Page 218 - Soil Degradation, Conservation and Remediation

6.2 Sources of Soil Pollutants 207

metals such as Cu, Pb, and Zn (Bell et al. 1988 ). Wastewaters containing high
concentrations of heavy metals have also been shown to inhibit mycorrhizal
infection of soybean, especially in soils with low pH (Angle and Heckman 1986 ).
Lead and copper are less mobile than Zn and Cd. Ni, Cd, and Zn are potentially
more serious contaminants of soil solutions than Cu and Pb (Biddapa et al. 1982 ).

Absence of nitrogen fixation in clover crops grown on soils contaminated with heavy
metals over a long period of time has been found to result from a survival only of
ineffective rhizobial strains (Giller et al. 1989 ).

Toxicity of Heavy Metals to Plants

Plants absorb heavy metals, essential or nonessential, from soils. Copper, molybdenum,
zinc, and nickel are essential trace elements required by plants in extremely small
amounts. If absorbed in relatively large amounts, all essential or nonessential heavy
metals become toxic to plants. According to Dan et al. ( 2008 ), heavy metals are
potentially toxic for plants; phytotoxicity results in chlorosis, weak plant growth,
yield depression, reduced nutrient uptake, disorders in plant metabolism, and, in

leguminous plants, a reduced ability to fix molecular nitrogen. Metals inhibit seed
germination, seedling growth, photosynthesis, and enzyme activity, but the effects
vary with the metals, their concentrations, as well as speciation and the plant species.
In a study Fargasova ( 1994 ) observed that Cr, Cd, Hg, Pb, and As all reduced seed
germination of mustard ( Sinapis alba ), but As was the most inhibitory. Cadmium
was found to be less toxic for germination of Sinapis alba seeds, but it is highly toxic
to mung bean ( Vigna radiata ) seeds. Spinach, soybean, and curly cress, for instance, were
sensitive to Cd, whereas cabbage and tomato were resistant. Toxicity symptoms of
some heavy metals to plants are summarized in Table 6.9 .

Heavy Metal Pollution and Earthworms

Earthworms are wonderful creatures of the soil. They constitute the largest
terrestrial faunal biomass. They live in soil, they modify soil, they ingest soil, and
they enrich soil. But they themselves are affected by the adverse conditions of
soil. For example, earthworms can be exposed to elevated metal levels in soil by
direct dermal contact or by ingestion of pore water, polluted food, and ingested
soil particles (Lanno et al. 2004 ). Saxe et al. ( 2001 ) estimated that earthworms
Eisenia andrei uptake more than 96 % of Cd and Cu and 82 % of Zn by der-
mal contact. Hobbelen et al. ( 2006 ) observed bioaccumulation of Cd, Cu, and
Zn by the earthworms Lumbricus rubellus and Aporrectodea caliginosa in soils
with high binding capacity. In the transfer of pollutants towards other trophic
levels, earthworms occupy a key position (Granval and Aliaga 1988 ).
Earthworms were found to have a high potential for Cd accumulation in pol-
luted floodplains (Hendriks et al. 1995 ). They have been considered useful for

assessing heavy metal pollution in soils (Menzie et al. 1992 ) because earth-
worm biomass and abundance were found to be more sensitive to pollution in
comparison with other indicator taxa (Spurgeon et al. 1996 ).