Multistrain vs. Single Strain Rhizobium japonicum Inoculants for Early Maturing (00 and 000) Soybean Cultivars: N2 Fixation Quantified by 15N Isotope Dilution

Early maturing soybean cultivars (groups 00 and 000) adapted to western Canada are now available but since the soils contain no indigenous Rhizobium japonicum, it is important to determine if strains selected for efficient N₂ fixation with American soybean cultivars are also efficient with Canadian cultivars and if single strain are superior to multistrain inoculants. Accordingly, a 2-year evaluation was made of the effect of up to eight single strains and two commercial multistrain combinations of R. japonicum granular inoculants on maturity, seed yield, oil and protein content, and N₂ fixation (as determined for the first time by ¹⁵N isotope dilution) for three promising soybean lines. Rhizobium japonicum strains 61A148 (USDA 142) and 61A118 (USDA 138) were consistently superior in N₂ fixation with all soybean cultivars, resulting in seed yields and seed protein higher than those of the other strains. Strain 61A101 was consistently an inefficient N₂ fixer. These differences in efficiency between strains were not related to documented ability to take up H₂ evolved by nitrogenase because, although strain 61A148 is considered Hup⁺, strain 61A118 is Hup⁻. Several strains resulted in percent plant N derived from the atmosphere exceeding 50% with N₂ fixed as high as 151 kg N ha⁻¹ for strain 61A148 on King Grain line X005. However, over all strains, Maple Amber was superior for supporting N₂ fixation, averaging 91 kg while X005 had 83 kg and Maple Presto 60 kg N fixed ha⁻¹. Line strain combinations resulting in lower levels of N₂ fixation assimilated more soil N such that total N yields averaged over lines and strains were generally similar. These strains selected for U. S. soybean cultivars are also acceptable for Canadian soybeans. Interstrain competition occurred with one multistrain inoculant, to the detriment of plant yield. A commercial mix containing four strains (61A101, 61A118, 61A124, 61A148) had low N₂ fixation similar to that of strain 61A101. This suggested that strain 61A101 was highly competitive and may have formed a majority of the nodules thereby excluding the highly efficient strains 61A118 and 61A148. However, a new commercial mix containing three strains which were clones of 61A101, 61A118, and 61A124 did not exhibit any detrimental interstrain competition. Thus, the new multistrain soybean inoculant can be recommended for Canadian soybean cultivars.