Soil Pollution

One form of pollution that has affected soil microbial communities and activities for many decades is acid deposition. This is caused by acid precipitation, the result of the release of nitrogen oxide (NOx) and sulfur dioxide (SO2) into the atmosphere, where they are oxidized to SO4 and NO3. Despite the efforts made to reduce the primary sources of acid input, its effects are still apparent in many regions. The effect of acid deposition on the soil ecosystem depends on the concentrations of SO4 and NO3, the amount of precipitation, and the buffering capacity of the soil (the cation exchange capacity via bases). The nitrogen and sulfur provided by acid rain may stimulate the growth of some soil microorganisms. On the other hand, even low-level but prolonged acid rain will result in soil acidification, which may have adverse effects on soil bacteria, whereas its effect on fungi seems to be minor (Pennanen et al. 1998a).

The effect of acid deposition can be direct or indirect. The lower pH and reduced concentrations of divalent cations (Ca2+, Mg2+) can lead to the mobilization and increased bioavailability of heavy metals and other toxic compounds (Francis 1986). Acidification of soils may also reduce the solubility of organic matter and thereby reduce substrate availability for microbes. Increased soil acidity does not seem to affect prokaryotic biomass to any significant extent; instead, it reduces prokaryotic growth rates and activity (Francis 1986; Pennanen et al. 1998b).

Reduced activities of a number of soil enzymes, such as dehydrogenases, ure-ases and phosphatases, have been observed upon significant pH reductions (Killham et al. 1983). The reduced microbial growth observed with increased acidity may indicate that more metabolic energy is used for maintenance rather than for the biosynthesis of cell materials. It has been suggested that an increased metabolic quotient (ratio of basal respiration to microbial biomass) indicates a shift in energy use from growth to maintenance, and that this increased energy demand is a sensitive indicator of physiological adaptation to environmental stress (Post and Beeby 1996; Liao and Xie 2007).

Soil can have naturally high concentrations of heavy metals as a result of the weathering of parental material with high amounts of heavy metal minerals (e.g., mineral sulfides). Other sources include contaminations associated with mines and metal smelters, which have led to increased soil concentrations of heavy metals, such as zinc, cadmium, copper and lead. Sewage sludge may also contain heavy metals, and it has been demonstrated that the long-term application of heavy metal containing sewage sludge to agricultural soils can have profound effects on the microbial diversity and community composition (Sandaa et al. 1999; Gans et al. 2005).

The effect of heavy metal toxicity depends on soil factors such as organic matter and clay content, divalent cation concentrations (cation exchange capacity), and pH (Giller et al. 1998). These factors influence complex formation and the immobilization of heavy metals. However, the relative toxicities of different metals, namely Cd, Cu, Zn, and Pb, appear to be the same irrespective of soil type (Baath 1989). In soil contaminated for 40 years with high concentrations of Cr and Pb, the microbial biomass and activity were reduced and soil organic carbon had accumulated. These results indicated that Pb exerted a greater stress on soil microbes than Cr.

Soil microorganisms vary widely in their tolerance to heavy metal contamination, and the proportion of culturable resistant microorganisms can range from 10% to nearly 100%. The activities of enzymes in soil may serve as indicators of heavy metal contamination, as there are generally high correlations between reduced enzyme activities (of, e.g., dehydrogenases, acid phosphatases and ureases) and increased heavy metal contamination (Baath 1989). It has been reported that heavy metal contamination has different effects on soil bacteria and fungi (Rajapaksha et al. 2004). Metal addition decreased bacterial activity but increased fungal activity, and fungal activity was still higher in contaminated than in control soil after 35 days. The different effects of heavy metals were also demonstrated by an increase in the relative fungal/bacterial ratio (estimated using phospholipid fatty acid analysis) with increased metal concentrations.

Mechanisms for metal resistance include stable complex binding (chelation) with organic ligands (extracellular or intracellular sequestration), transportation out of the cells, and biotransformation of the ions to less bioavailable or less toxic metal species. Genes for metal resistance (e.g., mercury resistance) are often present in plasmids and can easily be disseminated through a population or community in response to selection pressure associated with toxic metal exposure.

Hydrocarbon contamination of soils caused by human activities is increasing around the world. Petroleum is a rich source of carbon, and most hydrocarbon components can be biodegraded by microorganisms. The rate of degradation is normally rather low, since crude oil has low concentrations of phosphorus and nitrogen, which does not permit the extensive growth of indigenous hydrocarbon-degrading microorganisms in petroleum-contaminated soils. However, growth can be stimulated by the addition of phosphorus and nitrogen fertilizers. In many extreme environments there are hydrocarbon-polluted areas (Margesin and Schinner 2001). Bioremediation success in such environments depends on the presence of biodegrading microbes that are adapted to the prevailing environmental conditions.

Pesticides are classified according to their primary target organisms; i.e., into herbicides, fungicides, and insecticides (Johnsen et al. 2001). Normally the pesticides are very specific and restricted to a narrow range of target organisms. However, they can be modified in the environment and can become toxic to non-target organisms. For instance, triazines, which normally target photosynthetic enzymes in C3 plants, can be chlorinated in the triazine ring and thus become toxic to a wide range of organisms. The effect of pesticides on soil microbes depends on their bioavailability, which in turn is influenced by the crop being grown, as well as soil properties affecting the sorption and leaching of pesticides. Microorganisms can develop resistance to pesticides through their ability to decompose or transform them into less toxic compounds.

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