Page 139 - Enzymes in Tropical Soils

Enzymes in Tropical Soils 127

In general, heavy metals in the soil environment are originated from two
major sources: (a) natural sources such as rocks rich in heavy metal containing
minerals and (b) anthrogenic sources such as various industrial wastes, fertilizers,
and pesticides containing heavy metals (Lagerwerff, 1982; Wang et al., 1982;
Kardoz et al., 1986; Leung, 1988; Hegstrom and West, 1989; Alloway, 1990c;
Davies, 1990; Kiekens, 1990; Dowdy et al., 1991; Boon and Soltanpour, 1992;
Herrero and Martin, 1993; Sweet et al., 1993; Nicholson et al., 1994; Flegal and
Smith, 1995; Vile et al., 1995; Gimeno-Garcia et al., 1996; Salam et al., 1996; Yeh et
al., 1996; Salam et al., 1997a; Juracek and Ziegler, 2006; Biasioli et al., 2007; Benke
et al., 2008; Berenguer et al., 2008; Lin et al., 2008; Cakmak et al., 2010; Wang et
al., 2010; Tu et al., 2012). Nriagu (1979) as cited by Tiller (1989) estimated that the
anthropogenic emissions of heavy metals were much higher than those of natural
emissions (Table 9.1). Heavy metals may spread to the environment through food
chains, soils, rivers, and seas.

Table 9.1. The estimates of worldwide natural and anthropogenic
emmision of some heavy metals.

Natural Anthropogenic
Cd 0.83 7.3
Cu 18 56
Ni 26 47
Pb 24 450

Zn 44 310
3
Adapted from Nriagu (1979) as cited by Tiller (1989); in x 10 Mg
-1
year

As explained by Salam (2012), heavy metal contaminants come into soils
from various sources. Gimeno-Garcia et al. (1996) showed that Cd, Cu, and Zn from
agricultural superphosphates entered the soil system at the rates of 1,322, 7,500,
and 30,000 mg per hectare per year while those from pesticide Antracol were 7.79
mg, 20.00 mg, and 1,100g per hectare per year for Cd, Pb, and Zn, respectively
(Table 9.2). The heavy metal spreading through the soil system are partly
immobilized by adsorption or precipitation processes. Another part may enter the
ground water through water percolation, which then may enter the food chains

Abdul Kadir Salam - 2014