In-Furrow Inoculation and Selection for Higher Motility Enhances the Efficacy of Bradyrhizobium japonicum Nodulation

The issue of competition for nodulation has received much attention in studies on soybean [Glycine max (L.) Merr.] crops because native soil rhizobia often preclude nodulation of inoculated high-quality strains. In this work, soil placement and enhanced motility of Bradyrhizobium japonicum were investigated as strategies to improve the competitiveness of soybean inoculants applied in the presence of large populations of native (or indigenous) soybean-nodulating rhizobia. The first strategy might allow for the enhanced distribution of inoculant rhizobia in the surface soil, compared with inoculant that is directly applied to soybean seed, while the second strategy is expected to enhance root colonization. The effects of soil placement and enhanced bacterial motility on percentage nodule occupation, grain yield, and grain N contents were examined during the 2004-2005 soybean season at three locations in Argentina, comprising soils classified as entic hapludol, argillic pelludert, and typical argiudol. The experimental design was a randomized complete block design, and main effects and interactions were also analyzed with a factorial design. Strain LP 3008, previously selected for increased motility, was more effective than the commonly inoculated strain E 109. While E 109 occupied, on average, 13.2% of nodules with seed inoculation, LP 3008 occupied 28.2%. In comparison, in-furrow inoculation yielded 24.2 and 37.2% average nodule occupancies for E 109 and LP 3008, respectively. However, these gains in nodulation efficacy lead to a modest, nonsignificant yield increase, and grain N content was unaffected.