Improved Nitrogen Fixation of Soybean by Novel Seed Treatments.

Previous studies from our laboratory had shown that co-inoculation of kanamycin/neomycin-resistant Bradyrhizobium japonicum mutants (KNI) derived from the strain of USDA 110 and Streptomyces kanamyceticus (S.kanamyceticus) ATCC #12853 on soybean seeds (cultivar LS90-1920) led to greater nodule occupancy of KNI strains and shoot nitrogen content of host plant under greenhouse conditions. New seed treatments intended to improve the nodule competency by a selected inoculant against native bradyrhizobia were evaluated with cultivar LS90-1920 grown in southern Illinois soils. The soybean seeds in this study were treated with combination of various seed coating materials and bacterial inoculants. Although no significant differences in terms of nodule biomass, shoot biomass and nodule number were determined between all treatments, the linear regression of nitrogen fixation rates and nodule occupancy of the B. japonicum inoculant (KNI-3) from soybeans receiving inoculum treatments with KNI-3 was highly significant (P<0.01). Seeds coated by graphite and vermicompost gave rise to higher nodule occupancy of KNI-3 than by the other coating materials. When graphite and vermicompost were used as seed coating material, the combined mean nitrogen fixation rate of soybean receiving S.kanamyceticus and Pseudomonas putida strain G11-32 or 17-29 as co-inoculants with KNI-3 was significantly higher than those soybeans inoculated with KNI-3 alone (P=0.0192) or without inoculation (P=0.0074). The presence of Pseudomonas putida strain with this seed treatment also significantly improved the nitrogen fixation specific activity of soybean. The nitrogen fixation rate was still significantly higher when the combined mean of these co-inoculant  treatments were compared to of the combined treatment mean of KNI-3 alone (P=0.024) or without inoculation (P=0.02) regardless of  seed coating material. These results suggest Pseudomonas putida strain G11-32 or 17-29 in combination with the Streptomyces kanamyceticus in seed treatment can improve the efficiency of nitrogen fixation by soybean.