Nitrogen Transformations

The nitrogen cycle in soil, although well known in all its general aspects, still is incomplete in many details. It is predominantly, if not entirely, a biological cycle, but the populations or organisms responsible for the various steps differ greatly in degrees of specialization. The application of physiochemical methods to the study of the biochemistry of some of the chief transformations has been quite productive. The availability of the stable isotope of nitrogen, N15, is now permitting a greater refinement of such studies. The latter has so far been applied mainly to questions relating to nitrogen fixation by Rhizobium (bacteria which cause nodules to form on the roots of legumes and which fix nitrogen only when inside the nodules) and Azotobacter (bacteria which are free living in the soil and independent of plants). The technique will be of equal usefulness in attacking problems relating to immobilization or release of nitrogen in decomposition. Absolute values can now be obtained for recovery of nitrogen by a crop from some previous crop residues incorporated in the soil.

Study of the behavior of a large number of plant residues in soil in Australia has led to the conclusion that it is highly improbable that any material with a total nitrogen content of less than 1.5 percent will give a positive return of nitrogen in one season, and that only when the nitrogen exceeds 2.5 percent is a large early release secured coincident with the demand of a crop planted shortly after incorporation. In terms of the carbon-nitrogen ratios, these limits may be expressed approximately as 27-1 and 16-1, respectively.

Nitrate is the form of nitrogen ordinarily used by plants. The decomposition processes in soil result in the liberation of nitrogen as ammonia, and therefore the final step of conversion of this ammonia to nitrate, which is known as nitrification, is an important process. This change has been little studied in the last decade except in India where a number of purely chemical reactions said to be able to bring this change about have been under investigation. The activity of microbial nitrifying systems has, however, not been disproved, and there is no good reason for abandoning the long-established theories about the part played by this group of bacteria in soil. It must be admitted that the conditions under which the appearance of nitrate in soils occurs cannot be entirely reconciled with those under which the classical nitrifying bacteria carry out the oxidation of ammonia in pure culture in the laboratory.

Nitrogen has recently been supplied to irrigated crops by dissolving ammonia gas in the irrigation water. The subsequent oxidation of the ammonia to nitrate has been found to proceed only to the nitrite stage in certain alkaline desert soils in Arizona. The ammonia in solution is not toxic to the bacteria that should complete the oxidation even at a concentration as high as 300 parts per million; but if the soil is more alkaline than pH-7.7 the nitrite is not transformed to nitrate, and the crop cannot benefit properly from this unusual nitrogen fertilization.

The subject of nitrogen fixation has continued to attract many investigations with diverse interests and objectives. Some of the most refined techniques that have been devised for studying the physiology of bacteria have been applied to this problem, which is a challenging one because nitrogen-fixing organisms can accomplish readily what the chemist can accomplish only under extremely high pressures and temperatures. It is easier to work with Azotobacter than with the rhizobia, which fix nitrogen only when this bacteria is in the nodules of a living leguminous plant. The first step in the process is believed to be the combining of nitrogen from the atmosphere with some component of an enzyme system, called the azotase system, in the organism. Very fundamental investigations on the characteristics of this system have resulted in a unification of the subject of nitrogen fixation that did not appear probable a few years ago. They make it highly likely that the biochemical mechanism of fixation is identical in Azotobacter and in nodulated legumes.

Attempts have also been made to find out the nature of the chemical steps involved in the fixation process and a plausible theory, partly supported by experimental evidence, has been proposed by A. I. Virtanen of Finland. He identified some organic compounds which under certain circumstances pass out or are excreted from the roots of leguminous plants into the soil or sand in which they are growing. As a result he suggested that hydroxylamine was first formed and that this then combined with oxalacetic acid produced by the plant. This next is converted to aspartic acid, an amino acid that could be used in protein building. Others have maintained that ammonia is a key intermediate. Experiments by P. W. Wilson and others at Madison, Wis., in which Azotobacter was grown in the presence of various nitrogen compounds in a nitrogen atmosphere enriched with the heavy isotope, N ", showed that ammonia or compounds readily converted to ammonia are used by the bacteria to the exclusion of the nitrogen of the atmosphere.

Further studies of the distribution of species of Azotobacter in soils have indicated that this organism is found in all parts of the world but that its local distribution is erratic and limited primarily by the pH of the soil. Azotobacter rarely are found in soils, the pH of which is <6.0. The acid-tolerant species A. indicum, originally isolated from certain Indian soils, does not occur generally in acid soils. That an environment of pH 6.0 is apparently limiting in distribution is strong presumptive evidence for the view that Azotobacter in soil are dependent on the fixation process, because this same pH value has been proved to be the limit below which fixation does not occur. When supplied with combined nitrogen, these organisms can develop under considerably more acid conditions.