Nitrogen cycle

Where it happens

The nitrogen cycle happens across the atmosphere, soil, and living organisms, and is essential because nitrogen is needed to build proteins and nucleic acids, yet most organisms cannot use the nitrogen gas that makes up most of the atmosphere directly.

Each stage has its own location. Nitrogen fixation takes place inside the root nodules of legumes such as groundnut and soya bean, in free-living soil bacteria, and in the air during lightning.

Nitrification and ammonification happen in well-aerated topsoil where bacteria and fungi have oxygen. Assimilation happens in root hair cells and then inside plant and animal cells.

Denitrification is concentrated in waterlogged soil and mud with little oxygen, which is why flooded padi fields lose nitrate faster than dry fields.

Inputs and outputs

• Input: nitrogen gas from the atmosphere, fixed by bacteria or lightning.
• Input: dead organisms, faeces and urine that carry protein and urea back to the soil.
• Intermediate: ammonium ions produced by fixation and ammonification.
• Intermediate: nitrite ions, the short-lived product of the first nitrification step.
• Output to plants: nitrate ions absorbed by roots and built into amino acids, proteins, chlorophyll and nucleic acids.
• Output to the atmosphere: nitrogen gas released by denitrifying bacteria.

The steps

1. Nitrogen fixation by lightning: the energy of a lightning flash makes nitrogen and oxygen in the air combine into nitrogen oxides, which dissolve in rain and reach the soil as nitrate.
2. Nitrogen fixation by bacteria: nitrogen-fixing bacteria, such as Rhizobium in the root nodules of legumes and free-living Azotobacter in the soil, convert nitrogen gas into ammonium compounds.
3. Nitrification, first step: nitrifying bacteria such as Nitrosomonas convert ammonium ions into nitrite ions.
4. Nitrification, second step: nitrifying bacteria such as Nitrobacter convert nitrite ions into nitrate ions, the form plants absorb most readily.
5. Assimilation: root hair cells absorb nitrate and ammonium ions by active transport and the plant uses them to build amino acids, proteins, chlorophyll and nucleic acids; these compounds pass to animals when the plants are eaten.
6. Ammonification: decomposers, mainly bacteria and fungi, break down proteins in dead organisms, faeces and urea, releasing ammonia and ammonium ions back into the soil.
7. Denitrification: denitrifying bacteria, especially in waterlogged, oxygen-poor soil, convert nitrate ions back into nitrogen gas, which returns to the atmosphere and completes the cycle.

Why it matters and how it is controlled

Because nitrogen must pass through bacteria before plants can use it, the nitrogen cycle links the atmosphere, soil organisms, plants and animals together. Without nitrogen-fixing and nitrifying bacteria, the nitrogen locked in the atmosphere would remain unavailable to almost all living things.

The balance of the cycle is set by soil conditions. Oxygen favours nitrification and ammonification, so ploughing, draining and loosening soil raise the nitrate available to crops.

Waterlogging removes oxygen and favours denitrification, which strips nitrate from the soil. Farmers therefore rotate crops with legumes, add compost or manure to feed ammonification, and drain fields to hold nitrate in the root zone.

Excess nitrate fertiliser that is not absorbed leaches into rivers and lakes, feeding algal blooms and eutrophication, so the cycle also links to the pollution topics in the same chapter.

How it is examined

You may be asked to name the type of bacteria responsible for each stage of the nitrogen cycle, to explain why legumes are often grown to improve soil fertility, or to trace the path of a nitrogen atom from the atmosphere into a protein and back again.

Diagram questions give an unlabelled cycle with lettered arrows and ask you to name the process at each arrow and the organism responsible. Essay-style questions often ask you to explain how human activities, such as clearing legumes, over-using fertiliser or flooding fields, change the amount of nitrate in the soil.

Marks go to precise names: naming Rhizobium for fixation, Nitrosomonas and Nitrobacter for the two nitrification steps, and stating that denitrification removes nitrate. Writing 'bacteria' with no type earns less than naming the group.

Common misconceptions

Worked exam-style question

Question. A farmer grew maize on the same field for three years and noticed that yields fell each year. In the fourth year the farmer planted groundnut, a legume, and ploughed the plants into the soil after harvest.

The maize crop the following year was larger. (a) Name the bacteria found in the root nodules of groundnut and state the process they carry out.

(b) Explain, with reference to the nitrogen cycle, why ploughing the groundnut plants into the soil improved the maize yield. (c) The field later became waterlogged after heavy rain.

Explain how this would affect the nitrate content of the soil.

Model answer. (a) Rhizobium, which carries out nitrogen fixation, converting nitrogen gas from the air into ammonium compounds. (b) Maize absorbs nitrate ions from the soil each year, so repeated planting depletes soil nitrogen.

Groundnut roots contain fixed nitrogen in their nodules and proteins. When the plants are ploughed in, decomposers break down the proteins by ammonification, releasing ammonium ions.

Nitrifying bacteria then convert ammonium into nitrite and then nitrate, which the maize absorbs by active transport to build amino acids and proteins, so growth improves. (c) Waterlogged soil contains little oxygen.

Denitrifying bacteria thrive in these conditions and convert nitrate ions into nitrogen gas, which escapes to the atmosphere, so the nitrate content of the soil falls.

Source:SRC-DSKP-EN

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