Dr. Brian Klubek: Research

 

Enhancement of Soybean Inoculation for Maximizing Yields

The goal of this research is to develop a new method of legume inoculation to enhance the nodulation of soybean by a desired strain with an increase in the total N content of the host plant. Eight thousand nodules were screened to assess the antibiotic sensitivity of the native Bradyrhizobium japonicum population. The inhibition by each of the selected antibiotics was the order: kanamycin > tetracycline = oxytetracycline > rifampicin > neomycin. Using successive cycles of mutation, 90 mutant strains of B. japonicum were isolated and characterized for symbiotic competency in greenhouse studies. Ten of these strains (resistant to either kanamycin + neomycin or kanamycin + tetracycline) were chosen for soil-pot greenhouse studies to assess their competitiveness against native bradyrhizobia when soybeans were co-inoculated with Streptomyces kanamyceticus. Two strains (KNI-110-1 and KN3I1b-2) when used with S. kanamyceticus showed a significantly higher nodule occupancy (45 to 55%) and a significant increase in shoot N content (22 to 25%). Based on these results, a total of six strains were chosen for co-inoculation (field) studies with S. kanamyceticus at Carbondale, Illinois during the 2000 growing season. Significant differences in nodule occupancy and soybean yield by strains KNI-110-3 and KT3I1B-3 over the non-inoculated were determined. For all six inoculum treatments, a 15 to 28% increase in yield was achieved.

Additional field studies were conducted in 2001 and 2002 in Belleville and Carbondale, IL and a demonstration field trial (2001) in Nashville, IL, evaluating the effects of co-inoculation on soybean yield. For the Belleville and Carbondale field trials, nine treatments were used: a non-inoculated control, a Streptomycetes kanamyceticus treatment, and seven co-inocula of selected antibiotic resistant strains of B. japonicum and S. kanamyceticus. Data were collected regarding early nodulation, nodule number, nodule occupancy, leaf nitrogen composition, yield, seed nitrogen composition and accumulated seed nitrogen. For the Carbondale 2001 field study, significant differences between treatments were observed at the 5% level for early nodulation and nodule occupancy. The Belleville 2001 field data showed significant differences at the 5% level for nodule occupancy and leaf nitrogen content. However, soybean yield for the demonstration trial at Nashville, IL (Kurwicki Farm) displayed a 17% increase by co-inoculation versus the non-inoculated control (32' by 825' harvest area).

Due to the lack of timely rainfall, the Carbondale 2002 field study only showed a significant difference at the 10% level for early nodulation, while the Belleville 2002 study had a significant difference in nodule occupancy at the 5% level. However, a scanning electron microscopy (SEM) study showed that after 35 days of soybean growth, there was viable S. kanamyceticus filaments present on the soybean roots and nodule tissues.

For the 2003 growing season, we are evaluating the tri-inoculation of soybean employing B. japonicum (resistant to kanamycin + neomycin or kanamycin + tetracycline) S. kanamyceticus and four plant-growth promoting rhizobacteria (PGPR) that are compatible with S. kanamyceticus. It is hoped that the PGPR inocula will promote soybean germination and root growth, thereby enhancing early nodulation and nodule occupancy by the applied B. japonicum strains, resulting in higher soybean yield and seed nitrogen content.