Both climate change and the adverse effects of

The H2 is an obligate by-product of N-fixation. Recycling of H2 through uptake hydrogenase (Hup) inside the root nodules of leguminous plants is often considered an advantage for plants. However, many of the rhizobium-legume symbioses found in nature, especially those used in agriculture are shown to be Hup−, with the plants releasing H2 produced by nitrogenase activity from root nodules into the surrounding rhizosphere. Recent studies have suggested that, H2 induces plant-growth-promoting rhizobacteria, which may explain the widespread of Hup− symbioses in spite of the low energy efficiency of such associations. Wild legumes grown in Nova Scotia, Canada, were surveyed to determine if any plant-growth characteristics could give an indication of Hup choice in leguminous plants. Out of the plants sampled, two legumes, Securigera varia and Vicia cracca, showed Hup+ associations. Securigera varia exhibited robust root structure as compared with the other plants surveyed. Data from the literature and the results from this study suggested that plants with established root systems are more likely to form the energy-efficient Hup+ symbiotic relationships with rhizobia. Conversely, Hup− associations could be beneficial to leguminous plants due to H2-oxidizing plant-growth-promoting rhizobacteria that allow plants to compete successfully, early in the growing season. However, some nodules from V. cracca tested Hup+, while others were Hup−. This was similar to that observed in Glycine max and Pisum sativum, giving reason to believe that Hup choice might be affected by various internal and environmental factors.

The long-term success of hydrogenase uptake negative legume-rhizobia associations, in spite of their apparent inefficiency, may be explained by the positive effects of H₂ release to soil. A primary benefit of H₂ release to soil is the stimulation of H₂-oxidizing, plant growth promoting rhizobacteria (PGPR) [1]. Two such previously isolated strains were tested as seed inoculants for barley and spring wheat; there were significant differences between treatments and controls in tiller and grain head production, supported by data from greenhouse trials. T-RFLP analysis of barley soil samples, supported by DNA sequencing data, successfully distinguished both species inoculated. Successful re-isolation indicates that these isolates can reproduce themselves in soils and can be used as effective inoculants with peat as the standard carrier. This study showed that we are able to achieve some of the beneficial effects of crop rotation without the need to implement actual crop rotation