Protozoa and the Nitrogen Cycle.
In partially-sterilised soil from which protozoa were absent Russell and Hutchinson obtained an increased ammonia production, a result also obtained by Cunningham. Hill, on the other hand, concluded that protozoa have no effect on ammonification, but his technique is open to criticism.
Lipman, Blair, Owen and McLean’s work[17] contains many figures obtained by adding dried blood, tankage, soluble blood flour, cottonseed meal, soy-bean meal, wheat flour, corn meal, etc., to soil. It is difficult to understand how accurate results could be expected when, to an already little understood complex substance, such as soil, is added a series of substances whose effects are practically unknown.
Free nitrogen-fixation in soils is an important process, more especially in soils of a light sandy nature, from which crops are taken year after year without any application of manure. The effect of protozoa on the organisms causing this process has in the past received little attention. Recently, however, Nasir[20] has studied the influence of protozoa on Azotobacter, both in artificial culture and in sand. From a total of 36 experiments done in duplicate or triplicate, 31 showed a decided gain in nitrogen fixation over the control, while only 5 gave negative results.
Fig. 18.—Showing the highest fixations of nitrogen above the control recorded for Azotobacter in the presence of different species of Protozoa. (From Ann. Appl. Biol., vol. ii.)
X-axis (left): Artificial Media C A F AF AC ACF
X-axis (right): Sand Cultures C A AF AC
Legend: C represents CILIATES.
A -do.- AMOEBAE.
F -do.- FLAGELLATES.
As might be expected, the fixation figures varied from culture to culture, the highest recorded being 36·04 per cent. above the control and this in a sand culture ([Fig. 18]). Reference to the details of the experiments shows that the criticisms made against similar work done in the past do not hold here, and one must conclude that Azotobacter is capable of fixing more atmospheric nitrogen in the presence of protozoa than in their absence.
At present it is impossible to say how this occurs, but it is highly improbable that the protozoa are themselves capable of fixing nitrogen. A more likely explanation is that the protozoa, by consuming the Azotobacter, kept down the numbers, and transfer the nitrogen to their own bodies. This will tend to prevent the bacteria from reaching a maximum density, and reproduction, involving high metabolism, will be maintained for a longer period than would have otherwise occurred. This and other possible explanations, are being tested.
Little has been said regarding the application of protozoology to the question of soil partial sterilisation. As already pointed out, in the past much work has been done, but the results were conflicting. In view, however, of our recently acquired knowledge of the life of protozoa in ordinary field soil, most of the early experiments require repeating. A beginning has already been made, but the work is not sufficiently advanced to warrant discussion.
What is urgently needed, however, is to increase our knowledge of the general physiology of these unicellular animals. Until we know what are the inter-relationships between the members of the micro-organic population of normal soil it is almost impossible to hope that means will be devised by which they can be controlled.
At present we are almost entirely ignorant of the simplest of physiological reactions, such as the exact effect of various inorganic salts found in the soil.
Also some experiments in Germany and the States indicate that amœbæ are selective as regards the bacteria they ingest. If this is substantiated it may prove of importance to economic biology.
It has been shown that the flagellates occur in the soil in large numbers, and many of them feed on bacteria. It is probable, however, that certain of them feed saprophytically and must therefore exert some influence on the soil solution, though what this may be is entirely unknown.
Finally, as Nasir has shown, the protozoa play a part in the complicated nitrogen cycle, and work of this type needs extending.
Such, then, are a few of the outstanding problems that confront the soil protozoologist; but he must always remember that the organisms he studies are but a small fraction of the total, and that any influence affecting one part of the complex will be reflected in another. As Prof. Arthur Thomson said in his Gifford Lectures, “No creature lives or dies to itself, there is no insulation. Long nutritive chains often bind a series of organisms together in the very fundamental relation that one kind eats the others.” Such nutritive chains obtain in the soil as markedly as in other haunts of living creatures.