Effects of Bradyrhizobium strain and host genotype,nodule dry weight and leaf area on groundnut (Arachis hypogaea L. ssp. fastigiata) yield

Effects of inoculating four Arachis hypogaea ssp. fastigiata cultivars with 17 Bradyrhizobium spp. strains were studied in a glasshouse experiment using a sandy soil devoid of an indigenous Bradyrhizobium population. Firstly, a wide range of parameters, indicative of symbiotic performance, were assessed for their influence on seed yield, by correlation and statistical analyses. It was found that nodule dry weight and leaf area were relevant parameters concerning seed yield. Secondly, the effects of host and strain genotype on those parameters were described.Variations in nodule dry weight did not have an effect on seed yield, except for cultivar Natal Common at lower nodule dry weight values. Therefore, it was concluded that the quantity of nitrogen fixing tissue met the demand for combined nitrogen and did not limit seed yield. This conclusion was further supported by the observation that at low nodule numbers per plant the nodule size increased to generate sufficient nitrogen fixing tissue.Leaf area, which comprises components for both photosynthetic capacity and plant development, was found to correlate well with seed yield. An increase in leaf area resulted in significant seed yield increases for all three spanish-type cultivars, but not for the valencia-type cultivar. Leaf area, thus, appeared as a factor limiting seed yield of spanish-type groundnuts.Cultivar performance concerning seed yield was significantly better for Natal Common compared to the other three cultivars, while Natal Common had a significantly lower plant (biomass excluding seed) dry weight value.Inoculation with different strains of Bradyrhizobium resulted in significantly different nodule dry weight values, but hardly led to significant differences in seed yield. This agreed with the finding that the amount of nitrogen fixing tissue appeared not to limit the availability of combined nitrogen.A large quantity of nitrogen was partitioned to the groundnut seeds: 62% to 76% of total accumulated nitrogen was located in the seeds.This study showed that testing for symbiotic effectiveness in the groundnut Bradyrhizobium symbiosis should include assessment of final (seed and biomass) yield, because parameters measured at stages prior to maturity, like nodulation parameters, may lead to flawed effectiveness ratings.     

Peanut (Arachis hypogaea) response to inoculation with Bradyrhizobium sp. in soils of Argentina

Soil bacteria (rhizobia) of the genus Bradyrhizobium form symbiotic relationships with peanut root cells and fix atmospheric nitrogen by converting it to nitrogenous compounds. Inoculation of peanut with rhizobia can enhance the plant’s ability to fix nitrogen from the air and thereby reduce the requirement for nitrogen fertiliser. We evaluated three Bradyrhizobium sp. strains for effect on root nodulation and on pod yield of peanut in Argentina soils, using laboratory and field experiments. Of these, strain C‐145 was the most effective in laboratory studies. In‐furrow inoculation with this strain produced increased nodule number, relative to seed inoculation. However, pod yield was not increased significantly by either type of inoculation. In view of the inconsistent response of peanut to inoculation, we examined the effect of indigenous strains of bradyrhizobia. The high degree of nodulation and nitrogen fixation produced by indigenous rhizobia were sufficient for maximal yield under the field and inoculation conditions used in this study. The data are important for future investigation of alternative inoculant strains and conditions for improving peanut production.     

Competition between effective and ineffective bradyrhizobia nodulating peanuts (Arachis hypogaea)1

The competitive ability of effective and ineffective isolates of Bradyrhizobium to form nodules on four peanut (Arachis hypogaea) genotypes from which they were originally isolated was investigated in a greenhouse study. Pregerminated seeds of each genotype were inoculated with five ratios of ineffective effective bradyrhizobia: 10⁴:0, 10⁴:10² 10⁴:10⁴ 10²:10⁴ and 0:10⁴. Plants were harvested 35 days after planting and nitrogenase activity, total nodule number per plant, plant shoot dry weight and the proportion of nodules formed by each Bradyrhizobium isolate were determined. Nodulation suppression occurred on plants inoculated with the mixed cultures, especially on those inoculated with the 10⁴:10² ineffective effective inoculum ratio. More than 85% of the nodules on plants inoculated with the 10⁴:10⁴ and 10²:10⁴ ineffective effective Bradyrhizobium mixtures were formed by the effective isolates. The number of effective nodules per plant, nitrogenase activity and plant shoot dry matter production all increased with the proportion of the effective bradyrhizobia in the inocula.     

