The triphenyl formazan (TPF) was then extracted with methanol and determined colorimetrically at 485 nm. Acid phosphomonoesterase
(E.C. 3.1.3.2.), -glucosidase (E.C. 3.2.1.21) and
arylsulphatase (E.C. 3.1.6.1) activities were determined by incubating
soils with a substrate containing p-nitrophenyl sulphate
solution and quantified spectrophotometrically, measuring the
optical density of its coloration and comparing it with the
optical density of a standard solution of p-nitrophenol liberated
by an enzymatic hydrolysis. In these cases, the enzymatic
activity is expressed in mol p-nitrophenol g−1 h−1. Acid phosphomonoesterase
activity was determined by using p-nitrophenyl
phosphate as substrate and incubating it at pH 5.0 (Modified Universal
Buffer) and 37 ◦C. After 30 min, 2 M CaCl2 was added (to stop
the reaction and to avoid the brown coloration caused by organic
matter) and the liberated p-nitrophenol was extracted with 0.2 M
NaOH (