Understanding the Nitrogen Cycle (Explained Simply)

Why nitrogen matters, and why it's hard to get

Nitrogen is one of the elements every living organism needs, because it is a key building block of proteins and nucleic acids such as DNA. Without a supply of nitrogen in a usable form, an organism cannot grow or repair its cells.

This creates a puzzle: about 78% of the air around us is nitrogen gas (N₂), yet almost no plant or animal can use it in that form. The two nitrogen atoms in N₂ are joined by an extremely strong triple bond, which makes the gas very unreactive.

Plants can only take up nitrogen once it has been converted into a reactive, dissolved form, such as nitrate ions, that their roots can absorb from the soil.

This is also why a shortage of nitrate in the soil causes visible problems in plants, such as stunted growth and yellowing leaves, nitrogen is needed to make chlorophyll as well as proteins, so a plant that cannot get enough of it struggles to grow and photosynthesise normally.

How nitrogen becomes usable

1. Nitrogen fixation: certain bacteria, including nitrogen-fixing bacteria living in the root nodules of leguminous plants such as peas and beans, as well as free-living bacteria in the soil, convert atmospheric nitrogen gas into ammonium compounds that plants can eventually use.
2. Fixation by lightning: the energy released by a lightning strike can also combine nitrogen gas with oxygen in the air to form nitrogen oxides, which dissolve in rainwater and reach the soil as nitrates. This route adds far less nitrogen to the soil than nitrogen-fixing bacteria do.
3. Nitrification: nitrifying bacteria in the soil then convert these ammonium compounds first into nitrites, and then into nitrates.
4. Assimilation: plant roots absorb nitrate ions from the soil and use them to build proteins and nucleic acids; animals obtain their nitrogen by eating plants or by eating other animals that have eaten plants. Unlike plants, animals cannot build proteins directly from nitrate, so they depend entirely on the nitrogen compounds already assembled in their food.

Returning nitrogen to the soil and air

Nitrogen does not stay locked inside plants and animals forever. When organisms die, or produce waste such as urea and faeces, decomposers, mainly bacteria and fungi, break down these nitrogen-containing compounds through decomposition.

This releases ammonium compounds back into the soil, where nitrifying bacteria can convert them into nitrates again, ready for plants to absorb.

Some of the nitrate in the soil, particularly in waterlogged conditions with little oxygen, is converted by denitrifying bacteria back into nitrogen gas, which escapes into the atmosphere. This process, denitrification, is what returns nitrogen to the air and completes the cycle.

This constant recycling of nitrogen through decomposition and nitrification is what keeps soil fertile enough to support continued plant growth, generation after generation.

The cycle at a glance

Common mix-ups

How is the nitrogen cycle tested in exams?

Exam questions usually take one of three shapes. A diagram question shows the cycle with arrows and asks you to name the process at each arrow and the type of bacteria responsible.

A structured question gives a scenario, such as a waterlogged field, and asks you to explain how it changes nitrate levels. An essay may ask you to describe the whole cycle in sequence.

The marks most often lost come from naming a substance where a process is wanted, or the reverse. When an arrow points from nitrogen gas to ammonium compounds, the answer is 'nitrogen fixation', and the agent is 'nitrogen-fixing bacteria'.

Naming each process and its bacteria precisely is what earns full marks.

Why do farmers plant legumes and rotate crops?

Leguminous plants such as peas, beans and groundnuts carry nitrogen-fixing bacteria in their root nodules, so a legume crop adds usable nitrogen compounds to the soil as it grows. When the crop is harvested and its roots are ploughed back in, that nitrogen enriches the soil for the next crop.

This is why farmers often rotate a nitrogen-hungry crop with a legume, rather than growing the same crop year after year. It reduces the need for artificial nitrate fertiliser, which is costly and can be washed into rivers where it causes problems.

Understanding this link between the cycle and farming is a common application question.

What happens when the cycle is disturbed?

The cycle depends on a balance between the bacteria that add nitrate and those that remove it. Waterlogged soil has little oxygen, which favours denitrifying bacteria, so nitrate is lost as nitrogen gas and the soil becomes less fertile.

Draining and ploughing soil lets in air, which slows denitrification and keeps more nitrate available to plants.

Adding too much nitrate fertiliser causes a different problem. Rain washes surplus nitrate out of the soil and into rivers and lakes, where it makes algae grow rapidly.

When the algae die, decomposers use up the oxygen in the water, and other aquatic organisms may die from the shortage. This shows why the natural recycling of nitrogen matters for both soil and water.

Source:SRC-DSKP-EN

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