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Early Maturing Soybean Nodulation and Performance with Selected Bradyrhizobium japonicum Strains

Univ. of Mennesota, Northwest Exp. Stn., Crookston, MN 56716
Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, St. Paul, MN 55108

^* Corresponding author.

Seed inoculation with Bradyrhizobium japonicum (Kirchner) Jordan is a common production practice for most soybean [Glycine max (L.) Merr.] growers in northwest Minnesota; however, plants seldom are well-nodulated. Our objectives were to (i) identify strains of B. japonicum that enhance nodulation and agronomic performance of Maturity Group 00 soybean cultivars grown on soils having a range of NO₃^––N concentrations; and (ii) assess cultivar-by-strain specificity. Six cultivars and five B. japonicum strains were evaluated in six field trials during 1988 to 1990. Nonfertilized (1988–1990) and N fertilized (1990), uninoculated controls were included. Soil NO₃^––N concentrations (0-60 cm) at planting ranged from 52 to 175 kg N ha^–1. Residual soil NO₃^––N values were low and substantially less than initial values in only three environments. Nonetheless, significant increases in response to inoculation were observed for grain yield, seed weight, and grain N concentration in 17 of 18 comparisons. In 1990 trials, fertilizer N (168 kg ha^–1) increased seed weight and grain N concentration significantly more than inoculation. Nodule number (r = –0.89; significant at P = 0.05) and dry weight (r = –0.97; significant at P = 0.01) were inversely correlated with soil NO₃^––N (0-60 cm) at planting. Estimated (difference method) N₂ fixation (r = –0.93; significant at P = 0.05) and relative increases in grain yield (r = –0.93; significant at P = 0.05), seed weight (r = –0.97; significant at P = 0.01) and grain N concentration (r = –0.95; significant at P = 0.05) were inversely correlated with soil NO₃^––N (0-60 cm) at harvest. Although similar responses were observed among all cultivars, B. japonicum strains 61A152 and 61A212 generally outperformed other strains. Cultivar-by-strain specificity could not be detected. Soybean producers in northwest Minnesota should realize significant increases (10% or greater) in grain yield in response to inoculation of rhizobia-free soils, provided initial soil NO₃^––N concentrations (0-60 cm) are less than 110 kg N ha^–1 and N mineralization during the growing season is minimal.

Received for publication April 3, 1991.