Desiccation Tolerance of Clover Rhizobia in Sterile Soils

Procedures were developed to examine the effect of low soil moisture on clover rhizobia ( Rhizobium leguminosarum biovar trifolii , formerly R. trifolii ). A study was then conducted to determine the feasibility of identifying desiccation‐tolerant rhizobia as a possible means of improving inoculant quality for cool‐season, annual clovers ( Trifolium L.). Five clover rhizobia isolates were tested for response to three water potential regimes in two sterile soils over a 42‐d incubation period. Initial soil water potential for all treatment combinations was −0.1 MPa. The three water potential regimes imposed were a constant −0.1‐MPa treatment and two progressive desiccation treatments having equilibrium potentials of ca. −70 and −500 MPa, respectively. These treatments were obtained by using sulfuric acid solutions to manipulate the relative humidity levels of the atmosphere surrounding the inoculated soils. The two soils used (0–10 cm) were an Altavista (Aquic Hapludults) loamy sand and a Cecil (Typic Hapludults) sandy clay loam. Overall survival of the rhizobia was highest, and relative persistence of the isolates was similar, in both soils at the −0.1 MPa treatment. Population levels were generally lowest under the −70 MPa desiccation regime. Population trends under the desiccation regimes were strongly affected by soil type and appeared to reflect the differential desiccation rates observed between the physically dissimilar soils. Although soil type also affected relative survival among the rhizobia, one isolate exhibited excellent survival at all treatment combinations. The more consistent estimates of relative desiccation tolerance were obtained with the finer‐textured Cecil soil, −500 MPa desiccation regime, and a minimum 21‐d incubation period.