An investigation has been made of the survival of root-nodule bacteria in autoclaved soils which, after inoculation, were dried at 30°C and subsequently exposed to higher temperatures. Rhizobium trifolii died in grey and yellow sands heated to 70° but survived in red sands and soils of heavier texture. Amendment of a grey sandy soil with 5% (w/w) of montmorillonite, illite, fly ash, or haematite protected R. trifolii from the lethal effects of exposing the dry soil to high temperatures. Kaolinite and goethite did not protect the clover root-nodule bacteria. After three successive exposures at 50° for 5 hr R. trifolii disappeared in a grey sandy soil, but still survived after four exposures in the presence of montmorillonite. The greater survival in heavy-textured soils and red sands can be attributed to the presence of appreciable amounts of illite or haematite or both in these soils, the grey and yellow sands containing only kaolinite and possibly goethite, neither being protective. Another fast-growing species of the root-nodule bacteria, R. meliloti, also failed to survive heating to 70° in a grey sandy soil. On the other hand, the slow-growing R. lupini and R. japonicum were comparatively resistant to the effects of high temperature and desiccation in the same grey sand, a fact which may explain the survival of R. lupini in certain field soils where R. trifolii fails to survive.
Polyphasic characterization of forage legumes root nodule bacteria isolated from semiarid region in Brazil
