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Growth of Rhizobium in Soil Amended with Organic Matter¹

ABSTRACT

Small populations of Rhizobium meliloti, R. phaseoli, R. japonicum, and a strain of Rhizobium nodulating cowpeas failed to grow in moist unamended Lima silt loam at 29°C. The numbers of the four rhizobia increased if 0.5 or 1.0 but not 0.01 or 0.1% mannitol was added. The population of R. phaseoli and R. meliloti but not the other two bacteria then fell markedly after 2 d. A second addition of mannitol on day 4 did not result in a rise in the R. meliloti population. The addition of 1% mannitol also resulted in an increase in the total number of bacteria and protozoa. Addition of corn leaf residues to the soil did not stimulate R. japonicum or R. lupini but caused a fall in the abundance of R. meliloti and R. trifolii. Additions of alfalfa residues led to a reduction in the numbers of R. meliloti, R. phaseoli, R. japonicum, and the cowpea Rhizobium and an increase in the abundance of protozoa, but bdellovibrios, bacteriophages, and myxobacteria either could not be detected or did not respond. If the soil was supplemented with streptomycin, erythromycin, actidione, and nystatin, to which these rhizobia were resistant, the four strains proliferated, but growth in such treated soils was much greater if 0.1% mannitol was also added. Stimulation of the four rhizobia was greater in soils treated with the two antibacterial compounds than in Lima silt loam amended with the two compounds effective against eukaryotes. It is suggested that competition with bacteria and possibly grazing by protozoa limit the growth of small populations of Rhizobium in soil.

¹ Contribution from the Dep. of Agronomy, Cornell Univ., Ithaca, NY 14853. This study was supported by Consejo Nacional de Ciencia y Tecnologia, Mexico, Escuela Nacional de Ciencias Biologicas, IPN, Mexico, and U.S. Agency for International Development grant AID/DSAN-G-0090. The views and interpretations in this publication are those of the Authors and should not be attributed to the Agency for International Development or to any individual acting in its behalf.

² Graduate Student and Professor of Soil Science, respectively. The Senior Author is currently at Centro de Investigacion y de Estudios Avanzados del Instituto Politecnico Nacional—Unidad Irapuato, Apdo. Postal 629, Irapuato, Gto. 36500, Mexico.

Received for publication July 14, 1982. Accepted for publication December 8, 1982.