Soil enzymology: Role of protective colloid systems in the preservation of exoenzyme activities in soil

Abstract

The effects of repeated freezing and thawing, heating and drying, changes in pH and the presence of various concentrations of ZnSO 4 and CuSO 4 (1–50gl −1) on the activities of selected soil enzymes (protease, cellulase and amylase) were studied in humus samples and their alkaline extracts from the humus layer of a grass-herb forest in Kiiminki, northern Finland. In order to elucidate the anticipated protective effect of the clay and humus colloids present in the soil in preserving the enzyme activities against changes in physical and chemical conditions, the enzyme activities were measured in untreated humus samples, their extracts and the protein fraction formed in Sephadex G-200 gel filtration using 0.5 mol 1 −1 Na 2CO 3-NaOH for elution. The fractions identified in gel filtration were: clay colloids (~200kDa mol. wt), proteins (~150kDa mol. wt), mucins (50–160kDa mol. wt) and humus colloids (5–50 kDa mol. wt). The enzyme activities were inhibited in the fractionated samples by freezing and thawing, heating and drying, changes in pH and the presence of zinc and copper sulphates, while they remained relatively constant in the original (untreated) humus samples. Dilution of the soil extract reduced K m from about 3.0 to 0.1 and V max from 5.0 to 1.5. The effects of the various treatments were relatively weak in the original humus samples, which suggests that the fractionation and dilution dismantle or weaken a protective mechanism probably built up by the clay and humus colloids present in the untreated and unfractionated samples and which enable soil to preserve its enzyme activities against certain environmental and other physico-chemical changes.