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.