2.3 Soil sampling and laboratory analyses
Soil moisture (W) was quantified by an oven-dry method, and soil bulk density (ρb) was quantified by a cutting ring (5-cm diameter and 5-cm high) in each 2 × 2 m quadrat. Five soil samples were collected at a depth of approximately 15 cm from the center and four corners of each 2 × 2 m quadrat and mixed into a single composite sample. The composite soil samples were air-dried at room temperature, ground and passed through a 100 mesh plastic sieve to determine the chemical properties.
Soil chemical properties including pH, total carbon (TC), total nitrogen (TN), total phosphorus (TP), total potassium (TK), available nitrogen (AN), available phosphorus (AP), available potassium (AK), calcium (Ca) and magnesium (Mg) were analyzed for each quadrat using standard soil test methods (Liu et al. 2012). Soil pH was measured in a 1:2.5 (w/w) ratio of soil and deionized water at 20°C. Soil TC and TN concentrations were determined by combustion on a Vario EL cube CN Elemental Analyzer (Elementar Analysen Systeme GmbH). After acid digestion in a mixture of HNO3, H2O2 and HF (7:2:1) in a microwave digestion system, soil TK, Ca and Mg concentrations were determined by an inductively coupled plasma mass spectrometer (ICP Spectrometer, Thermo Scientific), and soil TP was measured by an ultraviolet spectrophotometer (UV2550, Shimadzu). Soil AN was quantified using the diffusion-absorption method. Soil AK was measured as described for TK measurement after extraction using 1.0 mol∙L-1CH3COONH4 solution. Soil AP was determined via molybdenum-antimony colorimetry after extraction using 0.5 mol∙L-1 NaHCO3 solution (UV2550 Spectrophotometer, Shimadzu).