Degree Type


Date of Award


Degree Name

Doctor of Philosophy




This research investigated the competitiveness of introduced Bradyrhizobium japonicum strains when added at standard rates for granular inoculants, evaluated the effect of nitrogen and cultivars on serogroup distribution in soybean Glycine max (L.) Merr. nodules, and characterized indigenous populations of bradyrhizobia that introduced strains must compete against in Iowa soils. In the competition studies, 22 introduced strains were evaluated by adding seven to 12 strains that contained antibiotic markers at 10('6) viable cells cm('-1) row on two to three soils each year during 1983 and 1985. Inoculant strains competitive against native bradyrhizobia were found with recoveries ranging up to 46% nodule occupancy. Strains from serogroup 123 were among the more competitive strains but the introduced strains did not increase grain yields. Increased grain yields and forage N of 30 to 40% for nodulating over nonnodulating isolines, however, demonstrated important contributions of bradyrhizobia to soybean production. These strains appear to hold promise for genetic manipulation to improve N(,2)-fixation as possible future inoculants. Nodule serology was not affected by additions of 165 or 330 kg N ha('-1) as urea, by the cultivars Corsoy or Clark, or by commercial inoculum. The serology of field-grown nodules was surveyed across 32 soils from 1982 to 1984, and 90 isolates from 10 of these soils were screened for tolerances to pH (4.5, 6.5, 8.5), temperature (5, 15, 25, 35(DEGREES)C), and salt (1, 10, 20, 30 g NaCl L('-1)) and assayed for their nodulation and relative N(,2)-fixation efficiencies. The more frequently observed serogroups were 123 (68%), 142 (31%), 122 (16%), 110 (10%), and 76 (10%). High nodule occupancy of serogroup 142, in 1984 has not been previously reported in Iowa. Some soils had isolates that were sensitive to pH 4.5 or to pH 8.5, but isolates from other soils grew well at both pHs. Three soils had isolates that tolerated only a narrow temperature range (15 to 25(DEGREES)C), whereas other soil isolates grew well at all temperatures, including 35(DEGREES)C. Isolate growth decreased as salt concentration increased; several isolates were sensitive to 1 g NaCl L('-1) and most were inhibited by 30 g NaCl L('-1). Nitrogen-fixation efficiencies of isolates ranged from 20 to 65% increases above the noninoculated controls and most soils had isolates that averaged 40 to 60% increases.



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Robert Keith Berg, Jr.



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175 pages