Phenotypic characterization of soybean bradyrhizobia in two soils of North Carolina

Serotypic composition of nodules (480 per soil type) from five soybean cultivars grown on two (Dothan and Cape Fear) soils of the Atlantic Coastal Plain of North Carolina was characterized. Symbiotic N2-fixation efficiency, capacity for induction of foliar chlorosis symptomatic of rhizobitoxine production and antibiotic resistances of isolates purified from these nodules were also determined. While host plant cultivar had no significant effect on the serotype distribution, soil type had a large effect on the distribution and diversity of serotypes. Forty-six serotypes were identified among nodules from the Cape Fear soil, but only serotype 4676 (8%), 76 (11%), 94 (9%) and 122124 (12%) occurred in more than 5% of the nodules. Thirty percent of nodule occupants were not identified with the eleven antisera used. Twenty-four serotypes were identified among nodules from the Dothan soil. Of these serotypes 3194 (32%), 4676 (16%), and 76 (23%) occurred in more than 15% of the nodules. Five percent of the nodule occupants were not identified. Major serotypes did not change, but their frequency changed when fields were sampled at different growth stages in the same season and at the same growth stage in different seasons. Isolates serotyped as 3194, 4676, and most of the isolates serotyped as 76 generally exhibited higher levels of resistance to streptomycin and erythromycin than isolates serotyped as 24, 94 and 122124. Five percent of the isolates from the Cape Fear soil (all serotyped as 3194) and 18% of the isolates from the Dothan soil (serotyped as 3194 or 76) induced foliar chlorosis when cultivar Brim was the host. Only 12–14% of the isolates from the two populations had N2-fixation capacity equal to or greater than that of the efficient reference strain MN110. However, four improved soybean cultivars grown in the same fields and year that isolates were obtained did not exhibit a significant seed yield response to application of 150 kg N ha−1 when yields in the minus N treatment ranged from 3.2 to 3.7 Mg ha−1. A significant seed yield response by a non-nodulated cultivar indicated that these soils were N limited. Therefore, the symbiotic N2-fixation capacity of these bradyrhizobial populations did not limit soybean seed yields despite the low percentage of isolates with high N2-fixation efficiency and the presence of isolates with the capacity to induce leaf chlorosis symptomatic of rhizobitoxine production.