Verify that biological N fixation (BNF) occurs in canola, wheat and soybean inoculated with a commercial strain of Gluconobacter nitrogen-fixing bacterium. Evaluate effects of inoculation on crop yield and N2O emissions.
We evaluated a microbial inoculant containing a free-living nitrogen-fixing bacteria (Gluconoacetobacter diazotrophicus) for its ability to biologically fix N for wheat, soybean and canola. Three inoculation methods were tested: direct application to the seed, foliar inoculation; and soaking the seeds in inoculant as they germinate. Plants were place in a sealed chamber and 15N2 released into the chamber. Because the 15N2 gas is the only source of 15N for the plants, any 15N that we measured in the plant tissues had to be fixed by the bacteria. We also had plants analyzed for DNA segment copies unique to the bacteria. If the bacterial DNA was detected in the plant it provided evidence that the bacteria had successfully colonized the plant tissue. Many of the tissues tested positive for colonization, but most of the 15N analyses did not show N fixation. In many cases the %N in the plants that were colonized was lower than the control (not inoculated) plants. The lower % N was not due to the N being diluted through enhance growth (biomass) as there were no differences in biomass production among any of the inoculation treatments. Unfortunately we only measured good 15N enrichment (indicating biological nitrogen fixation) in 3 soybean plants, 2 of which were inoculated foliarly. Of the different inoculation methods tested, soaking the seed during germination was most successful in delivering bacteria that colonized. Foliar application also tended to be a successful delivery method, but directly applying the inoculant to seed was the least effective. While soaking the seed during germination was the most successful delivery method, it is also the least practical, as it is unlikely to be implemented on a field scale. More work on improving the method of delivery of the inoculant and stimulating the inoculant bacteria to fix N is needed.