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).