Luciano Kayser Vargas

Biomassa microbiana e produção de C-CO2 e N mineral de um podzólico vermelho-escuro submetido a diferentes sistemas de manejo: Microbial biomass and C-CO2 and mineral nitrogen production in paleudult soil cultivated under different management systems

Os sistemas de manejo, com diferencas no revolvimento do solo e na composicao dos residuos vegetais, alteram as propriedades biologicas do solo, com reflexos na qualidade do solo e na produtividade das culturas. Com vistas em medir estas alteracoes nas propriedades biologicas do solo, a biomassa e a atividade microbiana foram avaliadas em um Podzolico Vermelho-Escuro, em Eldorado do Sul (RS), utilizando diferentes preparos (convencional, reduzido e plantio direto) e dois sistemas de sucessoes de culturas (aveia preta + vica/milho + caupi e aveia/milho). As avaliacoes foram realizadas em quatro epocas, durante 12 meses, e em duas profundidades (0-5 e 5-15 cm). O carbono da biomassa microbiana foi analisado pelo metodo de fumigacao-incubacao, e a atividade microbiana, pela producao de C-CO2 e N mineral, apos 60 dias de incubacao. As diferencas na biomassa e na atividade microbiana, entre os sistemas de manejo, foram mais pronunciadas na camada de 0-5 cm. Nesta camada de solo, observaram-se os maiores valores de biomassa e de atividade nos preparos conservacionistas e no sistema aveia + vica/milho + caupi. Dentre as variaveis estudadas, a mineralizacao de N mostrou-se a mais sensivel aos manejos, a profundidade e a epoca de amostragem.

The effect of plant growth-promoting rhizobacteria on the growth of rice (Oryza sativa L.) cropped in southern Brazilian fields

Background and AimsSeveral strains of rhizobacteria may be found in the rhizospheric soil, on the root surface or in association with rice plants. These bacteria are able to colonize plant root systems and promote plant growth and crop yield through a variety of mechanisms. The objectives of this study were to isolate, identify, and characterize putative plant growth-promoting rhizobacteria (PGPR) associated with rice cropped in different areas of southern Brazil.MethodsBacterial strains were selectively isolated based on their growth on three selective semi-solid nitrogen-free media. Bacteria were identified at the genus level by PCR-RFLP 16S rRNA gene analysis and partial sequencing methodologies. Bacterial isolates were evaluated for their ability to produce indolic compounds and siderophores and to solubilize phosphate. In vitro biological nitrogen fixation and the ability to produce 1-aminocyclopropane-1-carboxylate deaminase were evaluated for each bacterial isolate used in the inoculation experiments.ResultsIn total, 336 bacterial strains were isolated representing 31 different bacterial genera. Strains belonging to the genera Agrobacterium, Burkholderia, Enterobacter, and Pseudomonas were the most prominent isolates. Siderophore and indolic compounds producers were widely found among isolates, but 101 isolates were able to solubilize phosphate. Under gnotobiotic conditions, eight isolates were able to stimulate the growth of rice plants. Five of these eight isolates were also field tested in rice plants subjected to different nitrogen fertilization rates.ConclusionsThe results showed that the condition of half-fertilization plus separate inoculation with the isolates AC32 (Herbaspirillum sp.), AG15 (Burkholderia sp.), CA21 (Pseudacidovorax sp.), and UR51 (Azospirillum sp.) achieved rice growth similar to those achieved by full-fertilization without inoculation, thus highlighting the potential of these strains for formulating new bioinoculants for rice crops.

Screening of plant growth promoting Rhizobacteria isolated from sunflower (Helianthus annuus L.)

Background and AimsThis study was aimed at assessing the diversity of putatively diazotrophic rhizobacteria associated with sunflower (Helianthus annuus L.) cropped in the south of Brazil, and to examine key plant growth promotion (PGP) characteristics of the isolates for the purposes of increasing plant productivity.Methods299 strains were isolated from the roots and rhizosphere of sunflower cultivated in five different areas using N-free media. 16S rDNA PCR-RFLP and 16S rRNA partial sequencing were used for identification and the Shannon index was used to evaluate bacterial diversity. Production of siderophores and indolic compounds (ICs), as well phosphate solubilization activities of each isolate were also evaluated in vitro. On the basis of multiple PGP activities, eight isolates were selected and tested for their N-fixation ability, and their capacity as potential PGPR on sunflower plants was also assessed.ResultsAll except three Gram-positive strains (phylum Actinobacteria) belonged to the Gram-negative Proteobacteria subgroups [Gamma (167), Beta (78), and Alpha (50)] and the family Flavobacteriaceae (1)]. Shannon indexes ranged from 0.96 to 2.13 between the five sampling sites. Enterobacter and Burkholderia were the predominant genera isolated from roots and rhizosphere, respectively. Producers of siderophores and ICs were widely found amongst the isolates, but only 19.8% of them solubilized phosphate. About 8% of the isolates exhibited all three PGP traits, and these mostly belonged to the genus Burkholderia. Four isolates were able to stimulate the growth of sunflower plants under gnotobiotic conditions.ConclusionsEnterobacter and Burkholderia were the dominant rhizospheric bacterial genera associated with sunflower plants. Inoculation with isolates belonging to the genera Achromobacter, Chryseobacterium, Azospirillum, and Burkholderia had a stimulatory effect on plant growth.

