2.4. Soil enzyme activities
Soil enzymes involved in organic C decomposition and N mineralization were selectively examined. To reduce variation for measurements of soil enzyme activities, triplicate measurements were performed for soil collected from each field plot and then the average values were calculated and used for statistical analysis. All enzyme assays were determined against two types of controls (i.e., substrate alone and soil filtrate alone). Soil enzyme activity is expressed as mmol of a product produced per h per kg of soil.
The activity of N-acetyl-β -glucosaminidase was determined by using p-nitrophenyl N-acetyl-β -glucosaminide as the substrate (Parham and Deng, 2000). Soil ( ̴ 0.5 g dry weight equivalent) was
incubated for 1 h at 37 ˚C with 1 ml of 10 mM glucosaminide and 4 ml of 50mMacetate buffer (pH 5.0). Reactionwas terminated and colorwas allowed to develop by adding 1 ml of 0.5M CaCl2 and 4 ml of 0.5 M NaOH. The filtrate was measured colorimetrically at 410 nm.
Cellulase activity was determined by the modified method of Von Mersi and Schinner (1996). Soil ( ̴ 2 g dry weight equivalent) was incubated with 0.9% of carboxymethyl-cellulose in 10 ml 0.5 M acetate buffer (pH 5.5) at 37 ˚C for 24 h. The filtrates of soilsubstrate slurries were subjected to color reaction and the produced Prussian blue was analyzed colorimetrically at 690 nm.
The activity of soil phenol oxidase was determined colorimetrically using L-3,4-dihydroxyphenylanaline (i.e., DOPA) as the substrate (Saiya-Cork et al., 2002). In brief, soil ( ̴ 0.5 g dry weight equivalent) was mixed with 1 ml of 10 mM DOPA and 4 ml of 50 mM acetate buffer (pH 5.0) and incubated at room temperature for 1 h by constantly shaking at 125 rev min-1. The reaction was terminated by centrifugation at 5000 × g for 5 min and then the optical density of the supernatant was measured at 460 nm.