The fluctuations of bacterial numbers and nitrate content of field soils

The changes in the numbers of bacteria and in biological activities in field soils at different seasons of the year have been studied by many workers. Hiltner and Stormer (1) found little difference between bacterial numbers in summer and winter. Remy (2) and Woitkiewicz (3) found highest bacterial numbers in spring, while Brown and Halversen (4) found two maxima, in February and in June. On the other hand Given and Willis (5) found highest numbers in September. Müntz and Gaudechon (6) and Lemmermann and Wichers (7) found nitrate production most active in spring, while Löhnis and Sabaschnikoff (8) found that urea decomposing, nitrifying and nitrogen fixing powers of soil were highest in spring and autumn, and Woitkiewicz (3) found that the nitrogen fixing and denitrifying powers of soil were highest in autumn. In spite of conflicting results, therefore, there is general evidence of increased bacterial numbers and activities in spring and in autumn. Russell and Appleyard (9), however, made bacterial counts and estimations of nitrate and CO2 from soil every fortnight, and found that large fluctuations in bacterial numbers and activity occurred at these intervals. A distinction between seasonal and more frequent changes in the micro-population was made by Cutler, Crump, and Sandon (10), who took samples from the dunged plot of Barnfield, Rothamsted, at daily intervals for a year and made counts of bacteria and of the encysted and active stages of two species of amoebae and five species of flagellate. They found that in spring and autumn an increase in both bacterial and protozoal numbers took place, but that, superimposed on these seasonal changes, striking fluctuations both in bacteria and protozoa took place at very short intervals: the numbers on consecutive days often differing by over 100 per cent. More recently, a series of daily counts from lawn soil were made at Washington, by Smith and Worden (11). Their data when re-examined by Thornton and Fisher (12) were shown to indicate similar significant diurnal fluctuations in the bacterial numbers RESP-68