Effects Of Copper Concentration In Soil On Proliferation And Survival Of Rhizobia, Nodulation And Biological Nitrogen Fixation In Beans, Cowpeas And Soybeans

ABSTRACT

A study was conducted in which two strains of the fast-growing bean rhizobia- PV1 and PV2 and two others of the slow-growing bradyrhizobia for cowpeas and soybean- CP1 & GM8, respectively, were used to test their ability to: - (a) proliferate in copper contaminated liquid media, (b) survive in copper contaminated soil, (c) nodulate respective host legumes and (d) fix nitrogen under increasing copper levels both in vitro and in vivo. Known population sizes of each of the strains were exposed to copper concentrations in the range: 0, 20, 40 60, 80 and 100 ppm in either Yeast - extract Mannitol Broth or in modified Leonard Jar assemblies or potted soil. Most Probable Number (MPN) studies were also done to estimate populations of the native strains of the fast - and slow - growing rhizobia in a Cu -contaminated soil. Results indicated that copper was more toxic to the slow – growing bradyrhizobia than to the fast – growing rhizobia. Reduction in population sizes in vitro was more significant (p = 0.05) for bradyrhizobial than for rhizobial strains. Reductions in fresh nodule volume, fresh nodule mass and total shoot nitrogen were more significant (p = 0.05) in the bradyrhizobial – than in rhizobial – legume associations. Copper depressed the populations of slow – growing strain CP1 and GM8 in vivo more than those of the fast – growing strains PV1 and PV2. There was no statistically significant (p = 0.05) effect of 82.5 mg Cu/kg soil on numbers (MPN) of native rhizobial strains, probably due to adsorption of Cu by soil colloids. This study showed that based on the solution culture, potted soil and soil survival experiments, slow - growing rhizobia were more susceptible to Cu toxicity than were the fast - growing rhizobia.