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Enzymes in Tropical Soils 137

3+
occurring in soil solutions of most soils but to decrease slightly by Al at
concentration 0.4 nM (Sakai and Tadano, 1993).

9.3 Indirect Effects of Heavy Metals on Soil Enzymes

As mentioned previously, the decrease in soil enzymatic activity as affected
by heavy metals may be indirectly caused by the interaction of heavy metals with
microorganisms, macroorganisms, and plant rooting system (Mathur et al., 1980;
Baath 1989; Hiroki, 1994; Husyman, 1994). This interaction ends with the decrease
in the microbial pupulations and activities. For example, Hiroki et al. (1994) found
that the number of actinomycetes and bacteria decreased significantly by heavy
metals. Actinomycetes was affected most strongly. The degree of tolerance to
heavy metals follows the order of: fungi > bacteria > actinomycetes. In accordance
with this findings, Huysman et al. (1994) reported that in general bacteria were
more sensitive to Cu and were fungi. The decrease in the number of colony
forming unit as affected by Cu in unmanured soil is depicted in Fig. 9.6.

60
Activity (nmol mg -1 2.5min -1 ) 40 CaCl2
50

30
MgCl2
20
ZnCl2
10

0
0 1 5 10 15 20
Concentration (mM)

Fig. 9.5. The effects of Zn, Ca, and Mg on the activity of phosphomonoesterase
(Redrawn from Park et al., 1992).

Abdul Kadir Salam - 2014