Population dynamics of Rhizobium japonicum strains used to inoculate three successive crops of soybean

Abstract

Three closely related strains of Rhizobium japonicum, equally effective in N2 fixation, were used to inoculate each of three successive crops of soybeans [Glycine rnax (L.) Merr. cv. Bragg] grown on the same block of land. The soil was a vertisol previously free of R. japonicum, and inoculant was applied at different rates by spraying a suspension of peat culture into the seed bed at time of sowing the seed. The populations of rhizobia that developed in rhizosphere and soil were counted at intervals during crop growth and in the fallow period between crops. There was usually a substantial decline in recovery of inoculant strains immediately after sowing. In soil initially devoid of R. japonicum, populations in the rhizospheres of young seedlings were related to rates of inoculation, but differences disappeared as the plants aged. Shortly after harvest, the soil contained large populations of rhizobia which increased up to 200 times during the fallow period between crops, probably due to release of bacteria from disintegrating nodules. The size of these populations was maintained up to the time of sowing the following crop. Although strains used for second- and third-year crops were dominated by already established rhizobia in rhizosphere colonization and nodule formation, the magnitude of the domination could be reduced by increased rates of inoculation. In soil already containing R. japonicum, the proportion of nodules formed by inoculant strains was greater than the relative number of inoculant rhizobia in the soil or the rhizosphere; this was ascribed to an advantage of specific placement of the inocula in that zone of the soil where infection foci first formed. The results are explicable in numerical terms and are discussed in relation to an inoculation strategy for maximum nodulation by applied inoculant in competition with rhizobia already established in soil.

Human selection and the relaxation of legume defences against ineffective rhizobia

“…This ability of certain plant genotypes to perform better under mixed conditions may be due to strain selectivity of better rhizobium strains into nodules or post-infection sanctions, in which plants detect and senesce (or at least reduce resource supply to) nodules containing less-effective symbionts (Denison 2000;Kiers et al 2003). The resulting differences among strains in nodulation success, and in rhizobia per nodule, are also key to future rhizobial strain fitness (Moawad et al 1984;Brockwell et al 1987). When cultivars were inoculated with only an ineffective strain in our laboratory experiment, we found no significant difference among genotypes.…”

Measurement of Nitrogen Fixation by Soybean in the Field Using the Ureide and Natural 15N Abundance Methods

“…It was significant also that the smallest response by Bragg to the high level of inoculation was at site 2 (Figs. 1, 2, and 3; see also [7], and that ntslO07, the super-nodulating mutant of Bragg, nodulated most prolifically at this site (Fig. 1) (Fig.…”

“…2). Although numbers of B. japonicum in the soil at each site were not quantified, it was likely that sites 1, 3, 4, and 5 were essentially free of B. japonicum and only site 2 which was cropped to nodulated soybean during the previous year contained a population of any size (6,7,23). Confirmation of the Bradyrhizobium status of the soils was obtained by sampling uninoculated Bragg which had been sown adjacent to the experimental plots.…”

“…The 6"N = 1000 (Rsample-Rair)/Rair (7) where R is the ratio mass 29/mass 28. P could then be determined, by the following: P (%) = I00(x -y)/(x -z) (8) where x is the 6'"N of plants deriving all of their N from the soil, in this case the unnodulated Hardee; y is the 6'5N of the nodulated soybean; and z is the 6'5N of soybean receiving all of its N from nitrogen fixation (26).…”

In Situ Phylogenetic Structure and Diversity of Wild Bradyrhizobium Communities

“…Least is known about rhizobia in bulk soil (not penetrated by plant roots). While rhizobia can persist for years in soil without host legumes (12,30,61), it appears that growth is often negligible in bulk soil (4,10,14,22,25). Rhizobia can also proliferate in the rhizosphere (soil near the root zone) of legumes (4,10,18,19,22,25,51).…”