Bradyrhizobium japonicum Inoculant Mobility, Nodule Occupancy, and Acetylene Reduction in the Soybean Root System

In the American Midwest, superior inoculant rhizobia applied to soybeans usually occupy only 5 to 20% of nodules, and response to inoculation is the exception rather than the rule. Attempts to overcome this problem have met with limited success. We evaluated the ability of Bradyrhizobium japonicum, supplied as a seed coat inoculant, to stay abreast of the infectible region of the developing soybean root system. The rhizoplane population of the inoculant strain declined with distance from site of placement, the decrease being more pronounced on lateral than on taproots. This decline was paralleled by a decrease in inoculant-strain nodule occupancy. Inoculant bradyrhizobia contributed little to nodulation of lateral roots, which at pod-fill accounted for more than 50% of nodule number and mass, and were major contributors to acetylene reduction activity. From these data, it appears that inoculant bradyrhizobia are competitive with indigenous soil strains at the point of placement in the soil but have limited mobility and so are incapable of sustaining high populations throughout the developing root system. The result is low nodule occupancy by the inoculant strain in the tapand lateral roots. Future studies should address aspects of inoculant placement and establishment.     

Effect of seed size and plant growth on nodulation and nodule development in lima bean (Phaseolus lunatus L.)

Lima bean (Phaseolus lunatus L.) cultivars vary widely in their growth habit and seed size. Preliminary experiments indicated that a large-seeded pole cultivar (King of the Garden) formed many more nodules than a small-seeded bush cultivar (Henderson). The relative importance of seed size and shoot mass in determining nodule number and mass was assessed in five lima bean cultivars differing in seed size and growth habit. Between cultivars, significant positive correlations between initial seed mass, plant weight and nodule number and mass were observed during the first four weeks after planting. Comparisons within cultivars indicated a strong correlation between nodule mass and shoot dry weight. The influence of plant morphology on nodule formation and mass was secondary to the effects of seed and shoot mass. As plants matured, the increase in nodule mass paralleled the increase in plant mass, while nodule number was relatively stable after day 18. These results suggest that the highly regulated process of nodule formation was under the influence of seed derived factors, while the continued accumulation of nodule tissue was related to shoot growth.     

Early seedling and seasonal N2-fixing symbiotic activity of two soybean [Glycine max (L.) Merr.] cultivars inoculated with Bradyrhizobium strains of diverse origin

In areas with a short growing season the poor adaptability of soybean [Glycine max Meer. (L.)] to cool soil conditions is considered the primary yield limiting factor. Soybean requires temperatures in the 25 to 30°C range for optimum N₂-fixation and yield. Field studies were conducted in 1990 and 1991 at Montreal, Quebec to determine whether adaptability to cool soil conditions, with respect to earlier symbiosis establishment and function, existed among either Bradyrhizobium strains or soybean genotypes. An early maturing isoline of the soybean cultivar Evans and the cultivar Maple Arrow were inoculated with one of four strains isolated from the cold soils of Hakkaido, northern Japan, or the commercially used strains 532C or USDA110, at two planting dates. Plot biomass and nodulation were assessed at seedling (V2), and flowering(R2) growth stages and harvest maturity. Soybean genotypes did not differ for pre-flowering nodulation or N₂-fixation in the cool spring conditions of the first year. Seasonal N₂-fixation rates were also determined at the final harvest by the N-balance and ¹⁵N-isotope dilution methods. Significantly higher symbiotic activity was found for two of the four Hakkaido strains and was reflected in higher final soybean seed yield and total N₂-fixation for the growing season, as compared to the two commercial strains. Planting 14 days earlier resulted in greater early vegetative and total seasonal N₂ fixation and yield in the second year when soil temperatures were warmer, emphasizing the need for the development of soybean-Bradyrhizobium combinations superior in nodule development and function under cool soil conditions.     

Application of genistein to inocula and soil to overcome low spring soil temperature inhibition of soybean nodulation and nitrogen fixation

In the soybean (Glycine max. (L.) Merr)– Bradyrhizobium japonicum symbiosis, suboptimal root zone temperatures (RZTs) slow nodule development by disruption of the interorganismal signal exchange between the host plant and bradyrhizobia. Two field experiments were conducted on two adjacent sites in 1994 to determine whether the incubation of B. japonicum with genistein prior to application as an inoculant, or genistein, without B. japonicum, applied onto seeds in the furrow at the time of planting, increased soybean nodulation, N fixation, and total N yield. The results of these experiments indicated that genistein application increased nodule number and nodule dry matter per plant and hastened the onset of N fixation during the early portion of the soybean growing season, when the soils were still cool. Because these variables were improved, total fixed. N, fixed N as a percentage of total plant N, and N yield increased due to genistein application. The interaction between genistein application and soybean cultivars indicated that genistein application was more effective on N-stressed plants.     

