Any examiner will tell you that the nitrogen cycle creates more difficulties for examination candidates than almost any other single topic in the A-level Biology syllabus. Despite the fact that you can almost guarantee that there will be a question on the nitrogen cycle, it always comes as a surprise that so few students appear to understand it. So, there is a message here. Make sure you really know your nitrogen cycle.

Let’s start by looking at the basic cycle. We can use exactly the same approach as we did with the carbon cycle. Start from the general nutrient cycle in Figure 3.2. Now modify it. At this stage we will not consider the processes of denitrification and nitrogen fixation. We will look simply at the way in which nitrogen in the organic molecules of say, a dead rat, is made available to producers in the form of nitrate. This is shown in Figure 3.4 (opposite).

There are a number of nitrogen-containing substances in a plant, including amino acids, proteins and nucleic acids. This nitrogen is passed from producer to primary consumer to secondary consumer in a series of stages which involve digestion and assimilation. When organisms die, the nitrogen-containing organic matter is digested by saprobiotic bacteria and ammonia is released.

Another group of bacteria, the nitrifying bacteria, then convert ammonia to nitrites and nitrites to nitrates. Nitrates are taken up from the soil by producers, and the cycle is complete.

Figure 3.4 The basic nitrogen cycle

The particular ways in which this part of the nitrogen cycle differs from the general pattern are:

• Nitrogen from consumers is made available to saprobiotic bacteria when they die and through excretory products such as urea. 

• Two steps are involved in converting organic nitrogen-containing molecules into the inorganic ions which can be taken up by the producers:
  (1) Conversion of organic nitrogen-containing molecules such as proteins to ammonia by saprobiotic bacteria.
  (2) Conversion of ammonia to nitrites and nitrites to nitrates. This is done by nitrifying bacteria.

This is the basic cycle; but there is a complicating factor. Under anaerobic conditions such as those that exist when the soil is water-logged, some bacteria are able to get the oxygen they need from nitrates. These are denitrifying bacteria. Effectively, what they do is to convert nitrates to nitrogen which escapes into the atmosphere. Unfortunately, nitrogen itself cannot be used by animals and plants, so denitrification removes nitrogen from the cycle.

However, atmospheric nitrogen can be made available to plants by the process of nitrogen fixation. Nitrogen fixation is the incorporation of nitrogen gas into organic nitrogen-containing compounds. There are several ways in which nitrogen can be fixed, and all of them require a considerable amount of energy:

• Lightning. Electrical energy in lightning combines nitrogen and oxygen in the air to produce various oxides of nitrogen. Rain washes these into the soil where they are taken up by plants as nitrates.

• Ammonium fertilisers. These are made by an industrial process which combines nitrogen directly with hydrogen.

• Nitrogen-fixing microorganisms living in the soil. Some soil bacteria produce the enzyme nitrogenase. These bacteria are able to use this enzyme together with energy from ATP to convert nitrogen to ammonium compounds and then to organic nitrogen-containing substances.

• Nitrogen-fixing microorganisms living in the root nodules of leguminous plants. This is similar to the previous process. The bacteria convert nitrogen into ammonium compounds, some of which pass to the plant. The bacteria gain carbohydrates from the plant.

Let us return to our diagram of the nitrogen cycle. It is better to add this information about denitrification and nitrogen fixation on as a sort of ‘bolt-on’ extra rather than end up with a very complicated diagram which you will have difficulty remembering. Figure 3.5 is a simple diagram of the full nitrogen cycle and incorporates this extra material.

Figure 3.5 The full nitrogen cycle