## 2 Materials and Methods
### Research in Xunyi County, Shaanxi Province  
(1) Overview of Xunyi County  
Xunyi County, under the jurisdiction of Xianyang City, Shaanxi Province, is located in the north of Xianyang City, with a total area of 1811 square kilometers. The exposed strata mainly include the Quaternary, Tertiary, Triassic, and Cretaceous systems. There are nine main types of soil in Xunyi County, among which loess is the most widely distributed, followed by cinnamon soil and black loess. The soil samples collected in this study mainly include black loess in Zhitian Town, black loess in Tuqiao Town, and yellow loess in the southeast of Zhangbaosi Town. (2) The situation of Keshan disease in Xunyi  
Shaanxi Province is a province with relatively severe Keshan disease. Xunyi County is located in the southern part of the Northern Shaanxi Plateau, and all 14 townships in the county are Keshan disease areas. In the mid-1960s, it was one of the historically severe Keshan disease areas (counties) in Shaanxi Province. In 1965, the incidence rate of acute and subacute Keshan disease in Xunyi County reached 86.8/100,000. Since 1990, Xunyi County has been listed as one of the first national Keshan disease surveillance sites in China [17]. There are 14 disease-stricken townships in Xunyi County, with a population of 265,448. A survey in 2005 showed that there were still 14 new cases of natural chronic Keshan disease, with an annual incidence rate of 0.53/10,000. Houviyang Village and Changshetou Village in Houzhang Township, and Dongcao Village in Tuqiao Town, Xunyi County, serve as national surveillance points for Keshan disease in Xunyi County. In 2003, there were still 50 potential cases and 3 chronic cases [18]. This time, Changshetou Village, Zaochi Village, and Dongcao Village in Xunyi County were selected as high-incidence study areas to conduct soil selenium research. (3) Sample collection and testing  
Soil sampling was carried out across the county in August 2017, with a sampling area of approximately 1811 km\({}^{2}\). The sampling depth was 0-20 cm. Sample testing was conducted by the Xinjiang Geological Laboratory. Elements and oxides such as Al\({}_{2}\)O\({}_{3}\), Cr, Ga, K\({}_{2}\)O, Nb, P, Pb, V, Rb, SiO\({}_{2}\), Ti, Y, Cl, Zr, Br, Cu, Ba, CaO, Co, TFe\({}_{2}\)O\({}_{3}\), MgO, Mn, Na\({}_{2}\)O, Ni, Sr, and Zn were analyzed using X-ray fluorescence spectroscopy (XRF). Elements such as Ag, B, Sn, and Mo were analyzed using emission spectroscopy (AES). Elements such as Cd, Mo, U, Th, TL, Bi, Ge, W, Ga, Rb, Nb, Pb, Cr, Y, Cu, and Ce were analyzed using plasma mass spectrometry (ICP-MS). Elements such as As, Sb, Hg, and Se were analyzed using atomic fluorescence spectroscopy (AFS). Elements such as Be, CaO, Ce, Co, TFe\({}_{2}\)O\({}_{3}\), La, Li, MgO, Mn, Na\({}_{2}\)O, Ni, Sc, Sr, Zn, Al\({}_{2}\)O\({}_{3}\), Ti, Ba, K\({}_{2}\)O, and V were analyzed using inductively coupled plasma emission spectroscopy (ICP-OES). The actual number of samples analyzed in the whole county was 460, and 25 sets of wheat and its rhizosphere soil and 20 soil profile samples were collected in the high-incidence area. (4) Characteristics of selenium in the soil of the whole county  
(1) 1:250,000 soil selenium characteristicsBased on the soil data of the whole county, the element characteristic table related to Keshan disease in Xunyi County is calculated, as shown in Table 1.  
The average selenium content in the soils of Xunyi County is 0.13 \(\times\) 10\({}^{-6}\), while the national average is 0.29 \(\times\) 10\({}^{-6}\)[19]. Compared with the national level, Xunyi County is a selenium-deficient area, with an enrichment coefficient of 0.45. The analysis results of the total selenium content in the surface soils of Shaanxi Province show that the range is 0.018-17.618 \(\times\) 10\({}^{-6}\), with an average of 0.118 \(\times\) 10\({}^{-6}\)[20]. Compared with the soils of Shaanxi Province, it is relatively stable.