2.3 Soil properties
Soil subsamples were air-dried and analyzed for chemical and physical parameters. Soil pH was determined in deionized water (soil : water, 1 : 2.5) using a pH meter (Mettler-Toledo, Switzerland)). Total SOC was measured by wet digestion with 133 mmol L-1K2Cr2O7 and concentrated H2SO4 at 170–180 ℃ Total N (TN) was determined using a semi-micro-Kjeldahl method. Available N (AN) was analyzed using a diffusion method. available P(AP) was extracted using a mixed solution of 0.03 mol L-1 NH4F and 0.025 mol L-1 HCl. Available potassium (AK) was determined by flame photometric method. Soil inorganic N (NH4+ and NO3) was extracted from soil samples with 1 M KCl and determined by colorimetrically using a segmented flow auto analyzer (Skalar, Breda, Netherlands) (Lu, 1999). The values ofδ 13C and δ 13N were determined using an isotope ratio mass spectrometer (IsoPrime 100,Germany) coupled with an auto elemental analyzer (vario MICRO cube,Germany).