Influence of the inoculum density on the growth and survival of Rhizobium leguminosarum biovar trifolii introduced into sterile and non-sterile loamy sand and silt loam

Abstract

After the introduction of Rhizobium leguminosarum biovar trifolii into natural loamy sand and silt loam, bacterial numbers increased only directly after inoculation. Thereafter, bacterial numbers decreased until an equilibrium was reached. This decrease was exponential on a log scale and could be described by the function Y = A + B − R′, where Y is the log number of rhizobial cells at time: T; A represents the lgo of the final population size; B is the difference between the log (initial number of bacteria) and A; R is the daily reduction factor of Y−A and t is time in days after inoculation. The final population sizes increased with increasing inoculum densities (10 4−10 8 bacteria/g soil). In sterilized soil, however, the populations increased up to an equilibrium, which was not affected by the inoculum density. The final population sizes were higher in silt loam than in loamy sand in natural, as well as in sterilized soil. The final population size was reached earlier in natural silt loam than in loamy sand. Also the growth rate in sterilized soil was higher in silt loam than in loamy sand. The growth rate of low inoculum densities in silt loam was exponential and approximately the same as in yeast extract mannitol broth. The growth rate in loamy sand could be improved by incresing the bulk density of the soil from 1.0 to 1.4 g/cm 3.