Enzyme Activities

Soil enzyme activities have been proposed as suitable indicators of soil quality due to their intimate relationship with soil biology, their ease of measurement, and their rapid response to soil perturbations and/or stress, including those caused by heavy metal pollution (Dick et al. 1996). It has also been suggested that soil enzyme activities are indicators of process diversity, providing information on the biochemical potential, possible resistance and resilience, and the potential for manipulation of the soil system (Taylor et al. 2002).

However, the use of soil enzymes as indicators of microbial function is controversial because the overall enzyme activity is derived from various fractions, i.e., growing microorganisms, dead cells, and extracellular enzymes associated with the clay-humus complex (Wallenstein and Weintraub 2008). On the other hand, although the soil enzyme activity is mainly of microbial origin, it also originates from plants and from soil meso- and macrofauna. In addition, soil enzyme assays generally provide a measure of potential activity (Speir and Ross 2002). Measurements of soil enzyme activity are usually based on the addition of an artificial substrate at a saturating concentration in order to achieve a reaction rate that is proportional to the enzyme concentration.

Many studies have used soil enzyme measurements to evaluate the effects of heavy metal pollution in soil. The types of enzymes that have been measured range from unspecific enzymes that provide information about general microbial activities, such as dehydrogenase or fluorescent fluorescein diacetate, to those involved in specific reactions related to nutrient transformations, including urease, amidases, phosphatases, phenol oxidades, b-glucosidases, cellulases, arylsulfatase, etc. For example, in our laboratory, studies of the effect of adding (heavy metal enriched) pyrite sludge to soil indicated a consistently negative effect on the activities of many soil enzymes (Hinojosa et al. 2008). Arylsulfatase activity showed the lowest ED50 values, agreeing with our previous results in the field (Hinojosa et al. 2004a,b). The activity of this soil enzyme was previously cited by many authors as being one of the most sensitive to trace element pollution (Dick 1997; Renella et al. 2005). On the other hand, b-glucosidase activity also showed a high sensitivity to pyrite sludge pollution, which correlates with statements by Kuperman and Carreiro (1997) and Kunito et al. (2001) suggesting that the activity of this enzyme could be a useful indicator of soil quality due to its important role in the degradation of organic matter. This conclusion do not agree with those of Effron et al. (2004) and Renella et al. (2004), who found that this enzyme was apparently insensitive to heavy metal contamination. Kizilkaya and Bayrakle (2005) reported that the activity of urease was the most strongly affected by Zn pollution. However, interestingly, in our studies this enzyme was one of the lest sensitive to pyrite sludge contamination, agreeing with the findings of Speir et al. (1995, 1999) and Karaca et al. (2002).

The wide range of sensitivities to heavy metals shown by the activities of different enzymes in different studies may be due to different experimental approaches. On the other hand, heavy metals could affect soil enzyme activities via various pathways: (a) inactivation of the produced enzyme; (b) inhibition of the biosynthesis of microbial enzymes, and; (c) changes in the specific compositions of microbial groups that produce extracellular enzymes.

In addition to these direct effects of heavy metals, other indirect effects include changes in soil pH (which heavily affect substrate-enzyme kinetics) and the availability of natural enzyme substrates.

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