## 1 Introduction  
Selenium (Se) is classified within the VIA group of elements of the fourth period and is an essential nonmetallic element for human physiology. It serves as a crucial component of glutathione peroxidase (GPx), facilitating human metabolism by catalyzing the conversion of reduced glutathione (GSH) to oxidized glutathione (GSSG), thus neutralizing toxic peroxides into non-toxic hydroxyl components [1, 2]. Despite its indispensability, the human body lacks the ability to synthesize Se, necessitating its acquisition through dietary sources, with plant-based foods constituting the primary reservoir. The Se content within plants directly influences Se absorption and subsequently impacts human health. Notably, the Se content within plants is contingent upon the concentration, form, and availability of Se within the soil, which, in turn, is shaped by regional soils' geochemical characteristics. However, the implications of Se on human health are dual in nature. An Se intake exceeding 400 \(\mu\)g per day can manifest typical symptoms of Se poisoning or alkalosis [3, 4]. Conversely, an intake below 17 \(\mu\)g per day may lead to conditions such as Kashin-Beck disease [5], cardiovascular complications ([6], or susceptibility to viral infections. With the burgeoning interest in the geochemical behaviors of Se and its biomedical functionalities, Se is garnering increasing global attention [7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18]. Present research endeavors concerning Se primarily concentrate on soil's Se absorption and enrichment by plants, alongside soil's Se availability. Existing studies have demonstrated that plant absorption and enrichment of Se predominantly hinge upon the Se forms in soil [19; 20; 21; 22; 23; 24; 25; 26; 27], soil's physicochemical properties [19; 20; 21; 22; 23; 24; 25; 26; 27], and the plant's Se enrichment and absorption capabilities [19; 20; 21; 22; 23; 24; 25; 26; 27]. The principal determinant influencing Se migration within the soil-plant system is the soil's available Se content, with soil's physicochemical properties also emerging as pivotal factors influencing this availability [7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18]. Several studies have confirmed that in alkaline soils with favorable aeration conditions, Se primarily exists in the form of selenite [7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18]. In this form, Se readily dissolves, is absorbed, and is utilized by plants, thereby enhancing its biological efficacy. Soil organic matter (OM) can immobilize Se through both biotic and abiotic mechanisms, consequently diminishing the Se bioavailability [7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18]. However, the majority of these investigations have been conducted within sedimentary rock regions with temperate climates, such as Greece [19; 20; 21; 22; 23; 24; 25; 26; 27], Northeast China, and the United Kingdom [19; 20; 21; 22; 23; 24; 25; 26; 27]. Studies within subtropical regions, particularly subtropical karst areas, remain scarce. This study is the first to investigate the geochemical behavior of Se, a beneficial trace element in the traditional Chinese medicinal herb _Gastrodia elata_ (_G. elata_), and it is the first to carry out a systematic study of the Se transport, transformation, and enrichment in the \"soil-plant\" environmental medium in a subtropical karst region and to explore the impact factors. The study also explores the influencing factors of Se. China is recognized as a global Se-deficient country, but there are some regions with relatively high soil Se content, such as Enshi region in Hubei province, which is home to the world's largest independent Se deposit. In 2019, a large amount of Se-rich arable land with Se concentrations ranging from 0.8 to 2.31 mg/kg was found in Banzhu Town, Zheng'an County, Guizhou Province, which is a typical karst region. In this area, a large number of _G.