## 2 Materials and Methods
### Comparison of Environmental Selenium Characteristics in Keshan County, Heilongjiang Province  
Keshan County is under the jurisdiction of Qiqihar City, Heilongjiang Province. It is located in the western part of Heilongjiang Province, with a total area of 3320 square kilometers. It is the earliest place where Keshan disease was discovered. The geological background of the county is mainly Quaternary black soil. The high-incidence area of Keshan disease in the county is Guorong Village. According to the survey, there were a total of 286 people in Guorong Village in the winter of 1935, including 205 men and 81 women. In just two months, 36 people died of Keshan disease [9]. (1) Comparison of soil selenium values in the entire area  
The county has conducted a 1:250,000 land quality survey. The survey results are compared with the characteristics of elements such as As, Cao, Cd, Corg, Cr, Cu, Hg, Mgo, Mn, Mo, Pb, Ph, Se, and Zn in Xunyi County, as shown in Table 5.  
Compared with Keshan County, the soil in Xunyi County has higher contents of As, Cd, Cr, Cu, MgO, Mo, Pb, Ph, and Zn, indicating that the soil in Xunyi County is more alkaline, with higher contents of Cd and Cr within the safe range; Xunyi County has lower Mn and Pb contents, and the mean value of selenium in the soil of this county is only 0.52 times that of Keshan County, as shown in Table 6.  
In the high-incidence areas of Keshan disease in the two counties, Xunyi County has higher Ph and Mo contents and lower organic matter and Se. (2) Comparative analysis of the correlation of soil selenium elements  
Based on the characteristics of root soil elements and the availability of soil selenium, the SPSS software (IBM spss statistics20) was used to draw a column chart of the correlation between the calculated soil selenium in Keshan County and various elements, as shown in Figure 7. \begin{table}
\begin{tabular}{c c c c c c c c} \hline  & **Pb (10\({}^{-6}\))** & **Zn (10\({}^{-6}\))** & **Cr (10\({}^{-6}\))** & **Mo (10\({}^{-6}\))** & **Se (10\({}^{-6}\))** & **Corg (10\({}^{-2}\))** & **pH** & **Mn (10\({}^{-2}\))** \\ \hline Xunyi County & 25.41 & 80.71 & 68.64 & 0.89 & 0.12 & 1.7 & 8.44 & 0.07 \\ Keshan County & 25 & 63.35 & 66.18 & 0.63 & 0.33 & 3.1 & 5.8 & 0.08 \\ ratio & 1.02 & 1.27 & 1.04 & 1.42 & 0.36 & 0.55 & 1.46 & 0.92 \\ \hline \end{tabular}
\end{table} 
Table 6: Comparison of soil selenium in the high-incidence areas of Keshan disease in the two counties. \begin{table}
\begin{tabular}{c c c c c c c} \hline  & As (10\({}^{-6}\)) & Cd (10\({}^{-9}\)) & Corg (10\({}^{-2}\)) & Cr (10\({}^{-6}\)) & Cu (10\({}^{-6}\)) & MgO (10\({}^{-2}\)) \\ \hline Xunyi County & 13.07 & 0.174 & 0.98 & 69.04 & 24.6 & 2.31 \\ Keshan County & 10.65 & 0.09 & 2.28 & 66.09 & 20.77 & 1.281 \\ ratio & 1.23 & 1.85 & 0.43 & 1.04 & 1.18 & 1.8 \\ \hline  & Mo (10\({}^{-6}\)) & Pb (10\({}^{-9}\)) & PH & Se (10\({}^{-6}\)) & Zn (10\({}^{-6}\)) & Mn (10\({}^{-6}\)) \\ \hline Xunyi County & 0.77 & 22.41 & 8.23 & 0.13 & 68.51 & 652.93 \\ Keshan County & 0.64 & 23.17 & 6.45 & 0.25 & 58.65 & 683.7 \\ ratio & 1.21 & 0.97 & 1.27 & 0.52 & 1.17 & 0.95 \\ \hline  & & & & & & \\ \end{tabular}
\end{table} 
Table 5: Comparison of soil element characteristics between Xunyi County and Keshan County. It can be seen from the figure that when the selenium content in the soil of Keshan County increases, As, Cr, Cu, Zn, Ph, Org.C, etc., also increase, while Mo decreases. The organic matter in the black soil area is higher, which can reduce the availability of selenium. In the area where the selenium content in the soil of Xunyi County increases, Corg, Cd, Mn, Hg, Pb, etc., also increase. Among them, the organic matter can increase the availability of selenium due to its lower content.