## 4 Discussion
### Differences in Soil Se Content Across Parent Rock Types and Land Use Categories
Influence of Soil Physicochemical Properties on Total Se in Soils Derived from Different Parent Rocks  
A Spearman correlation analysis, accompanied by a heatmap visualization (Figure 6), was conducted to investigate the relationships between total Se content and various soil physicochemical properties. This approach enabled a comprehensive assessment of the factors influencing Se distribution. The results revealed that in limestone-dominated regions, no significant correlation was observed between soil pH and total Se content. In \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Limestone** & \multicolumn{6}{c}{**Sandstone**} \\ \hline
**Explanatory Variable** & **Correlation** & **R\({}^{2}\)** & \(p\) & **Explanatory Variable** & **Correlation Coefficient** & **R\({}^{2}\)** & \(p\) \\ \hline V & 0.776 & 0.71 & \textless{}0.01 & SOM & 0.687 & 0.624 & \textless{}0.01 \\ SOM & 0.444 & 0.82 & \textless{}0.01 & V & 0.336 & 0.72 & \textless{}0.01 \\ Zn & \(-\)0.339 & 0.86 & 0.02 & & & \\ Elevation & 0.141 & 0.88 & 0.027 & & & \\ \hline \hline \end{tabular}
\begin{tabular}{c c c c c c} \hline \hline
**Explanatory Variable** & \
\begin{tabular}{c} Correlation \\ Coefficient \\ \end{tabular} & R\({}^{2}\) & \(p\) & Explanatory Variable &
\begin{tabular}{c} Correlation \\ Coefficient \\ \end{tabular} & R\({}^{2}\) & \(p\) \\ \hline SOM & 0.88 & 0.75 & \textless{}0.01 & SOM & 0.631 & 0.80 & \textless{}0.01 \\  & & & V & 0.366 & 0.86 & \textless{}0.01 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Stepwise multiple linear regression analysis of influencing factors on soil total Se under different land uses and parent rock controls. contrast, a strong negative correlation was found in sandstone-dominated areas (r = \(-\)0.47, \(p\) = 0.01), indicating that higher pH levels are associated with lower Se concentrations. In limestone regions (Figure 6b), the absence of a significant relationship between pH and total Se may be attributed to the dominant influence of other factors, such as SOM and mineral components, which possess a strong capacity to adsorb Se. Similar findings were reported by [PERSON] and [PERSON] [50], who suggested that external inputs of Se into the soil may mitigate the inhibitory effect of pH on Se accumulation. In general, soil pH exhibits a negative correlation with total Se content. As pH increases, the positive charge on hydroxides and clay minerals decreases, thereby reducing their capacity to adsorb Se [51; 52]. Another possible explanation is that lower pH levels suppress microbial activity, which in turn slows the decomposition of SOM and alters Se dynamics [27; 53; 54]. The negative correlation observed between pH and total Se in sandstone-derived soils aligns with this general trend, likely due to the relatively low adsorption capacity of these soils. Furthermore, the humid climate and high rainfall in Southwest China may exacerbate Se loss through leaching, thereby contributing to the overall decline in total Se content. Numerous studies have reported positive correlations between Se and both SOM and pH levels [55; 56; 57]. The findings of the present study are consistent with this trend (Figure 6a,b), showing that total Se is significantly positively correlated with SOM in both sandstone- and limestone-dominated areas (r = 0.715 and r = 0.472, \(p\) = 0.01, respectively). This relationship may be attributed to the formation of ternary complexes involving Se, SOM, and mineral elements such as V, Cr, and Zn, which can reduce Se leaching and enhance its retention in soils [58; 59; 60]. Figure 6: Heatmap showing the correlations between Se concentration and key influencing factors. * indicates a significant difference. In addition, SOM plays a crucial role in Se redox transformations by promoting the development of anaerobic microsites within soil aggregates, thereby facilitating the reduction and immobilization of Se [61; 62; 63]. Moreover, SOM increases the number of available adsorption sites for Se, enabling its direct binding to soil particles [62; 63; 64].