Nitrogen Cycle in Nature

In nature there exists a continuous cycle whereby nitrogen is first "fixed," and then subsequently liberated from combination with other elements. This natural cycle of operations may be conveniently represented by the diagram.

Nitrogen Cycle in Nature.

Thus it would appear that elemental nitrogen becomes fixed from the atmosphere in two ways. Firstly, direct combination of nitrogen and oxygen occurs by means of atmospheric electricity resulting in the production of oxides of nitrogen. These are dissolved and washed down by rain in the form of nitrous and nitric acids, which are converted into the corresponding salts by neutralisation with bases in the soil. Secondly, direct assimilation of free nitrogen takes place by leguminous plants (peas, beans, clover, etc.) with the help of bacteria. This method of nitrogen fixation is nature's compensation for the loss of combined nitrogen in the soil - the leak, as it were, being due to denitrifying bacteria. Elementary nitrogen is fixed by a large variety of azo-bacteria, which work in either a symbiotic or non-symbiotic manner, the former being the chief method. It has been known for many centuries that legumes have a very beneficial effect in restoring the fertility of the soil, but it was not until 1886 that proof was forthcoming of the presence of definite bacteria in the nodules on the roots of leguminous plants. The isolation of B. radicicula followed, and its symbiotic relation to the plant itself was established by recognising that nitrogen was given to the plant in return for carbohydrates and mineral salts from the plant to the bacterium. Non-symbiotic fixation means that bacteria work independently and not in association with other bacteria or higher plants. The bacteria are either of the anaerobic or aerobic type, and it has been generally considered that, although the anaerobic are more common and important, yet azotobacter and other aerobic nitrogen fixers have an important function. It has been recently suggested that the use of aerobic bacteria is overestimated, as the anaerobic type are capable of fixing twice the amount of nitrogen under similar conditions. Most plants (excepting legumes) require the nitrogen to be in some form of combination - most generally as nitrates. Broadly speaking, it is possible to trace two ways in which nitrogen is brought into a suitable condition for assimilation. In the first place, the death and decay of vegetable matter causes the complex nitrogenous substances (albumens, proteins) which have been produced during plant life to yield ultimately simple substances such as ammonia, nitrites, and nitrates. Secondly, the assimilation of vegetable products by animals results in their decomposition by metabolic processes, which produce urea, uric acid, and other faecal products. These are further broken down into simpler derivatives (ammonia, nitrates), which are now available again for plant assimilation, and the cycle starts afresh. As practically all the chemical changes are brought about directly by micro-organisms, a very brief account of these natural processes of ammonification and nitrification will be given.

Ammonification is the production of ammonia from proteins and their cleavage products by means of micro-organisms. Both fungi and bacteria cause extensive decomposition of albumens and proteins, first producing albumoses and peptones, then amino-acids, which are readily converted into ammonia. Amongst soil bacteria, one of the commonest is B. mycoides, which is very efficient in producing ammonia from organic nitrogenous materials. Energy relations are an important factor in the rapidity of decomposition of proteins. Thus both carbohydrates and proteins are utilised as sources of energy. The presence of much carbohydrate material will result in a relatively small amount of decomposition of proteins - just sufficient to provide the necessary nitrogen. In the absence of carbohydrates, proteins are quickly broken down in order to obtain a supply of carbon primarily, and the excess of nitrogen is converted into ammonia.

With reference to the products of animal metabolism, of which urea is typical, mention should be made of the enzyme urease which converts urea by hydrolysis into ammonium carbonate,

CO(NH₂)₂+2H₂O = (NH₄)₂CO₃.

Nitrification is the production of nitrates, either from ammonia or nitrites, by processes of oxidation. The experiments of Schloesing and Muntz in 1877 showed that ammonia was oxidised to nitrate by passing through a long tube filled with soil. No oxidation occurred if the soil was first sterilised, which showed that the chemical change was brought about by micro-organisms. Winogradski in 1890 identified and isolated specific bacteria which produced this nitrification, and showed that the oxidation proceeded in two stages, each of which was brought about by different species. Thus the first oxidation of ammonia to nitrite was the work of nitrous bacteria:

(1) 2NH₃+3O₂ = 2HNO₂+2H₂O;
while the second stage, resulting in the formation of nitrate, was caused by nitric bacteria (nitro-bacter):
(2) 2HNO₂+O₂ = 2HNO₃.

It would seem that nitrification depends upon a large number of factors, such as supply of oxygen, water, basic materials, mineral matter, etc. The importance of the amount and distribution of basic materials is due to the neutralisation of organic and mineral acids produced by bacteria, as, generally speaking, favourable conditions for both ammonifying and nitrifying organisms are only maintained if the soil is neutral or very slightly acid.

It has been mentioned that a certain loss of combined nitrogen in the soil occurs due to denitrifying organisms. Denitrification is the conversion of nitrates into nitrogen or oxides of nitrogen, and the bacteria bringing about these changes are chiefly of the aerobic type.

Hence insufficient aeration of the soil will force these organisms to obtain their necessary oxygen from nitrates, with the resultant loss of nitrogen. Various kinds of bacteria are able to bring about the changes represented by the following equations: -

2HNO₃ = 2HNO₂+O₂,
HNO₃ = NH₃+2O₂,
4HNO₂ = 2H₂O+2N₂+O₂.

The mutual reaction of nitrites and ammonia or amino-derivatives will also occur:

NH₄Cl+KNO₂ = N₂+KCl+2H₂O;

and van Iterson has suggested that certain bacteria are capable of causing the oxidation of carbon compounds:
5C+4KNO₃+2H₂O = 4KHCO₃+2N₂+CO₂,
3C+4KNO₃+ H₂O = 2KHCO₃+2N₂+K₂CO₃.

The prevention of denitrification is of paramount importance in market gardens and greenhouses, but appears to be of little account in field cultivation.

There are certain micro-organisms which are capable of reversing the ammonifying and nitrifying processes in order to utilise available nitrogen compounds for synthesising complex protein substances, which result in the withdrawal of nitrogen compounds for plant uses.