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.