Genetic variability in <Emphasis Type="Italic">Bradyrhizobium japonicum</Emphasis> strains nodulating soybean [<Emphasis Type="Italic">Glycine max</Emphasis> (L.) Merrill]

Brazil has succeeded in sustaining production of soybean [Glycine max (L.) Merrill] by relying mainly on symbiotic N₂ fixation, thanks to the selection and use in inoculants of very effective strains of Bradyrhizobium japonicum and Bradyrhizobium elkanii. It is desirable that rhizobial strains used in inoculants have stable genetic and physiological traits, but experience confirms that rhizobial strains nodulating soybean often lose competitiveness in the field. In this study, soybean cultivar BR 16 was single-inoculated with four B. japonicum strains (CIAT 88, CIAT 89, CIAT 104 and CIAT 105) under aseptic conditions. Forty colonies were isolated from nodules produced by each strain. The progenitor strains, the isolates and four other commercially recommended strains were applied separately to the same cultivar under controlled greenhouse conditions. We observed significant variability in nodulation, shoot dry weight, shoot total N, nodule efficiency (total N mass over nodule mass) and BOX-PCR fingerprinting profiles between variant and progenitor strains. Some variant strains resulted in significantly larger responses in terms of shoot total N, dry weight and nodule efficiency, when compared to their progenitor strain. These results highlight the need for intermittent evaluation of stock bacterial cultures to guarantee effective symbiosis after inoculation. Most importantly, it indicates that it is possible to improve symbiotic effectiveness by screening rhizobial strains for higher N₂ fixation capacity within the natural variability that can be found within each progenitor strain.     

Bradyrhizobium technology: a promising substitute for chemical nitrogen fertilizer in Bangladesh agriculture

Nitrogen is the most limiting element in Bangladesh soils and urea is the fertilizer commonly used for supplying it. Bradyrhizobium/Rhizobium inoculant was tried as a source of N nutrition for grain legumes in a number of field experiments. The inoculants markedly increased nodule number, nodule mass, shoot weight and yield of the crops compared to uninoculated control and urea-N treatments. For soybean (Glycine max), inoculation increased yield 113 percent over the control and 49 percent over the urea treament. For groundnut (Arachis hypogaea), the increases were 36 and 11 percent; for lentil (Lens culinaris), 30 and 13 percent; and for mungbean (Vigna radiata), 47 and 7 percent. The local inoculant strains were suitable for dependable inoculant production. The inoculant technology can be used as a promising and cheap substitute of urea for growing food legume crops in Bangladesh.     

Nitrogen-fixing potential of micropropagated clones of Acacia mangium inoculated with different Bradyrhizobium spp. strains

Five A. mangium seedlings of different shoot lengths were selected from a 600-seed screening experiment and micropropagated. Two-week-old rooted microcuttings of the 5 micropropagated clones were inoculated with 3 specific Bradyrhizobium spp. strains in 15 combinations. After 5 months of growth, nodule dry weight and shoot dry weight data showed significant effects of clone and Bradyrhizobium spp. strain. Clones RR-G₁ and IR-M₂ and Bradyrhizobium sp. Aust13c resulted in the highest dry-matter production and most efficient nodulation. No interaction was observed between clone and Bradyrhizobium spp. strain, which indicates that the Bradyrhizobium spp. strain and the host plant can be selected separately.     

Effect of Fusarium oxysporum f. sp. glycines and Sclerotium rolfsii on the pathogenicity of Meloidogyne incognita race 2 to soybean

Screenhouse studies were conducted to investigate the effects of Fusarium oxysporum f. sp. glycines and Sclerotium rolfsii on the pathogenicity of Meloidogyne incognita race 2 on soybean and the influence of the nematode on wilt incidence and growth of soybean. The interaction of each fungus with the nematode resulted in reduced shoot and root growth. Final nematode population was also reduced with concomitant inoculation of nematode and fungus or inoculation of fungus before nematode. While M. incognita suppressed wilt incidence in two nematode-susceptible cultivars of soybean (TGX 1485-2D and TGX 1440-IE), it had limited effect on wilt incidence in the nematode resistant cultivar of soybean (TGX 1448-2E). When F. oxysporumwas inoculated with the nematode, the mean number of nematodes that penetrated soybean roots decreased by 75% in TGX 1448-2E, 68% in TGX 1485-1D and 65% in TGX 1440-1E. Similarly when the soil was treated with S. rolfsii, the number decreased by 78% in TGX 1448-2E, 77% in TGX 1485-1D and 68% in TGX 1440-1E. The nematode did not develop beyond second-stage juvenile in TGX-1448-2E.     

