## 1 Introduction  
Selenium (Se), a metalloid first discovered by Swedish chemist [PERSON] in 1817, shares many chemical properties with sulfur [1]. As an essential trace element, Se plays a crucial role in numerous physiological and metabolic functions in humans and animals. It contributes to cancer prevention, antioxidative defense, immune system regulation, endocrine health, and the detoxification of toxic elements such as arsenic, mercury, and chromium [2, 3, 4]. However, Se exhibits a narrow range between deficiency and toxicity. While insufficient intake can lead to disorders such as Keshan disease and Kashin-Beck disease, excessive intake may cause selenosis and other health issues, including skin lesions and neurological damage [5]. To manage this delicate balance, the World HealthOrganization (WHO) and the Chinese Nutrition Society recommend a daily Se intake of 40-400 ug [6]. Soil is the primary source of Se in the human diet, as it enters the food chain through plant uptake [7; 8; 9; 10]. However, the distribution of Se in the Earth's crust is highly heterogeneous [11; 12] ([PERSON] et al., 2024; [PERSON] et al., 2018), leading to significant spatial variability in soil Se concentrations. Globally, soil Se levels are generally low, typically ranging from 0.01 to 2.00 mg\(\cdot\)kg\({}^{-1}\), with an average of approximately 0.40 mg\(\cdot\)kg\({}^{-1}\), and high-altitude areas tend to exhibit relatively higher Se concentrations [13]. This heterogeneity is influenced by multiple environmental factors, including parent material, land use, topography, and soil physicochemical properties. As a result, understanding the environmental controls on soil Se distribution has become a globally relevant scientific challenge. Among these factors, parent rock is a fundamental determinant of soil Se content, as it serves as the primary geogenic source [6; 14]. Different lithologies influence soil formation processes and elemental concentrations due to varying mineral compositions and weathering rates [15; 16]. For example, carbonate rocks and sandstones weather at different rates, leading to distinct trace element profiles in derived soils [17]. Notably, Se-rich black shales from the Permian Gufeng Formation in Enshi, Hubei Province, have produced some of the highest recorded Se concentrations in soils, earning the region the title of \"Se Capital of the World\" [14]. Similar patterns have been observed in Ziyang County, Shaanxi Province, where Se levels in soils reach up to 279 mg/kg due to black shale weathering [18]. This underscores the fundamental role of parent rock in the geochemical cycling of Se and other elements. In addition to geogenic sources, land use plays a critical role in modifying soil properties and influencing the mobility and bioavailability of Se [19; 20; 21; 22]. Land use affects soil organic matter, pH, redox potential, and microbial activity, all of which are key factors in Se transformation and retention [23; 24]. While numerous studies have examined the impact of land use on macronutrients such as nitrogen and phosphorus [25; 26], relatively fewer have focused on trace elements like Se [27; 28; 29; 30]. Moreover, existing findings are often inconsistent. For instance, Se levels in dryland soils have been reported to be higher than in paddy soils in parts of Guizhou [31; 32], whereas opposite trends have been observed in nearby regions [33]. Similarly, comparisons between agricultural and forest soils have yielded contradictory results [34; 35; 36; 37], suggesting that other interacting variables, such as parent material and topography, may play significant roles. Despite growing interest in the environmental behavior of Se, research on its cycling in karst landscapes remains limited, especially from a global perspective. Karst regions--formed by the dissolution of carbonate rocks--cover approximately 15% of the Earth's land surface and support nearly 25% of the global population.