The fluctuations of bacterial numbers and nitrate content of field soils

Abstract

Samples from a field soil were taken at 2-hour intervals and from them bacterial numbers were counted by plating and nitrate was estimated. Fluctuations in bacterial numbers greatly exceeding the variation in bacterial content of simultaneous samples were found to occur by day and night. A special plot was prepared by screening the soil to remove stones, and 2-hour samples were taken from it, counts and nitrate determinations being made from 2 halves of the plot on each occasion of sampling. Fluctuations in bacterial numbers greatly exceeding the variation between simultaneous samples were found during the course of the day. A comparison of the variance between samples taken at different times of day and that between simultaneous samples showed that results could not be due to uneven distribution of bacteria over the plot. There is some evidence suggesting that different groups of bacteria show different fluctuations and that a portion of the population does not fluctuate. No correlation between changes in bacterial numbers and soil moisture content could be found. In one series of daily counts, the fluctuations were correlated with rainfall. Soil temp. changes rarely showed any relation to the fluctuations. Baterial numbers were usually high at 10 a.m. and low in the middle of the day. There is evidence suggesting that the nitrate content also fluctuates during the day.