Soil organic carbon in Jinfo Ping

The research team conducted a comprehensive survey of the Jinfoping Watershed in Wuqi County. According to the area and topographic characteristics of the study area, sample sites were pre‑set using a uniform grid layout method with 700 m–800 m intervals, taking into account both representativeness and uniformity. Aiming to reflect the natural characteristics of the watershed, typical vegetation types (arbor, shrub, grassland) were preferentially selected at each sample plot. Finally, 57 sample sites were established. First, GPS was used to record the longitude, latitude, and altitude of each sample site. Sampling quadrats were set according to vegetation type: 10 m × 10 m for arbor, 5 m × 5 m for shrub, and 1 m × 1 m for herbaceous vegetation. For each vegetation type at a sample plot, 3 quadrats were established. Within each quadrat, 3 soil profiles were arranged in a triangular pattern. After removing surface litter, a 40‑cm‑deep soil profile was excavated, and soil samples were collected at three layers: 0–10 cm, 10–20 cm, and 20–40 cm. Three replicates were taken per layer, resulting in a total of 1539 soil samples. Soil samples from the same layer in each profile were mixed thoroughly. Impurities such as gravel and plant residues were removed.