Reinoculation of topdressing Rhizobium tropici combined or not with Azospirillum brasilense in common bean1

The nitrogen (N) supplying capacity of common bean plants through biological fixation is still questioned by farmers. This study aimed to investigate the nodulation, growth and production of common bean cultivars under field conditions, when subjected to topdressing Rhizobium tropici reinoculation applied at different rates, combined or not with Azospirillum brasilense. A randomized blocks design was used, in a 2 x 4 x 2 + 2 factorial arrangement, with four replications. The treatments consisted of two common bean cultivars (BRSEstilo and BRS-Esteio) reinoculated with four R. tropici rates (0-, 1-, 2- and 4-fold the reference rate), combined or not with A. brasilense in co-inoculation. The additional treatments consisted of nitrogen fertilizer applications (20 and 50 kg ha-1 at planting and as topdressing, respectively) for both cultivars. The variables evaluated at the R6 stage were nodulation (number of active nodules and nodule dry weight) and morphological plant characteristics (root length and dry weight, plant height, shoot dry weight and N content), while, at harvest, the number of pods per plant, number of grains per pod, 100-grain average weight and grain yield were evaluated. The topdressing reinoculation of R. tropici combined with A. brasilense at the V4 stage affected the nodulation process and agronomic characteristics of the common bean plants. When performed in addition to seed inoculation, it may totally replace the supply of mineral N in the crop, allowing the achievement of high yield levels.