Identification of Bradyrhizobium japonicum Nodule Isolates from Wisconsin Soybean Farms

One-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis was a more discriminating method than serotyping for identifying strains of Bradyrhizobium japonicum. Analysis of 543 nodule isolates from southeastern Wisconsin soybean farms revealed that none of the isolates were formed by any of the inoculant strains supplied by either of two inoculant companies. Twenty-nine indigenous strains and six inoculant strains were identified. Strain 61A76, the most competitive indigenous strain, formed 21% of the nodules. Indigenous strains 3030, 3058, 0336, and 3052 formed 15, 11, 9, and 9% of the nodules, respectively. These predominant strains were not associated with a particular soybean cultivar, soil type, or farm location.     

Inoculation of soybean (Glycine max. (L.) Merr.) with genistein-preincubated Bradyrhizobium japonicum or genistein directly applied into soil increases soybean protein and dry matter yield under short season conditions

In short-season soybean production areas, low soil temperature is the major factor limiting plant growth and yield. The decreases in soybean yield at low temperatures are mainly due to nitrogen limitation. Genistein, the most effective plant-to-bacterium signal in the soybean (Glycine max (L.) Merr.) nitrogen fixation symbiosis, was used to pretreat Bradyrhizobium japonicum. We have previously reported that this increased soybean nodulation and nitrogen fixation in growth chamber studies. Two field experiments were conducted on two adjacent sites in 1994 to determine whether the incubation of B. japonicum with genistein, prior to application as an inoculant, or genistein, without B. japonicum, applied onto seeds in the furrow at the time of planting, increased soybean grain yield and protein yield in short season areas. The results of these experiments indicated that genistein-preincubated bradyrhizobia increased the grain yield and protein yield of AC Bravor, the later maturing of the two cultivars tested. Genistein without B. japonicum, applied onto seeds in the furrow at the time of planting also increased both grain and protein yield by stimulation of native soil B. japonicum. Interactions existed between genistein application and soybean cultivars, and indicated that the cultivar with the greatest yield potential responded more to genistein addition.     

Genetic diversity of indigenous Bradyrhizobium nodulating promiscuous soybean [Glycine max (L) Merr.] varieties in Kenya: Impact of phosphorus and lime fertilization in two contrasting sites

While soybean is an exotic crop introduced in Kenya early last century, promiscuous (TGx) varieties which nodulate with indigenous rhizobia have only recently been introduced. Since farmers in Kenya generally cannot afford or access fertilizer or inoculants, the identification of effective indigenous Bradyrhizobium strains which nodulate promiscuous soybean could be useful in the development of inoculant strains. Genetic diversity and phylogeny of indigenous Bradyrhizobium strains nodulating seven introduced promiscuous soybean varieties grown in two different sites in Kenya was assayed using the Polymerase Chain Reaction-Restriction Fragment Length Polymorphism (PCR-RFLP) of the 16S-23S rDNA intergenic spacer region and 16S rRNA gene sequencing. PCR-RFLP analysis directly applied on 289 nodules using Msp I distinguished 18 intergenic spacer groups (IGS) I–XVIII. Predominant IGS groups were I, III, II, IV and VI which constituted 43.9%, 24.6%, 8.3% 7.6% and 6.9% respectively of all the analyzed nodules from the two sites while IGS group VII, IX, X, XI, XII, XIV, XVI, XVII, XVIII each constituted 1% or less. The IGS groups were specific to sites and treatments but not varieties. Phylogenetic analysis of the 16S rRNA gene sequences showed that all indigenous strains belong to the genus Bradyrhizobium. Bradyrhizobium elkanii, Bradyrhizobium spp and Bradyrhizobium japonicum related strains were the most predominant and accounted for 37.9%, 34.5%, and 20.7% respectively while B. yuanmigense related accounted for 6.9% of all strains identified in the two combined sites. The diversity identified in Bradyrhizobium populations in the two sites represent a valuable genetic resource that has potential utility for the selection of more competitive and effective strains to improve biological nitrogen fixation and thus increase soybean yields at low cost.     

