## 3 Results and Discussion
### Distribution Characteristics of Soil Selenium (Se) Forms  
Among the five forms of Se (Table 2), residual Se accounted for the highest proportion at 53.1%. Following this, selenide-/sulfide-bound Se represented the second-highest proportion, with an average of 36.23%. Notably, OM-bound Se exhibited the lowest proportion at 3.19%. The order of Se content in each form was as follows: residual Se (53.1%) > selenide-/sulfide-bound Se (36.23%) > exchangeable Se (7.13%) > water-soluble Se (3.73%) > OM-bound Se (3.19%). The \"effective Se,\" comprising water-soluble Se and exchangeable Se, which are readily absorbed and utilized by plants, accounted for 10.86% of the total Se. This proportion aligns closely with the Se form distribution observed in the soil of Shuangan Naore village in Ziyang, where sulfur-/selenide-bound Se constituted 65.74% of the total, followed by residual Se (24.85%), OM-bound Se (6.61%), exchangeable Se (1.69%), and water-soluble Se (1.11%) [45].  
The proportion of Se occurrences in the studied soils is clearly different from that in the soil in Ziyang. It was reported that Se in soil in the Ziyang area was mainly derived from weathering products of local black sedimentary rock series (mudstone, shale, and siliceous rocks, mainly), which was enriched in Se-containing pyrite. The soil's parent material in Ziyang area is different from that in the study area, where outcropped rocks are mainly composed of carbonate rocks. This discrepancy could be the reason for the difference in aspect of Se occurrences in the soils. Such morphological distribution characteristics significantly influence the uptake of Se by plants [46; 47; 48; 49; 50].  
Previous studies indicate that the reduction partition index (\(I_{R}\)) [51], utilized for analyzing heavy metal pollution, can provide insights into the bioavailability of Se to some  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Range** & **Average** & **SD** & **Proportion** \\  & **(mg/kg)** & **(mg/kg)** & **(mg/kg)** & **(\%)** \\ \hline
**Water-soluble** & 0.0223–0.0487 & 0.0377 & 0.0085 & 3.73 \\
**Exchangeable** & 0.0625–0.0785 & 0.0708 & 0.0039 & 7.13 \\
**OM-bound** & 0.0132–0.0428 & 0.0315 & 0.0066 & 3.19 \\
**Sulfur-/selenide-bound** & 0.276–0.434 & 0.359 & 0.0402 & 36.23 \\
**Residual** & 0.152–2.311 & 0.764 & 0.6773 & 53.1 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Five forms of Se in root soil of _G. elata_.  
extent. The calculation of \(I_{R}\) is grounded in sequential chemical extraction and assigns different weights to distinct Se components based on their mobility:  
\[I_{R}=\sum_{l=1}^{k}\left(F_{i}\times i^{2}\right)/k^{2}\]  
Here, \(i\) represents the number of extraction steps (1 = least aggressive, \(k\) = most aggressive), and \(i\) is assigned specific values to denote each step and component: water-soluble Se = 1, exchangeable Se = 2, OM-bound Se = 3, sulfide-/selenide-bound Se = 4, and residual Se = 5. Additionally, \(Fi\) signifies the fractional content of Se in the total amount extracted from component \(i\), and \(n\) is an integer (usually 1 or 2). In this study, \(k\) = 5 and \(n\) = 2; hence, the minimum \(I_{R}\) = 0.0003 and maximum \(I_{R}\) = 2.0  
The \(I_{R}\) value can provide insights into bioavailability to some extent, where a low \(I_{R}\) value suggests that Se exists in unstable forms (e.g., exchangeable), while a high \(I_{R}\) value suggests that Se mainly exists in stable forms. The \(I_{R}\) values of Se in nearly all soil samples exhibit a consistent trend (Figure 2), indicating that most Se is in a stable state that is challenging to utilize, with only a small portion being available for uptake.