Arthur Isaac Kendall
|9 Landwirthsch. Versuchsstat, xlv.
10 Bot. Zeitung, 1888, p. 725.
11 These nodules were first described by Hellriegel (Tageblatt Naturforsch. Vers., Berl., 1886, p. 290) and Willforth (Ibid., 1887, p. 362).
1 Ann Inst. Pasteur, 1897, xi, 44; 1898, xii, 1, 128; 1899, xiii, 145.
2 Compt. rend. Soc. de biol., 1893, cxvi, 1385; 1894, cxviii, 353.
3 Centralbl. f. Bakteriol., II Abt,, 1901, vii, 562.
It is a matter of common observation that soil left uncultivated gains in fertility from year to year and in 1875 Berthelot, and Nobbe and Hiltner9 made the important discovery that nitrogen from the air is fixed in the soil. It was found that soil heated to 100° C. lost its power of fixation of nitrogen, suggesting that microorganisms played a part in the process. In 1888 Beijerinck10 made the very important discovery that nodules11 upon the roots of leguminous plants contain pleomorphic organisms, Bacillus radicicola, which were able to fix atmospheric nitrogen. Mazé1 and others have confirmed this observation. Somewhat later Winogradsky2 isolated an anaerobic spore-forming bacillus, Clostridium pasteurianum, not depending upon plants for its sustenance, but free living, which accomplished the same transformation, and in 1901 Beijerinck3 isolated and described the very important group of Azobacteria, which are widely distributed in the soil and are able to fix atmospheric nitrogen. These organisms are most active when associated with other soil bacteria, but are fully able to fix nitrogen when grown in pure culture in artificial media free from nitrogenous compounds.
The oxidation of ammonia salts to nitrites and then to nitrates is effected through the activities of nitrifying bacteria, first isolated and described by Warrington and Winogradsky. Two organisms are concerned, a coccus, Nitrosococcus, which transforms ammonium salts to nitrites, and a small bacillus, Nitrobacter, which oxidizes nitrites to nitrates. These organisms do not thrive in the presence of complex organic matter and appear to derive their nutritive requirements chiefly from inorganic salts. The nitrates are taken up by chlorophyll-bearing plants and, with the energy of sunlight transform them, together with carbon dioxide, water, phosphates and various salts, into the complex vegetable proteins upon which the animal kingdom primarily subsists.
It is obvious, therefore, that there is a well-defined nitrogen cycle—an intricate series of changes which proteins and their derivatives undergo, through which complex, lifeless nitrogenous compounds are reduced through bacterial activity to simple, stable mineralized inorganic combinations of their elements. These elements are restored, chiefly through the synthetic activity of plant life, to the animal kingdom. The nitrogen cycle is, in a sense, a measure of the metabolism of the living earth, in which the anabolic or synthetic processes occur in plants and indirectly in animals; the catabolic or analytic process is brought about chiefly by bacteria.