## 3 Results and Discussion
### Factors Affecting the Total Amount of Selenium (Se) in Soil and the Various Forms of Se
Further analyzing the loading values, pH showed moderate loading on both PC1 and PC2, indicating that the acidity and alkalinity of the soil had some correlation with both principal components. The OM content showed higher loadings on both PC1 and PC2, especially more significantly on PC2, which reflects that the OM content is an important factor influencing soil's properties and has a strong correlation with both principal components. Similarly, the clay content (Clay) loadings on PC1 and PC2 were similar to pH and also showed moderate correlation. When examining the behavior of selenium (Se) in soils, it is crucial to consider factors such as the soil's pH, organic matter (OM) content, and clay content, which significantly influence Se's chemical form, bioavailability, and stability. These properties not only affect the cycling and transport of selenium through the soil but are also further complicated in karst areas due to the unique geological features. Specifically, the thin soil layer and poor water retention capacity, coupled with the frequent alternation between surface water and groundwater, can lead to the discharge of water-soluble Se from the soil surface into the ground, thereby reducing the original effective Se content and bioavailability in the region. The pH of the study area varied from 5.98 to 6.6, averaging 6.24 (Table 5). Typically, soils in karst areas tend to be alkaline [52]; however, in this study, the soil exhibited a moderately acidic condition. This deviation can be attributed to the prolonged impact of acid rain in the research area. The complex lithological background, comprising white limestone, siltstone, and mudstone, interacted with the continuous influence of acid rain, collectively contributing to the moderately acidic soil environment. While carbonate minerals in the white limestone are prone to dissolution, releasing alkaline substances to neutralize the soil's acidity, the intense acidity of the acid rain has eroded these rocks, resulting in a significant loss of alkaline components such as calcium and magnesium in the soil. Additionally, clay minerals and organic matter in the siltstone and mudstone have also undergone decomposition and dissolution under the influence of acid rain, further exacerbating the soil acidification [52]. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **PC1** & **PC2** \\ \hline
**pH** & 0.501 & 0.587 \\
**Organic matter (g/kg)** & 0.703 & 0.820 \\
**Clay * (\%)** & 0.504 & 0.581 \\
**Characteristic** & 2.4157 & 1.589 \\
**Contribution (\%)** & 45.434 & 33.063 \\
**Cumulative contribution (\%)** & 45.434 & 78.5 \\ \hline \hline \end{tabular} \({}^{*}\)Clay particles with a diameter of less than 0.002 mm.  
\end{table} 
Table 4: Principal component analysis of soil’s physical and chemical properties. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Range** & **Average** & \(t\)**-Test** & \(p\)**Value** \\ \hline
**pH** & 5.98–6.6 & 6.24 \(\pm\) 0.16 & 5.567 & 0.871 \\
**Organic matter (g/kg)** & 21.92–127.66 & 38.95 \(\pm\) 19.25 & \(-\)2.265 & 0 \\
**Clay * (\%)** & 50.73–76.19 & 63.33 \(\pm\) 8.06 & 2.115 & 0.443 \\ \hline \hline \end{tabular} \({}^{*}\)Clay particles with a diameter of less than 0.002 mm.  
\end{table} 
Table 5: The physicochemical properties of the soil in the study area (mean \(\pm\) SD). In acidic soils, Se predominantly exists as selenite, which readily forms stable chelate complexes or precipitates with iron and manganese oxides. Conversely, in alkaline soils, Se primarily exists as selenate, which is highly soluble in water and easily absorbed and utilized by plants. Moreover, under alkaline conditions, the fulvic acid-bound Se components of OM-bound Se can readily mineralize and decompose into inorganic Se and low-molecular-weight organic Se, facilitating absorption and utilization by plants. Fulvic acid, however, exhibits limited mineralization and decomposition in neutral or acidic environments, and even if it does decompose in acidic conditions, it is prone to reabsorption into the soil.