Leguminosas inoculadas com rizobios para remediação de solo contaminado com metais pesados por deposição de escórias

Abstract

Heavy metals are elements that are potentially toxic to natural ecosystems and to human health, and may originate from natural or anthropogenic sources. In Brazil an important case of environmental contamination involving heavy metals occurred in Santo Amaro, Bahia. One foundry company left an environmental liability of approximately 500,000 tonnes of metals containing mainly cadmium (Cd), lead (Pb) and zinc (Zn). Soils near the slag contain heavy metals in concentrations above permissible limits, even after years of interruption of metallurgical activity. In this sense, remediation techniques are needed to minimize the effects of metals on contaminated soils and restore ecosystem functionality around the factory. The present work has the objective of evaluating the potential of two legume species (mucuna and bean), inoculated with rhizobia, for use in phytoremediation of contaminated soils. In addition, the occurrence and morphological characteristics of the naturally established rhizobia populations in contaminated soils were evaluated using the two legume species cultivated without inoculation as bait plants. The legumes were cultivated in pots containing soil samples collected at points with increasing distances from the source of contamination, to represent the effect of different concentrations of the metals in the soil, and inoculated with nitrogen - fixing bacteria. Soils presented values of Pb and Cd above the value of investigation, signaling the need for remediation. The plants used presented symptoms of toxicity induced by the excess of heavy metals, however being tolerant, evaluated as a function of biomass production. The populations of native rhizobia of contaminated soils are tolerant and resistant to high concentrations of metals, such as: Cd, Pb and Zn. However, the high concentrations of heavy metals affected the nodulation and consequently the amount of N fixed. The phenotypic characteristics of these isolates showed a very high diversity, unusual, given the amount of isolates. Therefore, further studies with molecular tools to determine which species are able to nodulate in an environment as inhospitable as soils contaminated with heavy metals.