Nitrogenase activity and population dynamics of isolated free-living aerobic bacterial diazotrophs enriched from soil were monitored upon salt addition after 3 to 24 d of preincubation. Three series of salt treatments were employed in this study: (1) KCl, NaCl, CaCl2, and MgCl2 varying from 0 to 30 meq/100 g soil or 100 ml medium, (2) three anionic forms, Cl-, CO32- and SO42- combined with K+, Na+, Ca2+, and Mg2+ upon single addition (10 meq/100 g soil or 100 ml medium), and (3) different combinations of salt mixtures (homocationic and heteroionic) at the same single addition. In all cases, the applied salts inhibited diazotroph numbers; however, survival was evident even at the highest salt levels. The intensity of inhibition was higher in the salinized culture medium than in soil. Nitrogenase activity, as determined by acetylene reduction, followed a similar pattern as the cell densities of diazotrophs in the salinized soil. The extent of inhibition in regard to cell population density and nitrogenase catalysis was as follows: (1) among the Cl- salts, CaCl2 > KCl > NaCl > MgCl2; (2) coion treatments, CO32- > Cl- > SO42- with the degree of inhibition varying with accompanying cations, Mg2+ being the least inhibitory; and (3) combination of salt mixtures (homocationic) Cl- plus CO32- > SO42- plus CO32- and (heteroionic) Na2CO3 plus CaSO4 being the most inhibitory, and MgCO3 plus K2SO4 being the least.
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