Eficiência e diversidade de bactérias simbióticas fixadoras de nitrogênio isoladas de solos sob floresta secundária e pastagem na Amazônia Ocidental

Abstract

The aim of this work was to evaluate the symbiotic efficiency and phenotypic and genotypic diversities of cowpea [Vigna unguiculata (L.) Walp] nodulating bacteria isolated from soils old secondary forest and pasture in the Western Amazon. The bacterial strains used in our work were isolated from cowpea nodules obtained after the inoculation of plants with soil suspensions (10-1) from two land use systems (LUS): old secondary forest and pasture, being 122 and 159 isolates from old secondary forest and pasture soils, respectively. Authentication and symbiotic efficiency were evaluated in the same experiment. The authentication aimed to verify the isolates capacity of inducing nodule formation in cowpea. Three completely randomized design experiments were carried out with the cowpea cultivar BR-17, from February to August 2005. The analysis of variance and the Scott-Knott test were performed by the program Sisvar. The strains INPA 03-11B and Ufla 03-84 and controls with and without mineral nitrogen were included for comparison. The shoot dry matter weight (SDMW) and the number (NN) and dry matter weight of nodules (DMWN) were evaluated, and the relative efficiency was calculated with the following formula: Relative efficiency = (SDMW inoculated / SDMW with nitrogen) x 100. Symptoms of deficiency were evaluated 10, 20 and 3 days after sowing by the visual criterion. The phenotypic diversity was evaluated by total protein profiling – SDS-PAGE. The Shannon and Chao1 indexes were used to estimate diversity and richness, respectively. A rarefaction analysis was also performed in order to allow the comparison between the two LUS. Efficient isolates were found in the two LUS. The isolates 88AB3, 90C1, 90A4, 90A8, 88AB10a, 88AB6, 88A10 e 88AC2 (old secondary forest) and 83C3 (pasture) presented a performance similar to the recommended inoculant strain or the control with N. Chlorosis symptoms appeared 20 days after sowing in plants inoculated with inefficient isolates. A high diversity was revealed by the total protein profiles. Four and five groups with 100% of similarity were formed in the old secondary forest and in the pasture, respectively. Other groups were formed at the level of 92% (94 groups) and 90% (62 groups) of similarity in the old secondary forest and in the pasture, respectively, and the last groups formed, respectively, at 8% (1 groups) and 9% (3 groups) of similarity in the old secondary forest and in the pasture. The diversity shown by the dendrograms was confirmed by the Shannon index, since the number of isolates (sample size) was different between LUS. The results indicate that the diversity and richness are similar for the two LUS. The rarefaction curves and the accumulation curve of operational taxonomic units almort the asymptote. More singletons and uniques were found in the old secondary forest soils than in the pasture soils. The cultural- based and the total protein profile- based diversities were similar.