Nitrogen fixation with the soybean crop in Brazil: Compatibility between seed treatment with fungicides and bradyrhizobial inoculants

Biological nitrogen fixation with the soybean crop can be improved by seed inoculation with superiorBradyrhizobium strains, but factors that reducethe population of inoculated bradyrhizobiaon the seedwill directly affect the efficiency of the process. Seed treatment with fungicides has been broadly practiced as cheap insurance against seed-and soil-borne pathogens, but toxicity of most fungicides to bradyrhizobia has often been underestimated. The compatibility between seed treatment with fungicides in single or mixed applications (including Benomyl, Captan, Carbendazin, Carboxin, Difenoconazole, Thiabendazole, Thiram, Tolylfluanid) and bradyrhizobial inoculants was examined in laboratory, greenhouse and field experiments during five crop seasons in Brazil. Bacterial survivalon the seeds was severely affected by all fungicides, resulting in mortalities of up to 62% after only 2 h and of 95% after 24 h. Fungicides also reduced nodule number, total N in grains and decreased yield by up to 17%. The toxic effects of fungicides were more drastic in sandy soils without soybean inoculation and cropping history, reducing nodulation by up to 87%, but were also important in areas with established populations of soybean bradyrhizobia. Therefore, fungicides should be used only when the seeds or soil are contaminated with pathogens, otherwise biological N₂ fixation may be severely affected.     

Performance of phaseolus bean rhizobia in soils from the major production sites in the Nile Delta

The symbiotic and competitive performances of two highly effective rhizobia nodulating French bean P. vulgaris were studied in silty loam and clayey soils. The experiments were carried out to address the performance of two rhizobia strains (CE3 and Ph. 163] and the mixture thereof with the two major cultivated bean cultivars in two soil types from major growing French bean areas in Egypt. Clay and silty loam soils from Menoufia and Ismailia respectively were planted with Bronco and Giza 6 phaseolus bean cultivars. The data obtained from this study indicated that rhizobial inoculation of Giza 6 cultivar in clayey soil showed a positive response to inoculation in terms of nodule numbers and dry weight. This response was also positive in dry matter and biomass accumulation by the plants. The inoculant of strain CE3 enhanced plant growth and N-uptake relative to Ph. 163. However, the mixed inoculant strains were not always as good as single strain inoculants. The competition for nodulation was assessed using two techniques namely fluorescent antibody testing (FA) and REP-PCR fingerprinting. The nodule occupancy by inoculant strain Ph. 163 in both soils occupied 30-40% and 38-50 of nodules of cultivar Bronco. The mixed inocula resulted in higher proportions of nodules containing CE3 in silty loam soil and Ph. 163 in clayey soil. The native rhizobia occupied at least 50% of the nodules on the Bronco cultivar. For cultivar Giza 6, the native rhizobia were more competitive with the inoculant strains. Therefore, we suggest using the studied strains as commercial inocula for phaseolus bean.     

Siderophore production byBradyrhizobium spp. strains nodulating groundnut

EighteenBradyrhizobium spp. strains, fourRhizobium spp. strains and oneAzorhizobium caulinodans strain were grown under Fe limitation and assayed for siderophore production. It was further assessed if Fe accumulation in two groundnut cultivars was influenced by inoculant strain or nitrate fertilisation. Growth ofBradyrhizobium spp. strains nodulating groundnut was slow with mean generation times from 11–24 h. All strains, except MAR 967, showed a reduced growth rate when deprived of Fe; none of the strains showed starvation at 1 M Fe. In the CAS (chrome azurol S)-agar assay, all strains, which formed colonies, produced siderophores as visualised by orange halos around the colonies on blue plates.Bradyrhizobium strains produced much smaller halos than the referenceRhizobium meliloti strain. In the CAS-supernatant assay, all strains, except MAR 967, gave positive responses (measured as absorbance at 630 nm) when supernatants of Fe-depleted cultures were assayed with CAS-indicator complex in comparison with Fe-supplemented cultures. Responses of all fourRhizobium spp. strains were large, while responses of allBradyrhizobium strains, exceptB. japonicum MAR 1491 (USDA 110), were small and mostly insignificant. A small response, i.e. a low Fe-scavenging ability, implies either the production of small quantities of siderophores or the production of low affinity siderophores. Among theBradyrhizobium strains, MAR 1574 and MAR 1587 gave the largest responses taken over the two assays. Fe accumulation in groundnut cultivar Falcon was seven times larger than in cultivar Natal Common. No correlation was found between the quantity of nodule tissue and Fe accumulation, making it unlikely that bacteroids are involved in Fe acquisition by groundnuts. Nitrate-fertilised plants accumulated significantly more Fe, suggesting involvement of nitrate reductase in Fe assimilation in groundnut. The two most successful Fe-scavengingBradyrhizobium spp. strains were also the most effective in nodulating groundnut, the reverse also being true. Strain MAR 967, with the lowest Fe requirement, produced the largest nodule dry weight. These data indicate that improved Fe scavenging properties and/or reduced Fe requirement improve rhizospheric growth and with that nodulation effectiveness.     

