Biological N2 fixation represents the major source of N input in agricultural soils including those in arid regions. The major N2-fixing systems are the symbiotic systems, which can play a significant role in improving the fertility and productivity of low-N soils. The Rhizobium-legume symbioses have received most attention and have been examined extensively. The behavior of some N2-fixing systems under severe environmental conditions such as salt stress, drought stress, acidity, alkalinity, nutrient deficiency, fertilizers, heavy metals, and pesticides is reviewed. These major stress factors suppress the growth and symbiotic characteristics of most rhizobia; however, several strains, distributed among various species of rhizobia, are tolerant to stress effects. Some strains of rhizobia form effective (N2-fixing) symbioses with their host legumes under salt, heat, and acid stresses, and can sometimes do so under the effect of heavy metals. Reclamation and improvement of the fertility of arid lands by application of organic (manure and sewage sludge) and inorganic (synthetic) fertilizers are expensive and can be a source of pollution. The Rhizobium-legume (herb or tree) symbiosis is suggested to be the ideal solution to the improvement of soil fertility and the rehabilitation of arid lands and is an important direction for future research.

Rhizobium-Legume Symbiosis and Nitrogen Fixation under Severe Conditions and in an Arid Climate

Rhizobia are well known for their capacity to establish a symbiosis with legumes. They inhabit root nodules, where they reduce atmospheric nitrogen and make it available to the plant. Biological nitrogen fixation is an important component of sustainable agriculture, and rhizobial inoculants have been applied frequently as biofertilizers. In this review we present recently developed technologies and strategies for selecting quality inoculant strains by taking into consideration the complex interaction between the edaphic environment with the genotypes of both the legume and its microsymbiont. Enhanced competitive ability in an inoculant strain is a key requirement for successful colonization of plant roots, nodule formation, and subsequent N2-fixation. We discuss several avenues for the management and manipulation of rhizobial competition as well as genes that influence competition in the rhizosphere. The use of molecular techniques has greatly contributed to our knowledge of nodule-bacterial diversity and phylogeny. Approaches to the study of rhizobial diversity as well as mechanisms for the evolutionary diversification of nodulating bacteria are presented. Rhizobium genomes ranging from 5.5 to 9 Mb have been sequenced recently and deposited in public databases. A comparison of sequence data has led to a better understanding of genes involved in the symbiotic process as well as possible mechanisms responsible for horizontal transfer of genetic elements and symbiosis genes among rhizobia. Furthermore, rhizobia are frequent rhizosphere colonizers of a wide range of plants and may also inhabit nonleguminous plants endophytically. In these rhizospheric and endophytic habitats they may exhibit several plant growth-promoting effects, such as hormone production, phosphate solubilization, and the suppression of pathogens.

Advances in Rhizobium Research

The importance of soybean as a source of oil and protein, and its ability to grow symbiotically on low-N soils, point to its continued status as the most valuable grain legume in the world. With limited new land on which to expand, and emphasis on sustainable systems, increases in soybean production will come mostly from increased yield per unit area. Improvements in biological nitrogen fixation can help achieve increased soybean production, and this chapter discusses research and production strategies for such improvement.

https://www.ctahr.hawaii.edu/bnf/Downloads/Bibliography/Potential%20for%20Increasing%20Nitrogen%20Fixation,%20Keyser,%20Li,%201992.PDF

Potential for increasing biological nitrogen fixation in soybean

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Documentation of the problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 13 Distribution of B,~~cly~~l i i~oDi~~rri groups in the United States . . . . . . . . . . . . . . . . . . . . . . . . . 5 13 Effects of bacterial and plant genotype . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 14 Soil properties and levels of indigenous bacteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 15 Other factors in the field environment that may affect survival and (or) infection . . . . . . . . . . . . . . 5 15 Approaches to controlling infection in the field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 16 Use of high inoculant doses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 16 Use of bacteriocidal agents in conjunction with resistant strains . . . . . . . . . . . . . . . . . . . . . . . 5 16 Inoculant formulation or placement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 16 Modification or selection of host genotype . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 16 Modification or selection of bacterial genotype . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 17 Potential problems confronting researchers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 17 Genetic diversity of rhizobial soil residents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 17 Prediction of field dominance by studies of "competitiveness" . . . . . . . . . . . . . . . . . . . . . . . . 5 17 Antibiotic-resistant ibolates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 18 Sampling of field populations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 18 Summary and prospects for the future . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 18 Key worcls: competitiveness, nitrogen fixation, rhizobial ecology, Blncl~rliizobi~~rri.

Failure of inoculant rhizobia to overcome the dominance of indigenous strains for nodule formation

Inoculation of legumes under field conditions with superior nitrogen-fixing rhizobia does not always result in the desired yield increase. Often it is observed that the inoculum strain fails to occupy a significant proportion of the nodules. The introduced inoculant strains have to compete with the indigenous, often ineffective, nitrogen-fixing rhizobial population at different levels. The success of inoculation depends to a large extent on the ratio of the inoculant cells to indigenous rhizobia. However, intrinsic characteristics of the inoculant and indigenous rhizobia, and their responses to abiotic and biotic environmental variables, also influence the outcome of inoculation. In this review, the genetic basis for “efficient host-bacteria interaction” is reviewed. In addition, environmental factors that influence competition and saprophytic competence of rhizobia are discussed.

Factors Influencing Nodule Occupancy by Inoculant Rhizobia

The influence of seed and soil inoculation on bradyrhizobial migration, nodulation, and N2 fixation was examined by using two Bradyrhizobium japonicum strains of contrasting effectiveness in N2 fixation. Seed-inoculated strains formed fewer nodules on soybeans (mostly restricted to the tap and crown roots within 0 to 5 cm from the stem base) than did bradyrhizobia distributed throughout the soil or inoculated at specific depths. Nodulation was greater below the depths at which bradyrhizobial cells were located rather than above, even though watering was done from below to minimize passive bradyrhizobial migration with percolating water. The most profuse nodulation occurred within approximately 5 cm below the point of placement and was generally negligible below 10 cm. These and other results suggest that bradyrhizobial migration from the initial point of placement was very limited. Nevertheless, the more competitive strain, effective strain THA 7, migrated into soil to a greater extent than the ineffective strain THA 1 did. Nitrogen fixation resulting from the dual-strain inoculations differed depending on the method of inoculation. For example, the amount of N2 fixed when both strains were slurried together onto the seed was about half that obtained from mixing the effective strain into the soil with the ineffective strain on the seed. The results indicate the importance of rhizobial distribution or movement into soil for nodulation, nodule distribution, strain competitiveness, and N2 fixation in soil-grown legumes.

https://aem.asm.org/content/aem/55/7/1711.full.pdf

G. D. Bowen

Papers

Influence of Bradyrhizobium japonicum Location and Movement on Nodulation and Nitrogen Fixation in Soybeans.