You can use the following method for determining carboxymethylcellulase (cellulase) activity.
Incubate soil samples with 0.7% carboxymethyl-cellulose in 2 M acetate buffer (pH 5.5) as substrate, for 24 h at 50ºC. The released reducing sugars can be measured following the method described in Schinner and von Mersi, 1990.
F. Schinner, W. von Mersi, 1990. Xylanase-, CM-cellulase- and invertase activity in soil: an improved method. Soil Biol. Biochem. 22, 511-515.