In the Interaction of Rhizobium Whith Soil Biological factors (in Tropical Soils) as Modified by Soil Acidity and Water Potential

Barrion, Melinda M.
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Experiments were conducted to determine the influence of selected biological factors on Rhizobiurn survival and/or activity in tropical soils. The extent to which the interaction of Rhizobium with biological factors was modified by soil acidity and water potential was also evaluated. Attempts made to isolate bdellovibrios and bacteriophages from the Wahiawa and Waialua soils yielded negative results. Moreover, data from the study in sterile soil inoculated with filtrates from soils enriched for these parasites suggested that these parasites either do not exist in the soils tested or do not have influence on the survival of rhizobia. Toxin-producing actinomycete isolated from the Wahiawa soil caused the decline of rhizobia in vitro, but not in sterile soil. It is unlikely that this actinomycete will directly influence the survival of rhizobia inoculated into this soil. Survival studies done in culture medium amended with clay minerals showed that the inability of the actinomycete to significantly influence Rhizobium survival in sterile soil is not explainable by the presence of clay minerals. However, the tendency of the actinomycete to hydrolyze extracellular polysaccharides produced by the Rhizobiurn in culture medium may have important implications on the interaction of Rhizobiurn with its host in nature and on the ability of the microsymbiont to endure adverse environmental conditions. Studies conducted using the Wahiawa soil have demonstrated that indigenous rhizobia may not pose serious obstacles in the establishment of effective symbioses between the introduced rhizobia and host plants. To study the extent to which soil acidity may modify the interaction between rhizobia and other soil biotic factors, an attempt was made to compare the survival of Rhizobium in sterile and nonsterile soils maintained at different soil pH levels. However, this approach was precluded by the extreme toxicity to Rhizobium of sterile soils maintained at pH levels lower than 5.0. As an alternative to the above approach, studies designed to determine the extent to which pH may modify the interaction of Rhizobium with soil protozoa were undertaken. The decline of Rhizobium in nonsterile soil at pH 7.0 was barely detectable in both the Paaloa and the Wahiawa soils. However, the populations of Rhizobium declined drastically when introduced into the soils at pH 4.2. Inoculation of Rhizobium into the soils at pH 7.0 resulted in increases in the number of protozoa. The lack of appreciable decline in the population of Rhizobium at this pH in the face of predatory protozoa suggests that the rhizobia were multiplying at a rate fast enough to offset consumption by protozoa. The growth activity of soil protozoa in the Wahiawa soil at pH 4.2 was significantly lower than that at pH 7.0. However, there was a considerable protozoan activity in this soil even at pH 4.2 and thus the rapid decline of Rhizobium was caused by factors related to soil acidity as well as predation by protozoa. Protozoa did not seem to respond significantly to the decline of Rhizobium cells in the Paaloa soil maintained at pH 4.2. Since low pH was associated with increase in the concentration of extractable aluminum in the Paaloa soil while it was associated with the buildup of extractable manganese in the Wahiawa soil, the lack of protozoan growth in the Paaloa soil as opposed to the significant level of growth observed in the Wahiawa soil maintained at the same pH points to the greater sensitivity of the predators to Al toxicity than to Mn toxicity. Even though protozoa did not appear to grow at pH 4.2 in the Paaloa soil it is not feasible to conclude that they were not feeding upon rhizobial cells since a large population of viable cells was maintained in this soil. It is thus possible that predation by protozoa could have contributed to the drastic decline of Rhizobium cells observed in the Paaloa soil at pH 4.2. The above results suggest that the best way of insuring the survival of Rhizobium at high population is to insure that pH levels that are comfortable to rhizobia are maintained so that the bacteria multiply at a rate fast enough to compensate for predation by soil protozoa. The degree to which the interaction of Rhizobium with soil populations may be modified by soil water potential was determined by comparing the survival and growth of Rhizobium in sterile and nonsterile soils maintained at -0.10 bar or -15 bar water potentials. Rhizobium survived equally well at -15 bars and at -0.10 bar in sterile soil. In nonsterile soil, rhizobial populations showed greater decline in soil maintained at -0.10 bar than at -15 bars. The results suggest that the antagonistic influence of soil populations could be regulated by monitoring soil water potentials.
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