Survival of a genetically modified root-colonizing pseudomonad and rhizobium strain in an acidic soil

Maintaining threshold populations of inoculum microorganisms in the soil environment is important for such practical applications as biocontrol, plant growth-promotion, bioremediation, and nodulation. However, because of both technical and labor constraints in monitoring bacterial viability in nonsterile soils, few studies have reported on survival kinetics, particularly in relation to subtle alterations in soil acidity-related factors. A genetically modified strain Pseudomonas putida R20/lacZY or Rhizobium leguminosarum bv. trifolii 162S7a/ gusA was introduced into conditioned, nonsterile Gilpin (fine loamy, mixed mesic, Typic Hapludult) silt loam soil, limed at four low levels (pH, 4.71, 4.81, 4.92, and 4.99) or derivative soil solutions with highly correlated (R-2 >= 0.81) chemical properties. Immediate declines in viability of both strains were found in all soils, reaching 0.1 to 1% initial colony-forming unit (CFU) g(-1) soil in 35 h for P. putida and in 68 h for R. leguminosarum bv. trifolii. Death rate constants (kd) for both strains were directly related to lime level (soil pH), although differences were not significant (P > 0.05) for the rhizobium. Use of soil solutions gave similar responses for both strains, but over much shorter incubation times. As with soils, kd values for both strains in soil solutions were directly related to lime level (solution pH). In both soil and soil solution experiments, survival (kd) was negatively correlated (R-2 >= -0.914) with pH and basic cation (Ca and Mg), and positively correlated (R-2 >= 0.933) with AI, concentrations. This relationship of viability to soil solution chemistry was broadly confirmed for both bacterial strains by use of fluorescent probes, suggesting increased cell membrane damage at lower pHs. These results demonstrate not only the alternative utility of using soil solutions, rather than nonsterile soils, for bacterial viability assessments, but also the positive effect of low-level liming (similar to 0.28 pH unit increase) on survival of beneficial root-colonizing bacteria in acidic soils.