Page 109 - Enzymes in Tropical Soils

Enzymes in Tropical Soils 97

(Moyo et al., 1989). Previously, Kumar and Wagenet (1984) also showed that the
rate of urea hydrolysis increased with temperature following the first-order kinetics
in accordance with the increase in urease activity (Fig. 6.10). Most researchers
suggest that the activity of soil enzymes increases with the increase in soil
temperature until an optimum temperature, after which the activity decreases as
the enzyme proteins may be denatured, as the case of urease at temperature
o
above 60 C (Gianfreda et al., 1992). Dash et al. (1981) previously reported that the
activity of urease in two Indian soils increased with temperature, and decreased
o
after an oprimum temperature of 47 C (Fig. 6.11).

30

25

Activity (mg-N g -1 h -1 ) 20 Kilburn

Dagor
15 Nibley

10
5 15 25 35 45
o
Temperature ( C)

Fig. 6.10. The effect of temperature on the activity of urease in three soils
(Redrawn from Kumar and Wagenet, 1984).

It is suggested that the increase in soil enzymatic activities with soil
temperature is attributed to two importan reasons. First, the phenomenon is
related to a direct effect of heat on the microorganims producing enzymes. As
previously discussed, at low temperature the populations and activities of soil
microorganims are low and, hence, the production and activities of soil enzymes
are also low. Second, this phenomenon is a direct effect of soil heat on the enzyme
proteins. The rates of reactions generally increased 2 – 3 times with the increase of

Abdul Kadir Salam - 2014