## 4 Discussion
### Differences in Soil Se Content Across Parent Rock Types and Land Use Categories
The observed correlations between Se and mineral elements such as V, Cr, and Mn in this study further support the ternary complexation mechanism. Influence of Soil Physicochemical Properties on Total Se in Soils Under Different Land Uses in Limestone Regions  
As shown in the heatmaps in Figure 6c,d, no significant relationship is observed between soil pH and total Se content in either paddy fields or drylands. This result is consistent with the pattern observed in sandstone-dominated regions, as opposed to limestone-dominated areas. These findings suggest that land use type does not significantly influence the role of pH in regulating soil Se content. In contrast, a comparison of the heatmaps in Figure 6a,b indicates that parent material plays a critical role in modulating total Se content through its influence on soil pH. It is hypothesized that parent material not only serves as the primary source of Se in soils but also strongly affects the physicochemical properties of the overlying soil, thereby shaping Se dynamics [65; 66]. In limestone regions, SOM is significantly correlated with total Se content in both paddy and dryland soils, with correlation coefficients of r = 0.878 and r = 0.900 (\(p=0.01\)), respectively. These strong correlations underscore the critical role of SOM in influencing total Se concentrations. Compared to previously reported correlation coefficients for SOM in limestone regions, the observed values represent increases of 0.163 and 0.185, corresponding to relative increases of 22.8% and 25.8%, respectively. This suggests that land use positively influences SOM accumulation, thereby enhancing total Se content in soils. This phenomenon is likely attributable to the substantial impact of anthropogenic activities, which can alter key soil properties such as clay content, cation exchange capacity (CEC), and pH. These changes promote the accumulation of organic matter, which in turn facilitates Se enrichment in soils [67]. Influence of Topographic Factors on Total Se Under Different Parent Materials and Land Uses in Limestone Regions  
As shown in Figure 6a,b, in limestone regions, soil Se content shows no significant correlation with any topographic factors. In contrast, in sandstone regions, soil Se content exhibits a significant positive correlation with elevation, consistent with previous findings [12; 68]. The elevated Se concentrations at higher altitudes can be attributed to several factors beyond reduced anthropogenic disturbance. These include lower temperatures that slow the decomposition of soil organic matter, reduced Se mobility and transformation rates, decreased leaching losses, and diminished plant uptake of Se. Moreover, parent material plays a critical role in modulating these processes, thereby establishing an indirect link between elevation and soil Se content. As illustrated in Figure 6c,d, in limestone regions, the total Se content in paddy soils exhibits a significant negative correlation with slope, whereas no significant relationship is observed between total Se content in dryland soils and topographic factors. Slope is a key factor influencing soil erosion and leaching processes, with steeper gradients typically associated with more severe soil loss ([PERSON] et al., 2008) [69]. This not only alters the transport mechanisms of Se but also indirectly affects its spatial distribution through the redistribution of trace elements within the soil profile [12]. The study area, characterized by mountainous terrain, is particularly susceptible to intense soil erosion and nutrient depletion due to its complex topography. As shown in Figure 6, SOM content decreases with increasing slope, further exacerbating Se loss. These findings highlight the critical role of slope in regulating soil Se dynamics, particularly in paddy fields, where the combined effects of waterlogging, erosion, and topographic variability are more pronounced.