Nitrogen Cycle

The various reactions by which nitrogen is transformed to its oxides, trioxonitrate(V) acid, ammonium and trioxonitrates(V) occurs in nature where gaseous nitrogen is continuously circulated between the atmosphere and living systems in a process called the nitrogen cycle.

Nitrogen is an essential part of plants and animal proteins found in cell protoplasms. Though animals do not synthesize proteins, they could absorb needed proteins from other animals or plants. Plants on the other hand, cannot on their own trap free atmospheric nitrogen to synthesize proteins, hence absorbs trioxonitrate(V) or ammonium salts from the soil.

Apart from plants that absorb soil trioxonitrate(V), bacteria (dinitrifying bacteria) are also known to reduce the compounds to gaseous nitrogen in a process known as dinitrification.

The question is that, what is the exact chemical procedure for the conversion of atmospheric nitrogen to soil nitrogenous compounds?

There are three processes of replenishing soil trioxonitrates: oxidation of atmospheric nitrogen, nitrogen fixation by nitrogen-fixing bacteria and decay of organic matter.

Oxidation of atmospheric nitrogen

Electric discharge in the atmosphere, such as lightning, initiates oxidation reactions where nitrogen in air - that is about 78% nitrogen - is oxidized to oxides of nitrogen. Nitrogen(II) oxide, NO, is formed from the combination of nitrogen gas, N₂, with oxygen, O₂.

N₂(g) + O₂(g) ↔ 2NO(g)

Some of the nitrogen(II) oxide further oxidizes with more oxygen to form nitrogen(IV) oxide.

2NO(g) + O₂(g) ↔ 2NO₂(g)

Both nitrogen(II) oxide and nitrogen(IV) oxide then dissolve in rain water to form dioxonitrate(III) acid and trioxonitrate(V) acid respectively.

An example of how the acids are formed can be represented by th formation of trioxonitrate(V) acid, HNO₃.

4NO₂(g) + O₂(g) + 2H₂O(l) → 4HNO₃(aq)

Both the trioxonitrate(V) acid and dioxonitrate(III) acid are absorbed into the soil where they react with soil mineral salts to form trioxonitrate(V) compounds that can be absorbed by plants.

Nitrogen fixing bacteria (nitrobacter) are micro organisms such as the symbiotic (symbiot) bacteria - like Rhizobium - living in the root nodules of leguminous plants. They help to extract nitrogen from the atmosphere by converting it to organic nitrogenous compounds like simple proteins that are eventually used by the host plants.

Apart from the nitrogen-fixing bacteria, other soil free living micro-organisms such as Clostridium, Nostoc and Azobacter, also fix atmospheric nitrogen. Here, they convert the nitrogen to ammonium compounds by the process of ammonification, then to soil trioxonitrates(V).

The nitrogen absorbed by plants and animals are released into the atmosphere by the excretion of excretory products and the decay and decomposition of dead organic matter by decomposers. For example, the nitrogen absorbed through the fixation by nitrobacter can be releases back into the atmosphere when the leguminous plant is dead and decay.

Putrefying bacteria and soil fungi convert organic nitrogenous compounds to ammonium, NH₄⁺, in a process referred to as putrefaction. Then the ammonium undergoes nitrification where nitrifying bacteria oxidizes the ammonia first to dioxonitrate(III), NO₂^- ,and finally to soil trioxonitrate(V), NO₃^-.

Soil trioxonitrate(V) can either be absorbed by other plants to make plant protein or undergoes dinitrification by dinitrifying bacteria to atmospheric nitrogen.

The nitrogen cycle helps to balance the process by which nitrogen circulates in nature. But the natural equilibrium is destabilised by certain practices that includes soil erosion, pollution of the sea, leaching, e.t.c. Small wonder several methods are used in agriculture to enhance soil nitrogen level. This methods could be organic or synthetic. Organic methods include the use of compost manure where organic waste are like plant remains are decomposed and the adoption of sustainable agricultural practices like crop rotation where leguminous crops are rotated with other crops.

Artificial fertilizers like ammonium tetraoxosulphate(VI) and sodium trioxonitrate(V) are also used as sources of soil trioxonitrates for plants. The Haber process is significant here as it is used to convert atmospheric nitrogen to ammonia which is eventually used to produce the trioxonitrate(V) fertilizers.