The effects of different fertilization conditions on bacterial plant growth promoting traits: guidelines for directed bacterial prospection and testing

AimsIn this work, the effects of fertilization on diazotroph diversity and plant growth promoting traits were analyzed. An interaction model was then suggested and tested in field.MethodsOne hundred and ninety bacterial strains were isolated from rhizospheric soil and roots of rice cropped in three different fertilization conditions. Phosphate solubilization, indolic compound (IC) and siderophore production, and nitrogen fixation abilities of the isolates were screened and compared. Some isolates were selected for in vivo plant growth promotion in greenhouse and field experiments.ResultsWe found that fertilization had a limited effect on diversity but had a major effect on phosphate solubilization and IC production abilities. We demonstrated that plants select bacteria that present good phosphate solubilization ability for intimate root association in lieu of good IC production under nutrient-poor conditions and select good IC producers in lieu of good phosphate solubilizers under nutrient-moderate conditions. In nutrient-rich conditions, this selection preference seems to be deactivated. In the field trial, good phosphate solubilizers only contributed effectively to plant growth at nutrient-poor conditions and good IC producers only contributed to plant growth at nutrient-moderate conditions.ConclusionsFertilization affects the PGP traits of the diazotrophic community. These findings may be used for directed PGPR prospection and anticipated PGPR candidate selection.

Influência da inoculação de rizóbios sobre a germinação e o vigor de plântulas de alface

Os rizobios, conhecidos por sua capacidade de fixar N2 em associacao com leguminosas, tambem se mostram capazes de promover o crescimento de nao-leguminosas, especialmente pela producao de acido indol-acetico (AIA). Neste trabalho, objetivou-se selecionar rizobios produtores de AIA e avaliar o efeito de diferentes concentracoes deste fitormonio sobre a germinacao e o desenvolvimento inicial de plântulas de alface. Foram selecionados quatro isolados de Bradyrhizobium sp. e um isolado de Rhizobium leguminosarum biovar trifolii, os quais foram crescidos por quatro dias em meio levedura-manitol enriquecido com triptofano. Apos esse periodo, avaliou-se a producao de AIA e procedeu-se a inoculacao de sementes de alface com os isolados. O isolado TV-13, de R. leguminosarum biovar trifolii produziu 171,1µg mL-1 de AIA, causando prejuizos para o desenvolvimento das plântulas de alface. Por outro lado, os isolados de Bradyrhizobium sp. produziram entre 1,2 e 3,3µg mL-1 de AIA e aumentaram o vigor das plântulas em relacao ao tratamento sem inoculacao com rizobios. Para verificar se essas diferencas foram decorrentes das concentracoes de AIA, foram realizados mais dois experimentos, nos quais as sementes foram embebidas em culturas de TV-13 com ou sem a presenca de triptofano ou em doses crescentes de AIA sintetico. O isolado TV-13 crescido na presenca de triptofano causou danos progressivos sobre o desenvolvimento das plântulas de alface, o que nao ocorreu na ausencia de triptofano. Tambem foi verificado um retardo na germinacao das sementes quando submetidas a altas concentracoes de AIA sintetico. Os resultados indicam a influencia do AIA sobre os parâmetros de germinacao, de modo que a inoculacao de sementes de alface com rizobios que produzem baixas quantidades de AIA e uma pratica recomendavel.

Occurrence of plant growth-promoting traits in clover-nodulating rhizobia strains isolated from different soils in Rio Grande do Sul state

In the last decades, the use of plant growth-promoting rhizobacteria has become an alternative to improve crop production. Rhizobium leguminosarum biovar trifolii is one of the most promising rhizobacteria and is even used with non-legume plants. This study investigated in vitro the occurrence of plant growth-promoting characteristics in several indigenous R. leguminosarum biovar trifolii isolated from soils in the State of Rio Grande do Sul, Brazil. Isolates were obtained at 11 locations and evaluated for indoleacetic acid and siderophore production and inorganic phosphate solubilization. Ten isolates were also molecularly characterized and tested for antagonism against a phytopathogenic fungus and for plant growth promotion of rice seedlings. Of a total of 252 isolates, 59 produced indoleacetic acid, 20 produced siderophores and 107 solubilized phosphate. Some degree of antagonism against Verticillium sp. was observed in all tested isolates, reducing mycelial growth in culture broth. Isolate AGR-3 stood out for increasing root length of rice seedlings, while isolate ELD-18, besides increasing root length in comparison to the uninoculated control, also increased the germination speed index, shoot length, and seedling dry weight. These results confirm the potential of some strains of R. leguminosarum biovar trifolii as plant growth-promoting rhizobacteria.

