Are Rhizobium and soil enzyme activities good indicators of heavy metal soil contamination?

The widespread pollution of soils is an increasing urgent problem because of its contribution to environmental deterioration on a global basis. Several toxic compounds, such as heavy metals, often contaminate soils. The main sources of heavy metal pollution are mining, industries and application of metal-containing pesticides, fertilizers and sewage sludge. In recent decades there has been increasing concern with heavy metals, not only because of their toxicity to animals, plants and microorganisms, but also because they are highly toxic, mutagenic and/or carcinogenic to humans. Due to their small size, which provides a large contact area that can interact with the surrounding environment, microorganisms are the first biota showing the impact of toxic compounds. Microorganisms being in intimate contact with the soil environment are considered to be the best indicators of soil pollution. In general, they are very sensitive to low concentrations of contaminants and provide a rapid response to soil perturbation. Rhizobium spp. are ubiquitous gram-negative soil bacteria that have a profound scientific and agronomic significance due to their ability to establish nitrogen-fixing symbiosis with legumes, which is of major importance to the maintenance of soil fertility. There is increasing evidence of the adverse effects of heavy metals on soil microbial processes, including on soil enzymatic activities. Soil enzymes are the driving force behind all the biochemical transformations occurring in the soil. Enzymes catalyse all biochemical reactions and are an integral part of nutrient cycling and soil fertility. Therefore, this chapter evaluated the impact of heavy metals on Rhizobium populations isolated from a lead mine which activity ceased 50 years ago. In order to reach this goal some physicochemical parameters that influence metal bioavailability was determined, as well as metal concentrations in soils. Soil enzyme activities are highly affected by soil conditions and their evaluation may provide useful information on soil microbial activity and survival. For this reason, soil enzyme activities have been proposed as biological indicators of pollution, specially organic, but information about the influence of heavy metal on soil enzyme activities is scarcer. Thus, it was determined the activity of enzymes such as dehydrogenases, hydrolases, phosphatases, catalase and lipase in heavy metal contaminated soils. Metal tolerance of Rhizobium isolates was also screened in artificial media supplemented with different metals (Pb, Cd, As, Ni, Cu and Co) and their tolerance related to soil contamination and enzyme activities. This chapter can widen the knowledge about the pressure that soil microflora experience under the direct effect of different metals. Rhizobium and soil enzyme activities may be useful for the evaluation of agricultural soil pollution, which may be used on the improvement of soil productivity or on the reclaim of contaminated soils.