Page 185 - Enzymes in Tropical Soils

Enzymes in Tropical Soils 173

involving biotic and abiotic factors (Baligar et al., 1998). The first factor is the
presence and the amount of substrate in the soil system, which is represented by
organic matters. A direct relationship between the activities of soil enzymes and
soil organic C or organic matter contents and total N supports this theory
(Nannipieri et al., 1980; Harrison, 1983; Trasar-Cepeda and Gil-Sotres, 1987;
Bonmati et al., 1991; Tate III et al., 1991; Martens et al., 1992; Salam, 1996; Salam
et al., 1998d). The second is the amount of enzymes in soils. With the increase in
substrate concentration in soils, the rate of decomposition is more dependent on
the available catalysts, that decreases the energy of activation of the biochemical
reaction. The rates of the biochemical reaction may increase with the increase in
concentration of soil enzymes. Therefore, all factors controlling the activity of soil
enzymes will control the rates of the soil biochemical reactions. The third is soil
water content, which is important due to its roles as a reactant in hydrolysis
process involved in the soil biochemical reactions. In dry soils, the hydrolysis of
organic matter does not occur due to the absence of water. In general, the rates of
the soil biochemical reactions increase with the increase in soil water contents until
a particular value, after which further addition of water will decrease the rates of
the soil biochemical reactions (Salam, 1998a). The fourth is soil temperature
(Harrison, 1983; Neal, 1990), which affects the rates of the soil biochemical
reactions; in general the rates of reactions increase with the increase in soil
temperature until a maximum temperature, after which the increase in soil
temperature may decrease the soil biochemical reactions.
In addition to the above factors, the rates of the soil biochemical reactions
are controlled by soil pH (Malcolm, 1983; Reddy et al., 1991; Trasar-Cepeda et al.,
1991; Salam et al., 1998d; 1998i). For example, the activity of phosphatase
incrreases with the increase in soil pH until an optimum pH where the activity is
maximum and then decreases afterwhich (Frankenberger and Johanson, 1982;
Trasar-Cepeda and Carballas, 1991; Sakai and Tadano, 1993; Huang et al, 1995;
Salam et al., 1998d). Some workers also reported the effects of some soluble
elements in soil water on the rates of the soil biochemical reactions, among which
is the effect of P on the activity of phosphatase. The majority of researchers
reported that the addition of phosphate into soils decreased the activity of
phosphatase (Juma and Tabatabai, 1977; Pang and Kolenko, 1986; Fox and
Comerford, 1992).
These above controlling factors may be used to regulate the rate of the
biochemical decomposition depending on our purposes. In some cases, we need to
increase the soil enzymatic activities. For example, in the composting processes,
we can stimulate the production of more enzymes. To do this, we can manage the

Abdul Kadir Salam - 2014