João Carlos Pereira

Dinâmica das populações bacterianas em solos de Cerrados

Among tropical environments, Cerrados stand out because of its agriculture potencial. Although microorganisms play an important role on soil sustainability and crop production, few infor- mation is available on the effects of soil management systems on Cerradois microbial ecology. In this study the effects of environmental conditions and soil management practices on bacterial populations were evaluated. Bacterial population densities in soil under native vegetation were variable and diferentiated. Actinomycetes densities varied from 1.7 to 50 X 10 4 CFU/g dry soil in Sete Lagoas region, Brazil, whereas bacterial populations in both the first and second year of land use were similar and higher than those from soil under native vegetation in the Planaltina region. Soil management practices did not cause strong modifications in the equilibrium among actinomycetes populations obtained from both spores and hyphal. The spore/hypha ratios varied from 1.1 to 5.8. Correlation coefficients among actinomycetes and other bacteria populations were signifficant in soybeans rhizo- spheres. Results show that soil management practices used for soybean cultivation in Cerrado soils may influence the equilibrium of bacterial populations.

Influência da antibiose exercida por actinomicetos às estirpes de Bradyrhizobium SPP., na nodulação da soja

The aim of this work was to evaluate the antibiotic spectrum of actinomycetes from Cerrado soils and their influence on soybean nodulation. Strains BR 29, BR 33, BR 40, BR 85, BR 86, BR 96, 47/587, 3B-7 and 4A-5 of Bradyrhizobium spp. were characterized by their natural resistence to antibiotics produced by 204 actinomycete isolates. The strains BR 29 and BR 96 of B. elkanii were sensitive to 5.2% and 9.9% the products of actinomycete isolates, respectively, while BR 33 was sensitive up to 20.3%. The antagonistic effects caused by actinomycete exclusively to BR 29 and BR 33 were 1.6% and 5.7% respectively. This effect was not observed for strains BR 40 and BR 96. Single and multistrains inoculations in the presence or absence of actinomycetes affected soybean nodulation. On double strain inoculations, with BR 33 and BR 29 mixed with actinomycete 370 there was a decrease in nodule occupancy by BR 29 from 94.1% to 83.7% with concomitant increase in the occupying of BR 33 from 6.7% to 17.2%. The results indicate the importance of ecological studies on actinomycetes population in order to access their role in the establishment of efficient nodulation.