Ocorrência de bactérias diazotróficas em gramíneas forrageiras na microrregião de Patos–PB.

Abstract

Soil nitrogen (N) is one of the most limiting factors for plant growth in semiarid areas. Diazotrophic bacteria can supply part of N necessary to grass growth with no additional costs, while reduce the environmental toll involved in fertilizer production and use evil. African grasses prevail in cultivated pastures in the semi-arid area of Brazil due to their adaptation to the stressful edaphoclimatic conditions present in their native region. An experiment was carried out in the Laboratory of Microbiology at the Patos campus of the Federal University of Campina Grande with the objective to study the association between diazotrophic bacteria and forage grasses cultivated in the semiarid conditions of northeastern Brazil, as well as identify and purify isolates of bacteria with the potential to produce efficient inoculums for these grasses. Root samples of three forage grasses [Andropogon gayanus Kunth (andropogon), Panicum maximum Jacq. cv. Tanzania (tanzânia) and Cenchrus ciliaris (buffel)] grown in the Agrostology Sector/Animal Husbandry Master Program facilities, in Patos-PB, were collected during the rainy (March of 2008) and dry (August of 2008) seasons. The sampled roots were washed in water or superficially disinfested with sodium hypochlorite. The bacteria were isolated in NFb selective media (specific for Azospirillum spp.) and characterized according to their density and diversity. After purification and morphological identification, their potential for atmospheric N biological fixation (NBF) was evaluated by means of the acetylene reduction technique. From the 72 observed strains, 50 were considered of higher potential for NBF after in vivo trials. The endophytic diazotrophic bacteria were observed associated to andropogon, tanzânia and buffel grasses. Seasonal variation and the pre-treatment of roots before isolation affected density. The largest densities bacterial isolated dizotróficos on andropogon, tanzânia e buffel were obtained in the dry period. Isolates UFCG 22, 25, 26, 27 and 29; UFCG 31, 32, 33, 34, 36, 37 and 38; and UFCG 42, 45, 49 and 50, associated, respectively, with adropogon, tanzânia and buffel grasses showed to be highly promising, and their potential to BNF, with the objective to save N-fertilizer in the growth of forage grasses, should be further studied.