Page 146 - Enzymes in Tropical Soils

134 Enzymes In Tropical Soils

system. As a result, the presence of heavy metals may lower the soil enzymatic
activities.
A great deal of researchers reported that heavy metals negatively affect the
soil enzymatic activities (Juma and Tabatabai, 1997; Mathur and Sanderson, 1980;
Mathur et al., 1980; Stroo and Jencks, 1982; Dick and Tabatabai,1983; Stott et al.,
1985; Reddy and Faza, 1989; Reddy et al., 1987; Asthana et al., 1992; Landesmaki
and Piispanen, 1992; Park et al., 1992; Joshi et al., 1993; Sakai and Tadano, 1993;
Salam et al., 1997a; 1997e; 1997f; 1997n; Geiger et al., 1998a; Salam et al., 1999h).
Most of the researchers generally show that heavy metals decreased the activities
of various soil enzymes such as acid phosphatase, -glucodidase, dehydrogenase,
and urease (Juma and Tabatabai, 1977; Stott et al., 1985; Reddy et al., 1987; Eivazi
and Tabatabai, 1990; Park et al.,1992; Stott et al., 1992; Salam et al., 1997f; 1997n;
1999h). For example, Geiger et al. (1998a) found that the actiity of -glucosidase
was inhibited by dissolved Cu concentration exceeding 0.2 mM. The presence of
goethite reduced the inhibitory effect of Cu due to the strong affinity of Cu towards
goethite. Reddy et al. (1987) found that soil dehydrogenase activity was inhibited
by the presence of sludge borned Cu, Zn, Ni, Cd, Fe, and Mn.
The effectiveness of heavy metal inhibition on the activity of enzymes is
various. Comparing several heavy metals on the inhibition of pyrophosphatase,
which catalyzes the hydrolysis of pyrophosphates to orthophosphate, with a 25
mmol/g soils, Stoot et al. (1985) grouped the metal elements into three catagories:
the most effective (average inhibition > 50%), moderately effective (average
inhibition 30 – 50%), and weakly effective inhibitors (average inhibition < 30%. The
grouping of the metal cetions is shown in Table 9.7.

Table 9.7. The grouping of heavy metal enzyme inhibitors.

1 Most Effective Hg(II), As(V), Mo(VI), W(VI)
2 Moderately Effective Cd(II), Cu(II), Fe(II), B(III), V(IV)
3 Weakly Effective As(I), Cu(I), Mn(II), Ni(II), Pb(II), Sn(II),

Zn(II), Al(III), As(III), Cr(III), Fe(III),
Se(IV), Ti(IV)
4 Not Inhibitor Ba(II), Co(II)
Adapted from Stott et al. (1985)

Abdul Kadir Salam - 2014