Estudo da atividade funcional de bactérias diazotróficas e o potencial de uso como biofertilizante em genótipos de Brachiaria

Abstract

Pastures are of great importance for Brazilian cattle and dairy cattle, where about 260 million ha are occupied by native and cultivated pastures. The pastures are mainly occupied with grasses of the genus Brachiaria, due to their high adaptability and tolerance to the acidic soils and low fertility conditions, together with their protein forage value. The greatest evidence of the forage grasse importance is the fact that Brazil has the largest commercial cattle herd, is the largest producer and exporter of meat and the fifth largest milk producer. Despite its economic importance, it is estimated that from 50 to 70% of the pastures cultivated in Brazil are in the process of degradation, being nitrogen its main limitation cause. However, nitrogen fertilizer is applied to increase the production, thus, an alternative for the nitrogen supply, even partially, is the use of nitrogen fixing bacteria. In this sense, the present study aimed to analyze the diversity of associative and endophytic diazotrophic bacteria present in twenty Brachiaria genotypes kept in the Germplasm Active Bank (GAB) of the National Center for Research Gado de Corte (NCRGC) and to analyze the functional capacity of the isolated strains in solubilizing inorganic phosphate and zinc oxide, producing siderophores and acid-3-indoleacetic acid (AIA), and degrading cellulose. In addition, the inoculation effect of the isolates with contrasting functional characteristics was evaluated in twelve Brachiaria genotypes grown in soil boxes in a greenhouse. The bacteria were isolated from rhizospheric soil and roots with and without surface disinfestation samples of all twenty genotypes. Two semi-solid and semi-selective culture media were used: LGI (Nitrospirillum amazonense) and NFb (Azospirillum spp.). The functional capacity tests were performed in vitro and were used as selection criteria for the 32 bacterial isolates used in the greenhouse inoculation experiment. Three cuts were made during the plant cycle and the parametersdry mass of shoot and root, and N total accumulated in the plant were evaluated. A total of 222 isolates were obtained, most of which were isolated from rhizosphere soil and non-disinfested roots. The population size of viable diazotrophic bacteria, colonizing the 20 genotypes, ranged from 102 to 108 bacteria per gram of fresh tissue, the largest population was observed in disinfested roots of B. humidicula H47. However, the greatest diversity of bacterial species came from the genotype B. decumbens D24 / 27 and the greatest number of isolates was obtained from the genotype of B. decumbens X009. Sequencing the 16S rRNA gene from the bacteria confirmed the presence of the genera Azospirillum and Nitrospirillum, and in addition to the presence of 15 other bacterial genera. Out of the 222 isolates, 39.2% were able to solubilize inorganic phosphate and 9.5% of Zn, 32.4% presented cellulolytic capacity, 84.2% produced siderophores and 18.5% indole compounds under in vitro conditions while the presence of the nifH gene was observed in 85.14% of the isolates. The evaluation of functional characteristics showed that 0.9% had all six functions, 5.4% had five of the characteristics evaluated, 42.8% had at least three characteristics, 41.9% presented two functional characteristics, 7.7% presented only a functional capacity while 1.3% did not present any of the functional capacities. The strain inoculation studies showed that there was a significant difference at 0.1% in relation to absolute control in the biomass production and N accumulation in the plant of the twelve Brachiaria genotypes, these being up to 76.9% for the dry mass and up to 54.3% for the N accumulation in the plant. The results suggest that the use of inoculants with these bacteria may be a natural alternative in reducing the use of nitrogen fertilizer in order to reduce the costs in pastures. In addition, inoculation is a clean technology, which contributes to the sustainability of agricultural activities in Brazil, without compromising the environment. It may also help the country to achieve its global goals in reducing greenhouse gas emissions and global climate change.