Physicochemical properties of licorice rhizospheric soil

The rhizospheric soil samples closely adhered to the roots were collected, screened with a 2 mm sieve, and dried at room temperature. Each dried soil sample (0.2 g) was digested in a 10:1:2 mixture composed of 10 mL perchloric acid (12.7 mol/L), sulfuric acid (18 mol/L) and water using a Mars 6 microwave digestion system (CEM, Matthews, NC, United States) until a transparent liquid was obtained (He et al., 2019b). Soil organic matter was determined as the percentage of organic carbon oxidized by dichromate in H₂SO₄ (Rowell, 1994; Chen et al., 2021). The contents of available nitrogen (N) and available phosphorus (P) was determined by using alkaline hydrolysis diffusion (Rowell, 1994; Chen et al., 2021) and the chlorostannous-reduced molybdophosphoric blue method (Olsen et al., 1954; Chen et al., 2021) respectively. The content of available potassium (K) was determined by Flame Photometer (Jackson, 1973; Chen et al., 2021).