2.5 Soil chemistry
The soil samples were analyzed for pH, water content (SWC), total nitrogen (TN), total phosphorus (TP), available phosphorus (AP), ammonium-nitrogen (NH4+-N), and nitrate-nitrogen (NO3--N). Soil pH was measured with a pH meter (HI-9125, Hanna Instruments Inc, Woonsocket, RI) by mixing the air-dried soil sample with deionized water at a 1:2.5 ratio (w:v). SWC (%) was calculated from the mass loss after drying the fresh soil samples at 75°C to a constant weight, for at least 48 h. TN content was analyzed using the Kjeldahl digestion procedure (Gallaheret al. , 1976). TP was measured by ICP (ICAP6300). The AP, NH4+-N, and NO3--N were determined by a continuous flow analyzer (SEAL AA3, Norderstedt, Germany). Soil organic carbon (SOC) content was measured by a K2Cr2O7-H2SO4calefaction method (Nelson & Sommers, 1982). Soil C:N ratio (C:Nsoil) was calculated by SOC and TN. The measurements of TN, TP, and AP were all made on air-dried soil samples, the NH4+-N, and NO3--N content were made on fresh soil samples.
The activities of acid phosphatase (ACP) and protease (PR) were measured on the frozen-stored soil samples. ACP was determined withp -nitrophenol as a substrate (Schinner et al ., 1996), with the reaction mixture of 1 g fresh soil with 1 ml 100 mMp -nitrophenol. PR was determined with casein as a substrate according to Ladd and Butler (1972), with the reaction mixture of 1 g fresh soil with 5 ml casein solution (2%, w/v). The enzyme activities were expressed as µmol g-1 soil h-1.