Seasonal Changes.
Superimposed on the daily variations in numbers there are seasonal changes, as is clearly shown when fourteen day averages are made of the numbers for each species. Bacteria have long been known to show autumn and spring rises, but recent research has demonstrated that the protozoan population also rises to a maximum at the end of November, with a less marked spring rise at the end of March and beginning of April ([Figs. 14] and [15]).
It has sometimes been claimed that the numbers of soil organisms are closely linked with the soil moisture, but no support for this view was found during the course of the experiment. Similarly, as in the case of the daily variations, no connection could be traced between the seasonal changes and any of the external conditions considered.
It is interesting to note, however, that the seasonal variations in the numbers of soil organisms is very similar to those recorded for many aquatic organisms. Miss Delf,[8] for instance, found that in ponds at Hampstead the algæ are most numerous in spring and again in the autumn, and like changes are recorded in British lakes by West and West[25] and in the Illinois river by Kofoid.[14]
Fig. 14.—Fortnightly averages of total numbers of Oicomonas, Species γ, and Species α, and of bacteria, moisture, and temperature. (From Phil. Trans. Roy. Soc., vol. ccxi.)
X-axis: Fortnight beginning 1920. July. Aug. Sept. Oct. Nov. Dec. Jan 1921. Feby. Mch. April. May. June.
Y-axis (bottom left): Percentage of moisture
Y-axis (top left): Logarithms of numbers of active protozoa per gramme of soil
Y-axis (bottom right): Temperature F
Y-axis (top right): Bacteria in millions per gramme
Legend: Oicomonas
Species γ
Species α
Bacteria
Temperature
Moisture
It is difficult to resist the conclusion that these annual variations are produced by similar causes, from which it follows that the increase in the numbers of protozoa in the soil is not wholly conditioned by an increased food supply—the bacteria—for the algæ are not dependent on such a form of nourishment. This is substantiated by the fact that the numbers of protozoa, except those of Oicomonas, rose during March, whereas the corresponding increase in the bacteria was delayed till the early part of April.
Fig. 15.—Fortnightly averages of total numbers of Heteromita, Cercomonas, and Dimastigamœba and of bacteria, moisture, and temperature. (From Phil. Trans. Roy. Soc., vol. ccxi.)
X-axis: Fortnight beginning July 1920. Aug. Sept. Oct. Nov. Dec. Jan. 1921. Feb. Mar. April May June
Y-axis (bottom left): Percentage of moisture.
Y-axis (top left): Logarithms of numbers of active protozoa per gramme of soil.
Y-axis (bottom right): Temperature F
Y-axis (top right): Bacteria in millions per gramme
Legend: Heteromita
Cercomonas
Dimastigamoeba
Bacteria
Temperature
Moisture
Owing to the variations in the numbers of both protozoa and bacteria, little reliance can be placed on figures obtained from an isolated count, since on one day the total numbers of flagellates may be nearly 2,000,000 per gram and drop by more than half this figure in 24 days. It is certain, however, that the numbers recorded in the past are much too low, since the total flagellate and amœbæ species were lumped together in two groups. Some idea of the size of the soil population can be obtained, nevertheless, by using the fourteen-day averages mentioned above. In [Table VIII.] are tabulated the average total numbers of flagellates, and amœbæ for the two periods of the year when the population was at its maximum and minimum respectively. An endeavour has also been made to strike a rough balance sheet as to the amount of protoplasm represented by protozoa and bacteria in a ton of soil. For this purpose it has been assumed that the organisms have a specific gravity of 1·0 and are spheres of diameters, 6μ for the flagellates, 10μ for the amœbæ, and 1μ for the bacteria; and that they are uniformly distributed through the top nine inches of soil. The top nine inches of soil is taken as weighing 1000 tons.
TABLE VIII.
| Maximum Period. | Minimum Period. | |||||
|---|---|---|---|---|---|---|
| No. per Gram. | Weight in Gram per Gram. | Weight in Tons per Acre. | No. per Gram. | Weight in Gram per Gram. | Weight in Tons per Acre. | |
| Flagellates | 770,000 | 0·000087 | 0·087 | 350,000 | 0·000039 | 0·039 |
| Amœbæ | 280,000 | 0·000147 | 0·147 | 150,000 | 0·000078 | 0·078 |
| Bacteria | 40,000,000 | 0·000020 | 0·02 | 22,500,000 | 0·000012 | 0·012 |
It must be remembered that the above figures are minimum ones, as many species of bacteria and protozoa, known to occur in the soil, are not included in the statement owing to their not appearing on the media used for counting purposes.
Fig. 16.—Daily variations in the numbers of active individuals of a species of flagellate, Oicomonas termo (Ehrenb.) during March, 1921. (From Phil. Trans. Roy. Soc., vol. ccxi.)
X-axis: March
Y-axis: Active numbers per gramme of soil
Before leaving the discussion of daily variations in numbers of protozoa, reference must be made to the flagellate species. As already mentioned, their active numbers fluctuate rapidly, and for the most part entirely irregularly. One species, however, Oicomonas termo, is characterised by possessing a periodic change; high active numbers on one day being succeeded by low, which are again followed by high on the third day. This rhythm was maintained, with few exceptions, for 365 days ([Fig. 16]), and has been shown to take place in artificial culture kept under controlled laboratory conditions ([Fig. 17]).
Fig. 17.—Daily variations in the numbers of active individuals of Oicomonas termo (Ehrenb.) in artificial culture media kept at a constant temperature of 20° C. A, in hay infusion; B, in egg albumen.
X-axis: Days
Y-axis: Thousands
It was thought that an explanation of this phenomenon might be found in alternate excystation and encystation, since the latter is a constituent part of the animals’ life history (see [p. 73]). This, however, does not hold, for the cyst curve is not the inverse of that of the active; and, moreover, statistical treatment demonstrated that cyst formation is wholly unperiodic in character.
An explanation must therefore be sought in the changes in the organisms during the active period of their life, and the deduction can be drawn that, increased active numbers tend to be followed by death, conjugation, or both, while decreases in the active numbers are followed by rises in total numbers, i.e., reproduction, and this rhythmically.
This somewhat surprising conclusion appears to hold, in a lesser degree, for other soil protozoa, and is of sufficient importance to warrant further research. The direction in which this is being pursued is by a study of the reproductive rates of pure cultures of certain ciliates and flagellates under varying external conditions. Space does not admit of adequate discussion of this problem, but the results already obtained justify the view that such lines of work will elucidate some of the baffling problems of soil micro-biology.