## 4 Discussion  
### Differences in Soil Se Content Across Parent Rock Types and Land Use Categories  
The lithological distribution of Zheng'an County is shown in Figure 4. Figure 5 demonstrates that total Se content differs significantly between soils derived from limestone and sandstone (\(p<0.05\)), and exhibits a marginally significant difference between paddy fields and drylands (\(p<0.1\)). Within limestone-derived soils, total Se content varies significantly across land use types, with drylands exhibiting higher Se concentrations than paddy fields. In contrast, no significant difference is observed between land use types within sandstone-derived soils, indicating an interaction effect between parent material and land use. On average, total Se content in limestone regions is approximately 17% higher than in sandstone regions. Moreover, within limestone areas, Se content in drylands is, on average, 42% higher than in paddy fields. This disparity is primarily attributed to prolonged waterlogging in paddy fields, which accelerates the decomposition of organic matter. Consequently, organically bound Se is transformed into more bioavailable forms, such as fulvic acid-bound Se, which are readily absorbed by crops, leading to Se depletion in the soil [47]. Additionally, Se losses through leaching and percolation under flooded conditions further contribute to Se depletion in paddy soils [48]. Figure 4: Lithological distribution map of Zheng’an County. Figure 5: Comparative analysis of total Se content differences based on land use and lithology. * indicates a significant difference. The bars represent the mean \(\pm\) standard error (n = 20). An asterisk (*) denotes a statistically significant difference at \(p\) < 0.05 between limestone- and sandstone-derived soils, as well as between paddy fields and drylands. In soils derived from limestone, V and SOM are the strongest predictors of total Se content, jointly explaining 86% of its variation (Table 4). This suggests a strong association between Se and elements such as V and zinc (Zn), consistent with the findings of [PERSON] et al. [49]. Both V and Zn are partially indicative of clay mineral content, with V exhibiting a higher adsorption capacity for Se compared to Zn. Among the topographic variables, elevation shows a relatively weak influence on total Se content, with a correlation coefficient of 0.141. In sandstone-derived soils, SOM emerges as the primary explanatory variable, accounting for 62.4% of the variation in total Se content. A similar pattern is observed in dryland soils within limestone regions, where SOM explains 80% of the variation, indicating a stronger influence. In contrast, in paddy soils derived from limestone, SOM alone accounts for 75% of the variation in total Se content, suggesting that organic matter plays a dominant role in Se retention under flooded conditions. The analysis of Figure 5 reveals significant variations in total Se content between soils derived from different parent materials, whereas no substantial differences are observed among different land-use types. This indicates that parent material exerts a stronger influence on Se concentrations than land use. In limestone regions, dryland soils exhibit markedly higher Se levels than paddy soils, while overall Se concentrations are substantially lower in sandstone-derived soils. Notably, the elevated Se levels in limestone areas are primarily attributable to Se-enriched dryland soils. In contrast, Se concentrations in paddy soils from limestone regions are even lower than those in both paddy and dryland soils from sandstone regions. These distribution patterns may be shaped by additional factors, such as topography and soil physicochemical properties, that interact with both land use and parent material to influence total Se content.