physico-chemical properties and enzyme activities
Soil bulk density (BD) and moisture (SWC) were measured with a bulk
density ring and conventional dry wet weight methods, respectively. Soil
pH was measured at a soil-to-water ratio of 1:2.5 (w/v) with a pH meter
(FE20K, Mettler Toledo, China). Organic carbon (SOC) was determined
using the dichromate oxidation method. Dissolved organic carbon (DOC)
was extracted by potassium sulfate (0.5 mmol L−1).
Total nitrogen (TN) was determined according to semi-micro Kjeldahl
method. Ammonium nitrogen (NH4+) and
nitrate nitrogen (NO3-) were measured
by a continuous flow analyzer (Skalar Analytical B.V, SAN++). Total
phosphorus (TP) and available phosphorus (AP) were assayed by the
molybdenum antimony anti-colorimetric method. Total potassium (TK),
available potassium (AK), exchangeable calcium (Ca) and magnesium (Mg)
were determined using the EDTA titrimetric method and an inductively
coupled plasma spectrometer (ICP-AES, Thermo, USA). Potential soil
sucrase (SC), urease (UR), acid protease (ACPT) and glutaminase (GLS)
activities were determined using the soil enzyme kit from Solarbio
Science & Technology Co. (Beijing, China). Acid (ACP) and alkaline
phosphatase (ALP) activities were assayed by p-nitrophenyl phosphate
disodium method (PNPP) at pH = 6.5 and 11, respectively. Three parallel
measurements were performed for each sample to minimize experimental
errors.