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Growth and nodulation of Acacia mangium, Enterolobium contortisiliquum and Sesbania virgata in heavy metal contaminated soil

One of the great challenges in environmental research today is to find tolerant species of plants and microsymbionts to make viable the revegetation of contaminated areas with heavy metals. This experiment was carried out under greenhouse conditions at the Soil Science Department of the Federal University of Lavras (MG), from August to December 1998, to assess the tolerance to heavy metals and the capacity to establish rhizobia symbiosis with Enterolobium contortisiliquum (tamboril), Acacia mangium (acacia) and Sesbania virgata (sesbânia) in soil mixtures, containing different proportions of contaminated soil (PCS): 0, 15, 30, 45 and 60% (v/v) with Zn, Cd, Pb and Cu (18.600, 135, 600 and 596 mg dm-3 extracted by aqua regia, respectively), diluted in a low fertility Oxisol. The species were inoculated with recommended strains (S) and isolates from soil contaminated with heavy metals (ICS) and isolates from non contaminated soil (INS), whose tolerance to Cu, Cd and Zn was previously determined "in vitro". PCS increase in the mixtures inhibited vegetative growth, dry matter production and the nodulation of the three species. The tamboril-BR 4406 symbiosis was the most tolerant and acacia-BR 3617 symbiosis the most sensitive to soil contamination. Although the ICSs that were more tolerant "in vitro" formed effective nodules in non contaminated soil, nodules were ineffective in soil mixtures with heavy metal contaminated soil. In the PCS, 15% (Zn = 750; Cd = 22.1; Pb = 65.1 and Cu = 111 mg dm-3 extracted by DTPA) the specific activity of nitrogenase was stimulated, increasing 5 and 10 times in relation to the soil without contamination for the sesbânia-BR 5401 and tamboril-BR 4406 symbiosis, respectively. Rhizobia's "in vitro" tolerance to heavy metals was not related to symbiosis tolerance in contaminated soil.

Bradyrhizobium; Azorhizobium; biological N2-fixation; leguminous; soil pollution; heavy metal; phytotoxicity; phytoremediation


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