2.2 Soil physicochemical properties determination
Measurement of soil properties were carried out following the methods of the soil agrichemical analysis (Lu, 2000). Soil pH was measured with a METLER TOLEDO FE20 at a soil/water ratio (m/m) of 1:1. Soil moisture content was determined after drying at 105°C for 12 h. Soil organic matter was calculated with organic carbon determined with potassium dichromate and sulfuric acid. Total carbon (TC) and total nitrogen (TN) were measured by the elemental analyzer (Vario EL Ⅲ, Elementar). Dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) were measured by TOC/TN analyzer (Multi N/C Model 3100, Analytic Jena, Germany). Ammonium-N (NH4+-N) and nitrate-N (NO3--N) were extracted from 6.0 g fresh soil with 1.0 mol L-1 KCl solution (soil/extract ratio of 1:5) and shaken for 1 h at 180 rpm. The soil extract was analyzed by continuous flow analyzer (AA3, SEAL, Germany).