Molecular evidence for shifts in polysaccharide composition associated with adaptation of soybean Bradyrhizobium strains to the Brazilian Cerrado soils

Pyrolysis mass spectrometry (PyMS) and DNA fingerprinting (RAPD and RSα hybridization) were used to characterize soybean inoculant strains and root nodule isolates of bradyrhizobia from the Brazilian Cerrado soils. Most isolates were shown to be derived from the inoculant strains on the basis of genotype comparisons by DNA fingerprinting. Phenotypic analysis (using PyMS) of the strains and separately of the polysaccharides derived from them showed that the nodule isolates differed from the parental strains, suggesting adaptation to the Cerrado soil environment. The extent of the differences between the derivatives and inoculant strains was similar for comparisons made on the basis of whole-cell preparations or from the isolated polysaccharides, indicating that the adaptation was caused by changes in the composition of the polysaccharides produced.     

Methods To Alter the Recovery and Nodule Location of Bradyrhizobium japonicum Inoculant Strains on Field-Grown Soybeans

Three strains of Bradyrhizobium japonicum, I17, 110, and 61A76, were evaluated for their ability to form nodules on field-grown soybeans in soil with a highly competitive indigenous B. japonicum population. The predominant indigenous strain, 0336, in the field site used was unlike the more common isolates from Midwestern soils which belong to the 123 or 138 serogroups. This strain persisted in the soil for at least 30 years without any soybean crops. The three inoculant strains differed in their ability to compete with indigenous strains for nodule formation. Four different inoculation treatments were tested in three adjacent fields. When the amount of inoculum was increased, a higher proportion of nodules contained the inoculant strain. The most competitive inoculant strain was I17, a recent field isolate. Strain 61A76 was better than 110. There was no difference in recovery of the inoculant strains on the Hodgson or Corsoy soybean cultivars, nor was there a difference in recovery of the inoculant strains during the growing season. The vertical distribution of nodules containing the inoculant strains was affected by the method of adding the inoculant to the soil. Inoculant added to the seed furrow produced nodules mainly in the top region of the soybean root. Inoculant tilled into the soil produced nodules primarily in the bottom part of the root. The nodules that were produced in the bottom part of the root are younger and may contribute significant amounts of fixed nitrogen to the soybean during seed formation.     

Competitiveness of indigenous populations of Bradyrhizobium japonicum serocluster 123 as determined using a root-tip marking procedure in growth pouches

Soil Bradyrhizobium populations limit nodule occupancy of soybean by symbiotically-superior inoculant strains throughout much of the American midwest. In this study, the competitiveness of indigenous populations of B. japonicum serocluster 123 from Waukegan and Webster soils was evaluated in growth pouches using a root-tip marking procedure. The native rhizobia were from soils incubated 0–8 h in soybean root exudate (SRE) or plant nutrient solution (PNS) prior to inoculation. Populations of serocluster 123 strains in soil and nodule occupancy by these strains were assessed using fluorescent antibodies prepared against B. japonicum USDA 123. There were no significant differences in populations that came from SRE or PNS incubated soils: both populations increased in number over the incubation period. Nodule occupancy by both populations in growth pouches was similar to that previously encountered in field studies with these two soils. With the Waukegan soil, the serocluster 123 population dominated nodulation forming 69 and 62% of taproot nodules above and below the root tip mark, respectively. However, for the more alkaline Webster soil, serocluster 123 strains were much less competitive, producing only 9 and 13%, respectively, of the nodules formed above and below the root tip mark. In growth pouches, soil populations of bradyrhizobia from the Webster soil produced significantly more nodules than those from the Waukegan soil, but both strains and a pure culture of USDA 110 had a similar distribution of nodules.