Genetic variability of soybean bradyrhizobia populations under different soil managements

Due to their ecologic and economic importance, bradyrhizobia have been extensively studied in recent years. Since 1992, Bradyrhizobium elkanii SEMIA 587 and SEMIA 5019 and Bradyrhizobium japonicum SEMIA 5079 and SEMIA 5080 have been widely used in most Brazilian soybean fields. The objective of this work was to estimate the genetic variability of bradyrhizobial isolates recovered from soils under rhizobial inoculation and different soil managements. Only 25% of the isolates demonstrated high similarities to the original strains, and a strong correlation was obtained between the bradyrhizobial genetic variability and soil management. A high level of genetic diversity was observed both within isolates (H = 5.46) as well as among the different soil practices. Soil under no-tillage presented a higher bradyrhizobia diversity compared with bradyrhizobia isolated from soil under conventional tillage. Serological characterization also indicated that B. elkanii strains SEMIA 587 and SEMIA 5019 were more competitive and presented a higher nodular occupancy capacity than strains belonging to B. japonicum species in Southern Brazilian soils.

Indicadores microbianos de qualidade do solo em diferentes sistemas de manejo

Agricultural activity, by the application of different soil management practices, can cause impacts on soil production capacity that need to be assessed. In this study, different microbiological parameters were evaluated, to assess the relation of soil quality with different tillage practices and crop systems, in comparison to a natural reference soil. The activities of β-glucosidase, urease, acid phosphatase and arylsulphatase were evaluated, as well as soil microbial respiratory activity and microbial biomass. Conventional tillage (CT) and no-tillage (NT) were evaluated in two crop rotations and winter fallow. In addition, natural pasture (NP) was also evaluated as reference, for comparison with the other management systems. The analyses were carried out in four growing seasons in two years. The results of the four seasons indicated that the enzyme activities, as well as microbial biomass and respiratory activity, were lower in CT than in the other systems, while results in NP and NT tended to be similar, indicating the capacity of NT to maintain the original soil quality. On the other hand, the evaluated parameters were in general not significantly influenced by the crop systems.

Isolation and characterization of two plant growth-promoting bacteria from the rhizoplane of a legume (Lupinus albescens) in sandy soil

Two bacterial strains that amplified part of the nifH gene, RP1p and RP2p, belonging to the genus Enterobacter and Serratia, were isolated from the rhizoplane of Lupinus albescens. These bacteria are Gram-negative, rod-shaped, motile, facultative anaerobic, and fast-growing; the colonies reach diameters of 3-4 mm within 24 h of incubation at 28 oC. The bacteria were also able to grow at temperatures as high as 40 oC, in the presence of high (2-3 % w/v) NaCl concentrations and pH 4 -10. Strain RP1p was able to utilize 10 of 14 C sources, while RP2p utilized nine. The isolates produced siderophores and indolic compounds, but none of them was able to solubilize phosphate. Inoculation of L. albescens with RP1p and RP2p strains resulted in a significant increase in plant dry matter, indicating the plant-growth-promoting abilities of these bacteria.

Nitrogênio da biomassa microbiana, em solo sob diferentes sistemas de manejo, estimado por métodos de fumigação

The aim of this experiment was to evaluate the nitrogen of the microbial biomass under different soil management systems by the fumigation-incubation (FI) and fumigation-extraction (FE) methods. Soil samples were collected in four seasons, during one year, from a Paleudult soil in the Central Depression of Rio Grande do Sul State, Brazil. Soil microbial biomass N was analyzed in soil under three tillages systems (conventional, reduced and direct drilling) and two crop systems: oats (Avena strigosa) + vetch (Vicia sativa)/corn (Zea mays) + cowpea (Vigna sinensis) and oats/corn), at two depths (0-5 cm e 5-15 cm). The FE procedure was used in the last two evaluations, after soil tillage. The greatest values of microbial biomass N were found in soil from direct drilling and oats + vetch /corn + cowpea, in the upper 0-5 cm. The analyses by the fumigation-incubation method presented the lowest variation, showing that the method is useful in evaluations of soil management systems.