Isoelectric focusing of β-glucosidase humic-bound activity in semi-arid Mediterranean soils under management practices

Abstract

Changes in β-glucosidase enzyme–humic complexes and conventional parameters (pH, total organic C, total N, water-soluble C, and bulk density) were studied in an almond-cropped soil prone to erosion under a rehabilitation practice. The experimental plan included three soil slopes (0%, 2%, and 6%) and two type of fertilization (organic and mineral), with sampling of rhizosphere and inter-row soils. The enzyme humic complexes were extracted by pyrophosphate, purified by ultrafiltration of the organic extracts on molecular mass exclusion membranes (mol wt > 104) and fractionated by isoelectric focusing technique (IEF). The IEF on polyacrylamide rod gels with a restricted pH gradient ranging between 6.0 and 4.0 gave five humic bands on the basis of the little differences of their electric charges (pI). Under both organic and mineral fertilization, β-glucosidase activity bound to the fractionated humic substances, especially in the pH range 4.5–4.2 of the rhizosphere soil, was higher than that of the inter-row soil. This also occurred in 6% slope where the enzyme activity was lower than in soil with lower slopes. The higher number of the β-glucosidase active humic bands in rhizosphere than inter-row soil, particularly for the 0% slope, may be due to the presence of humic molecules capable of preserving the enzyme molecules in the active form, other than to the higher microbial activity synthesizing and releasing the tested enzymes.