Page 53 - Enzymes in Tropical Soils

Enzymes in Tropical Soils 41

immobilization. Huang et al. (1995) reported that a complete adsorption of
enzymes led to the formation of clay-enzyme complexes, that had lower activities
than the free enzymes. They stated that enzyme adsorption by clay was one of the
factors that can reduce the enzyme activity other than the amount of clay and the
increase in soil pH. Boyd and Mortland (1985) found that the behavior of the free
and bound enzymes were similar with respect to kinetics parameters, optimum pH,
and substrate selectivity. However, in general most researchers agree that the
major part of the active enzymes are those that are soluble in the soil water (Huang
et al., 1995). The enzymes in soils can be extracted, among which by 0.14 M
sodium pyrophosphate pH 7.1 that can extract phosphatase, urease, casein, and
proteases (Nannipieri et al., 1980).
The soluble forms of enzymes are in equilibrium with those adsorbed by soil
solids (Huang et al., 1995). Therefore, the concentrations of soluble enzymes are
also determined by the concentrations of the adsorbed enzymes. As the soluble
enzymes decrease due to some other mechanisms such as leaching through soil
body or enzyme decays, the adsorbed enzymes may dissolve to compensate the
equilbrium shifting. Conversely, the concentration of the adsorbed enzymes
increases as the concentration of dissolved enzymes increases. Dick and Tabatabai
(1987) reported that a part of adsorbed phosphatase also participates in the
organic P transformation. However, part of soil researchers suggest that the biggest
part of active phosphatases is those dissolved in soil water (Huang et al., 1995).
The activities of enzymes in soil are probably determined by the soluble enzymes.
Changes in the soil environment properties, such as pH, soil water, and so on, may
probably directly affect this form of enzymes.
The soluble enzymes may easily move with water through water percolation
and/or run-off while the adsorbed enzymes may move only with soil particles
through soil erosion or leaching (Tabatabai and Fu, 1992). Salam et al. (1998)
suggests that some soil enzymes might have moved from topsoils (0 – 30 cm) to
subsoils (30 – 60 cm). Plant roots and microorganisms generally concentrate in
topsoils and, therefore, the soil enzymes must concentrate in topsoils. However, it
was reported that these soil enzymes are also found in subsoils at lower
concentrations. The enzymes in subsoils were probably originated from topsoils
and moved to subsoils by leaching. Huysman et al. (1994) suggested that native
bacteria can also migrate downward in soil body for a considerable distances in
agricultural soils. This suggestion indicate that the subsoils enzymes are also
possibly produced by the migrated microorganims.

Abdul Kadir Salam - 2014