## 2 Materials and Methods
### Chemical Analysis  
#### 2.3.1 Determination of Total Selenium (Se)  
All samples underwent processing in accordance with national standards (GB 5009.93-2017 [33] and HJ 680-2013 [34]). Soil samples were digested using concentrated HCl and HNO\({}_{3}\) (V\({}_{1}\):V\({}_{2}\) = 3:1), while _G. elata_ samples were digested using concentrated HNO\({}_{3}\) and  
Figure 1: Digital elevation model (DEM) map of sampling points. (**(a)** Location map of Guizhou in China; (**b**) Location map of the sampling area in Guizhou; (**c**) Map of sampling sites in the study area). H\({}_{2}\)O\({}_{2}\) (V\({}_{1}\):V\({}_{2}\) = 2:1). The microwave digestion system (CEM Mars 6 microwave digestion system, manufactured by CEM Corporation, Matthews, NC, USA) was utilized for complete sample digestion, followed by cooling and volume adjustment. The digested samples were then analyzed using Atomic Fluorescence Spectrometry (AFS-8510 Atomic Fluorescence Spectrophotometer; manufactured by HAIGUANG Instrument, Beijing, China). All chemical reagents used were of high-purity grade. A sequential extraction method was employed to determine the Se forms. Sequential extractions of water-soluble, exchangeable, organic-matter-bound (including fulvic acid- and humic acid-bound Se), selenide-/sulfide-bound, and residual Se were conducted following an adapted version of the BCR's sequential extraction method [35]. The extraction steps were as follows:  
(1) Adding 0.5 g of soil and 20 mL of deionized water to a 50 mL centrifuge tube, shaking the mixture (200 cpm) for 1.5 h, and centrifuging (4000 rpm) for 30 min to separate the supernatant from the residue. (2) Adding 10 mL of phosphate solution to the residue from Step 1, shaking well for 1.5 h, followed by centrifugation. The supernatant was separated, and 10 mL of deionized water was added to the residue, shaken for 0.5 h, and centrifuged. The two supernatants were combined. (3) Adding 10 mL of NaOH to the residue from the previous step, shaking on a shaker for 0.5 h, heating in a thermostat water bath at 90 \({}^{\circ}\)C for 2 h, centrifuging, washing the sample with 10 mL of deionized water, shaking for 30 min, and centrifuging. The supernatant was collected. (4) Drying the remaining material from the previous step in an oven at 50-60 \({}^{\circ}\)C, crushing the residue, adding KClO\({}_{3}\) solids, mixing well, adding 10 mL of concentrated HCl, and heating. After completion of the reaction, 10 mL of deionized water was added, followed by centrifugation. The supernatant was transferred to a polytetrafluoroethylene beaker for 110 \({}^{\circ}\)C for 1 h using an electric boiling plate. (5) The remaining residue was dried and poured out, scraped from the inner wall, ground well in an agate mortar, weighed, and put into a sachet. Then, 0.05 g of residue was weighed out from the pouch and placed in an ablation tube and ablated with concentrated hydrochloric acid and HNO\({}_{3}\) (V\({}_{1}\):V\({}_{2}\) = 3:1), and the samples were completely digested using a microwave digestion system, followed by cooling and volume adjustment. Regarding the determination of selenium in sequential extracts, it was divided into two categories. One was the sulfide-/selenide-bound and residual Se of the extract, which can be directly diluted and then determined by atomic fluorescence. Meanwhile, the other category included the water-soluble, exchangeable, and organic-matter-bound Se of the extract. For this category, it was necessary to carry out the determination through oxidative reduction. This involved taking 1 mL of the extract and transferring it to a polytetrafluoroethylene beaker. Then, we added 1 mL of HNO\({}_{3}\) and 0.5 mL of H\({}_{2}\)O\({}_{2}\) and steamed until the volume reduced to 0.5 mL at 100 \({}^{\circ}\)C. Next, we added 0.5 mL of HNO\({}_{3}\) and steamed until nearly dry.