Isolation and Characterization of Rhizobia Bacteria from Midwest Soils

Symbiotic bacteria, known as rhizobia, can colonize the roots of legume plants and then convert atmospheric nitrogen into a usable form by the host through the process called biological nitrogen fixation (BNF). Many of these bacteria produce other plant-growth-promoting benefits, such as nutrient acquisition, phytohormone production, and the modification of plant stress responses. Agricultural inoculants made from these bacteria have helped reduce the need for man-made nitrogenous fertilizers, known causes of pollution and eutrophication. There are additional benefits in developing bacteria inoculants local to regional soils. The ability to survive and compete in Midwest soil and climates present an opportunity to optimize plant growth-promoting bacteria (PGPB’s) that have remained unexamined. In our first study, soil was collected across undeveloped regions across South Dakota. Through a pipeline of trap experiments and phenotype testing, we discovered several isolates that showed promising results in biomass and nitrogen production. In a similar study, we harvested soils from the northern Great Plains region near vetch plant roots to find field pea (FP) compatible rhizobia. Using a combination of saline, antibiotics, and plating techniques, rhizobia isolates were confirmed using 16S sequencing. Rhizobia isolates were tested for phenotypes such as saline resistance, indole-3-acetic acid (IAA) production, and phosphate solubilization. Greenhouse trials measuring plant biomass and nitrogen production tested to study rhizobia interactive effects. Five isolates were chosen due to their resistance to high saline stress and were then evaluated for their plant-growth-promoting effects in field trial settings. By isolating native rhizobia from regional soils in stressed environments, we attempted to develop microbial inoculants that performed favorably to commercial controls in regional field testing that could out-compete other soil bacteria and survive in drought-stressed conditions. Over a two-year research period, rhizobia inoculants produced a range of results showing greater yield or nitrogen fixation than commercial controls. Additionally, these rhizobia were shown to be statistically comparable to the commercial controls in comparison to the uninoculated control. This research may help further streamline a testing process for regional growers to identify and produce high-performing and resilient rhizobia that is adapted for their local climates and soil.