Early maturing soybean nodulation and performance with selected Bradyrhizobium japonicum strains.

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) identity strains of B. japonicum that enhance nodulation and agronomic performance of Maturity Group 00 soybean cultivars grown on soils having a range of NO3(-1)-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(-1)-N concentrations (0-60 cm) at planting ranged from 52 to 175 kg N ha-1. Residual soil NO3(-1)-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 wore 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 sod NO3(-1)-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(-1)-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 sod NO3(-1)-N concentrations (0-60 cm) are less than 110 kg N ha-1 and N mineralization during the growing season is minimal.