, HREEs have a smaller ionic radius and stronger hydrolysis ability than LREEs and therefore have a higher affinity toward iron (hydro) oxide. Previous studies also showed that the chondrite-normalized iron (hydro) oxide fractions are slightly enriched in HREEs via inner-sphere complexation and may play an important role in redistributing HREEs in the weathering crust [61; 62]. As shown in Figure 7b, LREE/HREE had a remarkable negative relationship with Fe\({}_{2}\)O\({}_{3}\) in profile 2, suggesting the critical control of iron (hydro) oxide on the REE fractionation. In the study area, profile 2 contained more abundant Fe\({}_{2}\)O\({}_{3}\) (4.92-9.49%, 7.95%) than those in profile 1 (Fe\({}_{2}\)O\({}_{3}\) = 4.40-7.47%, 6.22% ).