Agricultural and Environmental Applications of Nitrogen Fixing Organisms

Abstract

The wide-scale application of wild-type or genetically-modified nitrogen-fixing bacteria to agricultural production fields presents some challenging, and often insurmountable, problems. Faced with a new and potentially hostile environment, applied inoculants often fail to increase plant productivity. There are a large number of diverse environmental factors that have been thought to limit the effectiveness of inoculants. The abiotic factors include: nutrient limitations, soil moisture, pH, temperature, texture, and organic matter content, inoculant placement and mobility, and soil solute types and concentrations. On the other hand, biotic constraints have also been shown to influence the efficacy of nitrogen-fixing inoculants and include: habitat provision, bacteriocin, and antibiotic production, numbers and types of indigenous microorganisms, selective predation by protozoa, residence time in soils, and time in laboratory culture prior to reintroduction into soils. The majority of these studies have been done with inoculants containing the root nodule symbionts of legumes, Rhizobium, Bradyrhizobium, Sinorhizobium, Mesorhizobium, and Azorhizobium, collectively called “rhizobia”. Equal problems, however, are likely with inoculation of other N₂-fixing organisms, including Frankia, Acetobacter, and Herbaspirillum. While some of the factors affecting the efficacy of legume inoculants have been studied for decades, there has been only limited advancement, at the fundamental level, of our understanding of how these variables directly influence strain survival and competence.