Bacterial strains decrease solubility of potassium in the soil / Estirpes bacterianas reduzem potencial de solubilização de potássio no solo

This study aimed to verify whether inoculation with strains of diazotrophic bacteria, with proven ability to solubilize potassium (K) in vitro, contributes towards the release of K in the soil after fertilization with phonolite rock powder. The experiment was conducted in containers with 0.3 dm-3 of soil containing low potassium content. Fifteen treatments were used, namely, 12 inoculated with the bacterial strains, a control treatment (without phonolite and without inoculation), one containing phonolite without inoculation and one containing KCl, soluble fertilizer, without inoculation. In treatments with phonolite and KCl, the doses of these materials were applied to provide the soil with 195 mg dm-3 of K. A completely randomized design with four replications was used. The soil was incubated during 90 days at room temperature and humidity at about 70% retention capacity. After this period, the content of K+ (Mehlich and resin), pH value and potential acidity (H+Al) were evaluated. Phonolite, associated with inoculation with most bacterial strains, increased the availability of potassium in the soil, pH rate and reduced potential acidity. Among the strains tested, UNIFENAS 100-01, UNIFENAS 100-16, UNIFENAS 100-27, UNIFENAS 100-39 and UNIFENAS 100-93 were the most efficient for the solubilization of K+ of the phonolite. In spite of the observed results, K content released by the bacterial strains in the soil decreased when compared to in vitroconditions, thus justifying the need for studies on bio-solubilization of soil to select the most efficient strains in the process.

O objetivo desse estudo foi avaliar se a inoculação com estirpes de bactérias diazotróficas de comprovada capacidade de solubilizar potássio (K) in vitro, contribui para a liberação de K no solo, após adubação com o pó da rocha fonolito. O experimento foi conduzido em recipientes contendo 0,3 dm-3 de solo contendo baixo teor de potássio. Foram utilizados 15 tratamentos, sendo: 12 com fonolito inoculados com as estirpes bacterianas e 3 tratamentos controle, um sem fonolito e sem inoculação, um contendo fonolito sem inoculação e um contendo KCl, fertilizante solúvel, sem inoculação. Nos tratamentos contendo fonolito e KCl, foi aplicada doses desses materiais para fornecer ao solo 195 mg dm-3 de K. Foi utilizado delineamento inteiramente casualizado com quatro repetições. O solo foi incubado por 90 dias, em temperatura ambiente e a umidade foi mantida a cerca de 70% da capacidade de retenção. Após esse período, foram avaliados o teor de K+ (Mehlich e resina), valor de pH e da acidez potencial (H+Al). A utilização do fonolito, associado a inoculação com a maioria das estirpes bacterianas contribuiu para aumentar a disponibilidade de potássio no solo, o valor de pH e reduzir a acidez potencial. Dentre as estirpes testadas, destacaram-se UNIFENAS 100-01, UNIFENAS 100-16, UNIFENAS 100-27, UNIFENAS 100-39 e UNIFENAS 100-93, que foram as mais eficientes para a solubilização do K+ do fonolito. Apesar dos resultados observados, verificou-se que o teor de K liberado pelas estirpes bacterianas no solo foi reduzido quando comparado às condições in vitro, justificando assim, a necessidade de estudos de biossolubilização no solo visando selecionar as estirpes mais eficientes para desempenhar o processo

High diversity of Bradyrhizobium strains isolated from several legume species and land uses in Brazilian tropical ecosystems.

The genus Bradyrhizobium stands out among nitrogen-fixing legume-nodulating bacteria because it predominates among the efficient microsymbionts of forest, forage, and green manure legume species, as well as important species of grain legumes, such as soybean, cowpea, and peanut. Therefore, the diversity of Bradyrhizobium strains is a relevant resource from environmental and economic perspectives, and strains isolated from diverse legume species and land uses in Brazilian tropical ecosystems were assessed in this study. To accomplish this, sequences of four housekeeping genes (atpD, dnaK, gyrB, and recA) were individually analysed, with the first three also being considered using multilocus sequence analysis (MLSA). The sensitivity of the strains to different antibiotics, their tolerance to different levels of salinity, and their ability to nodulate soybean plants were also measured. The phylogenetic trees based on each individual gene, and on the concatenated housekeeping genes, revealed several strain clusters separated from any currently described species. The Bradyrhizobium strains studied were generally resistant to antibiotics. All strains were able to grow at salinity levels of up to 0.5% NaCl, whereas only strains UFLA03-142, UFLA03-143, UFLA03-145, and UFLA03-146 grew in the presence of 1% NaCl. Together, the results indicated that some of the strains studied were potential novel species, indicating that the various soils and ecosystems in Brazil may harbour an as yet unknown diversity of rhizobia.

Genetic and symbiotic diversity of nitrogen-fixing bacteria isolated from agricultural soils in the western Amazon by using cowpea as the trap plant.

Cowpea is a legume of great agronomic importance that establishes symbiotic relationships with nitrogen-fixing bacteria. However, little is known about the genetic and symbiotic diversity of these bacteria in distinct ecosystems. Our study evaluated the genetic diversity and symbiotic efficiencies of 119 bacterial strains isolated from agriculture soils in the western Amazon using cowpea as a trap plant. These strains were clustered into 11 cultural groups according to growth rate and pH. The 57 nonnodulating strains were predominantly fast growing and acidifying, indicating a high incidence of endophytic strains in the nodules. The other 62 strains, authenticated as nodulating bacteria, exhibited various symbiotic efficiencies, with 68% of strains promoting a significant increase in shoot dry matter of cowpea compared with the control with no inoculation and low levels of mineral nitrogen. Fifty genotypes with 70% similarity and 21 genotypes with 30% similarity were obtained through repetitive DNA sequence (BOX element)-based PCR (BOX-PCR) clustering. The 16S rRNA gene sequencing of strains representative of BOX-PCR clusters showed a predominance of bacteria from the genus Bradyrhizobium but with high species diversity. Rhizobium, Burkholderia, and Achromobacter species were also identified. These results support observations of cowpea promiscuity and demonstrate the high symbiotic and genetic diversity of rhizobia species in areas under cultivation in the western Amazon.

Cupriavidus necator isolates are able to fix nitrogen in symbiosis with different legume species.

The aim of the present study was to identify a collection of 35 Cupriavidus isolates at the species level and to examine their capacity to nodulate and fix N(2). These isolates were previously obtained from the root nodules of two promiscuous trap species, Phaseolus vulgaris and Leucaena leucocephala, inoculated with soil samples collected near Sesbania virgata plants growing in Minas Gerais (Brazil) pastures. Phenotypic and genotypic methods applied for this study were SDS-PAGE of whole-cell proteins, and 16S rRNA and gyrB gene sequencing. To confirm the ability to nodulate and fix N(2), the presence of the nodC and nifH genes was also determined, and an experiment was carried out with two representative isolates in order to authenticate them as legume nodule symbionts. All 35 isolates belonged to the betaproteobacterium Cupriavidus necator, they possessed the nodC and nifH genes, and two representative isolates were able to nodulate five different promiscuous legume species: Mimosa caesalpiniaefolia, L. leucocephala, Macroptilium atropurpureum, P. vulgaris and Vigna unguiculata. This is the first study to demonstrate that C. necator can nodulate legume species.