Page 103 - Enzymes in Tropical Soils

Enzymes in Tropical Soils 91

6.2 Soil Water and the Release and Adsorption of
Enzymes by Soil Colloids

As discussed previously in Chapter IV, enzymes in soil exist in two forms;
dissolved in soil water and adsorbed on soil solids. Dissolved soil enzymes are
active while those adsorbed on soil solids are inactive. These forms are in
equilibrium and, hence, the dissolved enzymes will be adsorbed by soil solids if
their concentrations in soil water are relatively high. Conversely, the inactive
adsorbed enzymes may dissolve and becomes active when the concentrations of
dissolved enzymes are low due to several mechanisms such as enzyme
denaturation.
The concentration of dissolved enzymes in an air-dry soil is low because the
most part of dissolved enzymes is adsorbed. As shown previously in Fig. 4.1 (Salam
et al., 1998a), a stepwise addition of water increases the concentration of dissolved
phosphatase as measured by Tabatabai method (Tabatabai, 1982). Baligar et al.
(1988) also showed that the phosphatase activity in topsoil and subsoils stored at
field capacity were 1.7 to 1.9 times higher than those stored at dry condition.
Klein and Koths (1980) previously also reported that the activity of phosphatase
was higher in no-tillage soils than that in conventional tillege soils, closely related
to the higher water content in no-tillage soils. The higher activities of
phosphatases are suggested as a result of enzymes released from soil solids.
However, the fact that higher water contents above the optimum value
lower the activity of soil enzymes may complicate the interpretation. The role of
water in dissolving adsorbed soil enzymes is less significant compared to the role of
O 2 diffusion to suffice the need of O 2 for organic matter oxydation by soil
microorganisms. The increase in soil phosphatase activity below the optimum
temperature (Fig. 4.1) may be also caused by the increase in the populations and
activities of phosphatase producing soil organisms. The time dependence of the
raising activities following the first order kinetics as depicted in Fig. 6.6 and Fig 6.7.
may also support this suspicion.

6.3 Soil Water and Temperature versus the Lives of
Microorganisms and Macroorganisms

Water is needed by soil microoganisms and macroorganisms for growing
and developing. Some experimental data show that the populations and activities

Abdul Kadir Salam - 2014