2.3.1 Root and soil sampling
One average individual plant was selected in each plot for sampling roots in late September 2019, 2020, and 2021, representing plant growth for the second, third, and fourth years, respectively. Soil cores (7 cm inner diameter) were used to sample root segments and soil from vertical soil profiles. In each plot, two cores were extracted at 10 cm from the plant base in different directions. Each soil core was sampled to a depth of 50 cm and divided into five layers (0–10, 10–20, 20–30, 30–40, and 40–50 cm) in the second and third years. Sampling of the 50–70 cm soil layer was performed in the fourth year. The root segments and soil of each soil layer from the two cores were mixed into one sample in each plot to estimate the root morphology and dry weight.