2.3 Soil properties analyses
A PHS-3C pH meter (Shanghai Lida Instrument Factory, Shanghai, China) was used to measure the soil pH via the potentiometric method in a soil-to-water ratio of 1:2.5 (w/v). The soil moisture was gravimetrically measured. An elemental analyzer (2400 II CHNS/O, Perkin-Elmer, Danbury, CT, USA) was used to measure the soil total carbon (C) and total nitrogen (N) via dry combustion at a combustion temperature of 950 °C. The soil inorganic C was volumetrically measured using an inorganic C analyzer (Calcimeter 08.53, Eijkelkamp, Giesbeek, Netherlands). The soil organic C content was indirectly determined as the difference between the total C and inorganic C contents. An automated segmented flow analysis (SmartChem140, AMS Systea, Roma, Italy) was used to colorimetrically measure the ammonium (NH4+) and nitrate (NO3-) contents in the soil via the salicylate/dichloroiso-cyanuric acid and cadmium column/sulphanilamide reduction methods, respectively. The available phosphorus (P) was extracted using 0.5 mol L-1 NaHCO3 (pH 8.5) and measured by a 722s visible light spectrophotometer (Shanghai Precision and Scientific Instrument Corporation, Shanghai, China).