158. Evaluating the Effects of Plant Growth Promoting Rhizobacteria in Bermudagrass (Tifton-85) on Nitrogen Use Efficiency, Drought Response, and Inoculation Frequency

Abstract Bermudagrass is a high-yielding warm-season C4 grass that is used in the southern US for grazing and hay production. Plant growth promoting rhizobacteria (PGPR) have emerged as a possible tool to help reduce the need of synthetic N fertilizer and to induce drought tolerance. Two greenhouse studies were conducted at Auburn University’s Plant Science Research Center; the first study evaluated the effects of PGPR on nitrogen use efficiency and drought response on dry matter yield (DMY). Based on those results a second study was necessary to evaluate inoculation frequency of PGPR on (DMY). The first experiment assessed the response of Tifton-85 to drought stress imposed for three weeks, compared against a well-watered control. Plants were inoculated with one of three PGPR strains—Methylobacterium symbioticum (Corteva), Bacillus subtilis DH267, or Paenibacillus riograndensis DH44—or left uninoculated as a control. Nitrogen fertilizer was applied at three levels: 100%, 50%, and 25% of the recommended agronomic rate using ammonium sulfate. The imposed drought produced marked reductions in DMY and visible stress symptoms, as well as an interaction between nitrogen rates and drought. PGPR, however, did not have an effect on DMY. It was hypothesized that the lack of effect by PGPR was due to the plants being already established and pot bound at inoculation, because of these findings a second experiment was conducted. The second experiment investigated the influence of repeated inoculation with DH44 on Tifton-85 growth across three harvest and inoculation cycles under reduced nitrogen fertilization (25% of the recommended rate). The experiment began when plants were transplanted to avoid them being established or pot bound. The experiment included six inoculation treatments: no inoculation, inoculation at cutting only, inoculation at cutting +post harvest, inoculation at cutting +7DAC, inoculation at cutting +14DAC, and inoculation at cutting +7&14DAC. DMY and root mass were quantified, with one-third of the pots destructively harvested each cycle to assess belowground development. Results showed a significant inoculation timing effect on DMY in the first and second harvests, with the inoculation at cutting +14DAC and inoculation at cutting +7&14DAC treatments yielding the highest. This positive effect of the PGPR in the first two harvests but not in the third is likely due to the plants establishing and benefiting from the extra growth provided by PGPR. Root mass increased throughout the experiment, with significant effects in the third harvest (P = 0.0111) for inoculation at cutting +7&14DAC. These findings suggest that DMY and root mass can be enhanced by multiple PGPR inoculations, which could potentially improve N use efficiency and drought resilience. Further research should evaluate whether pot size has an effect so that field scalability can be determined.