Rhizobium population dynamics in the pea rhizosphere of rhizobial inoculant strain applied in different formulations

The effect of inoculant formulation on the population dynamics of rhizobia in the pea rhizosphere was investigated using a streptomycin-resistant muta nt of Rhizobium leguminosarum bv. viceae NITRAGIN128C56G (128C56G strR). Th e isolate was formulated into liquid, peat powder, and granular peat carrie rs, and was tested on pea at field sites near Saskatoon, Saskatchewan, and Beaverlodge, Alberta, in 1996 and 1997. The liquid and peat powder formulat ions were applied to seed while the granular inoculant was applied to soil. In three out of four site years, population dynamics were similar among fo rmulations: an initial decline or lag period lasting 2-5 days followed by a n increase to approximately 10(5) colony-forming units (CFU)/seedling by 14 -28 days after planting (DAP) and, where sampled, a continuing increase fro m 10(7) to 10(8) CFU/plant at 63 DAP. In these same site years, nodule numb er (not determined at Beaverlodge in 1997) and nodule occupancy at 60 days were not significantly different among formulations. In contrast, soil popu lations of 128C56G strR from the liquid formulation declined to near zero b y 28 DAP at Beaverlodge in 1996, when soil moisture was excessive in spring because of high rainfall. Populations increased in this treatment after th is time, but remained significantly lower than the populations of the other two formulations throughout the sampling period. Pea seed yields were not significantly different among treatments in either year at Beaverlodge, but were significantly higher with granular inoculant than the noninoculated c ontrol in Saskatoon. Within inoculated treatments at Saskatoon, there were no significant differences in grain yield.