Short-period fluctuations in the numbers of bacterial cells in soil

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Abstract
The numbers of microorganisms in soil samples were detd. by (1) the plate count method, using soil extract or mannite salt agar; (2) the ratio method to determine total numbers. Indigotin particles added in known concn. to a measured volume of soil, and the carbol-erythrosin stained bacteria, were counted in 80 microscope fields for each soil sample. The latter method, though open to the objection that the dye stains "resting" and dead as well as living cells, enabled significant fluctuations to be detected which were not established by the plate count method, owing to disagreement between parallel plate counts. Changes in total bacterial numbers occur from day to day in soil taken from the field, the same soil after 30 mins. at laboratory temp., soil incubated under constant temp. and moisture conditions, and in sterilized soil inoculated with mixtures of soil bacteria or with a single spore-forming organism, incubated under constant temp. and moisture conditions. A difference of 10[degree] (50 and 60[degree]C) in incubation temp. had no significant effect on total bacterial numbers. In 1 expt., inoculation of sterilized soil with 2 non-sporing bacteria, one of which depressed the growth of the other, resulted in a suppression of short period fluctuations in the total count. Protozoa, soil NH3 and nitrate, temp., moisture were detd. as well as bacteria in one 8 A. M. to 10 P. M. expt. Bacterial total count fluctuated inversely with the protozoa count, and roughly followed the nitrate concn. The other factors, and time of day, were not correlated. In a 48 hr. expt., 2-hr, fluctuations in total count occurred. Total numbers increased in the soil from spring to summer, and declined in the autumn. On one occasion only large increases in moisture content of fresh soil, produced by rain, were correlated with a subsequent rise in total count. It is concluded that the fluctuations are due to an unstable equilibrium between the diverse groups of microorganisms in the soil. Moisture and other factors will exert an effect not necessarily directly on bacterial numbers, but indirectly according to their effect on this equilibrium.

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