Effect of Soil Temperature and Inoculum Rate on the Recovery of Three Introduced Strains of Rhizobium japonicum1

Abstract

AbstractIn order to determine the effect of soil temperature on inoculum recovery within soybean [Glycine max (L.) Merr.] nodules, an experiment consisting of four soil temperatures (15, 20, 25, and 30 °C), four soybean lines (Hodgson, Steele, ISZ.7, and 257.428), four inoculation rates (0,3 × 107, 1.5 × 108, and 3.0 × 108 cells/pot) and replicated four times was conducted in a controlled environmental chamber. Single strain inoculants of USDA 110, 138, and 140 on peat base carrier (l08 rhizobia/gram of peat) were mixed and added to each pot at the time of planting. Plants were then grown for 6 weeks. Soil temperature affected plant and nodule growth as well as the recovery of strains of Rhizobium. Plant heights, when averaged over four soybean genotypes, were 12, 15, 45, and 55 cm for soil temperatures 15, 20, 25, and 30 °C, respectively. Plant dry weight, nodule number and nodule fresh weight increased with increasing temperature until a maximum value was reached at the 25 °C soil temperature. Recovery of introduced strains of R. japonicum 110, 138, and 140 was dependent on soil temperature and inoculum rate. Introduced strains of R. japonicum 110 and 138 showed their highest recovery rates at the 15 and 30 °C soil temperatures. At the 20 °C soil temperature recovery of strains 110 and 138 were significantly lower for the cvs. Hodgson and Steele than for the genotypes 257.428 and ISZ.7. The indigenous rhizobia were most difficult to displace at 20 °C. Strain 140 was a poor competitor throughout the entire experiment.