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Survival of Rhizobium in Soils Undergoing Drying¹

ABSTRACT

Streptomycin-resistant mutants of Rhizobium were used to study Rhizobium survival in soils undergoing drying. The decline in numbers of R. japonicum in soil was biphasic. The initial, rapid phase coincided with the time of major water loss, and the subsequent slow rate of decline of viability occurred as the soil lost little water. Similar population charges were found with other species of Rhizobium. The rate of dying of R. japonicum and R. leguminosarum during the second phase in soil at relatively low moisture levels was governed by the water content of the soil. In 10 days of drying, the cell numbers of the six Rhizobium species and a cowpea Rhizobium were reduced by two logarithmic orders of magnitude, and no differences in susceptibility were noted between the fast- and slow-growing rhizobia. Exposure of R. japonicum to several cycles of wetting and drying resulted in a further reduction in population size with each cycle. The percentage survivors of R. japonicum was not greatly affected by the time required for the drying but was different in different soils. Rhizobia that proliferated in soil showed somewhat better survival than those grown in culture and added to soil. Decayed organic residues afforded little protection against desiccation, but placing the root-nodule bacteria in a suspension of the polysaccharide they elaborate allowed for somewhat better survival.

¹ Contribution from Dep. of Agronomy, Cornell Univ., Ithaca, NY 14853. These studies were supported by the Consejo Nacional de Ciencia y Tecnologia (CONACYT) Mexico, the Escuela Nacional de Ciencias Biologicas, IPN, Mexico, and the U. S. Agency for International Development (211(d) grant A1D/csd 2834).

² Graduate Student and Professor of Soil Science, respectively.

Received for publication March 22, 1979. Accepted for publication May 23, 1979.

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