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

Abstract

Seasonal changes in the numbers of micro-organisms in fresh soil were first reported at the beginning of the present century; their existence has since been confirmed by workers in many different parts of the world. (For references, see below .) More recently, short-period fluctuations in bacterial numbers were found to exist. Such fluctuations were found in plate counts from daily samples of field soil by Cutler, Crump, and Sandon (1922) and from 2-hourly samples by Thornton and Gray (1930). Periodic determinations of bacterial numbers in soils other than those taken from natural conditions have been few in number and have usually been made as checks on work of some other nature. In earlier work of Russell and Hutchinson (1909), soil incubated at room temperature showed fluctuations in microbial content over such a short interval as 8 hours and over as long a period as 60 days. In their later work, Russell and Hutchinson (1913) working with three soils of different moisture contents, dry, moist, and saturated, incubated at constant temperature, found changes in numbers between samples taken at from 5- to 8-day intervals. Such changes in numbers were not related to temperature, nor necessarily to moisture changes. A more intensive study was made by Allison (1917). He brought soil samples into the laboratory and made bacterial and fungal counts at short intervals of time. Samples taken during the winter showed a drop in numbers of as much as 40% during the first 1½ hours’ storage, followed by a large rise after some hours; in summer the initial decrease was less pronounced, this being attributed to the fact that at that season outdoor temperatures more nearly approached indoor temperatures. Cutler and Dixon (1927) found that, with soil stored at laboratory temperatures in deep narrow bottles, bacterial numbers decreased steadily over a period of 5 weeks. In soil stored in pots with a large surface area, fluctuations in bacterial numbers of as much as 100% were obtained from week to week, and the soil in general behaved as fresh soil.