1.2.3 Sample determination
Soil samples were air-dried at room temperature, and visible roots and other debris in the soil samples were removed. Each composite soil sample was sieved through a 2-mm sieve. SOC was determined by the Walkley–Black method (Nelson & Sommers, 1996). Total N was determined by the Kjeldahl acid digestion method (Foss Kjeltec 8400, FOSS, DK) (Nelson & Sommers, 1996). Total P was measured by Mo-Sb colorimetry (UV-2102C, UNICO, Shanghai, China) (Nelson & Sommers, 1996). Soil pH value: potential method (water/soil ratio: 2.5:1). Urease enzyme activity was measured using the phenol sodium-hypochlorite sodium colorimetric method (Soil Physics institute, 1978). Sucrase enzyme activity was calculated according to the amount of glucose (mg) that was generated in 1 g soil sample after cultivation at 37℃ for 24h (Soil Physics institute, 1978). Alkaline phosphatase enzyme activity was determined using the method of disodium phenyl phosphate (Soil Physics institute, 1978). Catalase was determined by KMnO4titration (Soil Physics institute, 1978).