Bradyrhizobia and Mychorrhizae Inoculation Influenced Soybean Growth, Yield and Seed Nutritional Quality.

In soil ecosystems, there are indigenous microorganisms with beneficial role in nutrient cycling and management that can efficiently fulfill the organic farming requirements.  Although it is believed that growing soybean does not require mineral N fertilization, the fact is that a large percentage of the soybean acreage in the US receives N and P fertilization.  This is due to the lack of efficient symbiosis with indigenous microorganisms such as bradyrhizobia and mychorrhizae. In this study, greenhouse and field experiments were conducted to examine the synergistic effects of bradyrhizobia and mychorrhizae, when combined together as an inoculant, on growth, seed yield and seed nutritional quality of soybean.  The interactions between five elite soybean bradyrhizobia and three diverse soybean genotypes were also examined.  Bradyrhizobia strains differed significantly in their symbiotic association with the soybean genotypes.  Strains GR03 and GR04 were better microsymbionts with soybean cultivars 94M30 and Mooncake, respectively.  Inoculation of 94M30 soybeans with both bradyrhizobia and mychorrhizae positively influenced most of the agronomic traits of soybean studied.  When soybean plants were inoculated with B. japonicum GR04 and mychorrhizae in the greenhouse, nodule number and shoot dry weight were increased by 15 and 6%, respectively, over plants inoculated with only bacterial strain GR04.  A similar trend was found when plants were grown in the field.  Combined inoculation of soybean with GR04 and mychorrhizae increased nodule mass, plant biomass and seed oil content by 27, 4 and 8%, respectively, over plants inoculated with strain GR04 alone.  On the contrary, seed yield and seed protein content were not significantly affected by the combined inoculation of soybean with these two microorganisms.  More studies are needed to further examine the synergistic effects of these beneficial microorganisms on soybean growth, yield and nutritional quality.