Diversity and efficiency of rhizobia communities from iron mining areas using cowpea as a trap plant

Abstract

Mining is an important economic activity. However, its impact on environment must be accessed, mainly on relevant processes for their sustainability. The objective of this study was to evaluate the diversity and efficiency of symbiotic nitrogen fixing bacterial communities in soils under different types of vegetation in the Quadrilátero Ferrífero: ironstone outcrops, Atlantic Forest, neotropical savanna, and a rehabilitated area revegetated with grass. Suspensions of soil samples collected under each type of vegetation were made in a saline solution to capture rhizobia communities that were then inoculated on cowpea [Vigna unguiculata (L.) Walp.], which was used as a trap plant. The symbiotic efficiency of the communities was evaluated in a greenhouse experiment and the data obtained were correlated to the chemical and physical properties of the soils under each type of vegetation. At the end of the experiment, the bacteria present in the nodules were isolated to evaluate their diversity. The highest numbers of nodules occurred in the treatment inoculated with soil samples from rehabilitated area revegetated with grass and neotropical savanna vegetation, and the lowest numbers were observed in the treatment inoculated with soil samples from ironstone outcrops and Atlantic Forest. In relation to root dry matter, the treatment inoculated with soil samples from Neotropical savanah was superior to those inoculated with soil samples from the other areas; already, in relation to the shoot dry matter, no significant difference among the treatments was observed. The soil properties with the greatest influence on the microbial communities were Al3+ content, considered as high in the Atlantic Forest and neotropical savanna vegetation, as intermediate in the iron outcrops, and as very low in the rehabilitated area revegetated with grass; organic matter, considered as very high in the ironstone outcrops and neotropical savanna, as high in the Atlantic Forest, and as low in the rehabilitated area revegetated with grass; and the pH, with intermediate acidity level in the rehabilitated area revegetated with grass, high level of acidity in the iron outcrops and neotropical savanna, and very high acidity in the Atlantic Forest. After isolation of the nodules, 380 bacterial strains were obtained and separated into 27 groups by cultural characterization analysis. Genetic diversity was evaluated by the 16S rRNA gene partial sequencing of 156 strains, which identified some bacteria belonging to nitrogen-fixing Leguminosae nodulating bacterial genera (Rhizobium, Bradyrhizobium, Burkholderia, and Cupriavidus), some representative of associative bacteria (Bacillus, Paenibacillus, Herbaspirillum, Pseudomonas, and Agrobacterium), and other genera (Brevibacillus, Novosphingobium, Chitinophaga, Dyella, Acinetobacter, and Stenotrophomonas). The highest genetic diversity of bacteria was found in the rehabilitated area revegetated with grass indicated that it was effective in soil rehabilitation