The Nitrogen Cycle

One of the most important nutrients in the soil is nitrogen, which is an essential element for all living things, and it plays a key role in the growth and development of plants and animals. Nitrogen makes up around 78% of the atmosphere. The nitrogen cycle is the process by which nitrogen is converted into different chemical forms and moves through the environment.

Nitrogen passes through both living and non-living things, including:

  • Plants
  • Soil
  • Water
  • Animals
  • Bacteria
  • The atmosphere

Nitrogen changes form as it moves through different parts of the cycle.

Processes Involved in the Nitrogen Cycle

Nitrogen fixation

Although there is plenty of nitrogen gas (N2) in the atmosphere, it is too unreactive. Therefore, it requires significant amounts of energy to break the bond between the two nitrogen atoms. This means that plants cannot absorb it directly from the atmosphere and use it to make proteins.

However, this nitrogen can still be absorbed from the air and converted into nitrate ions, which are easier to absorb. This can happen in two ways:

  • By nitrogen-fixing bacteria, which are present in the root nodules of leguminous plants (e.g., beans, clover and peas). They take nitrogen gas from the air and change it into nitrates in the soil.
  • Lightning can split the bond between the two atoms in N2 gas. This turns them into nitrous oxides (e.g. N2O and NO2) that eventually leach into the soil.

The Haber process

The Haber process converts nitrogen gas to ammonia, which is used in fertilisers. There are nitrifying bacteria in the soil that can convert ammonium ions into nitrates.

Fertilisers provide an artificial way to ensure that plants get the nitrates required for growth. It does not rely on external processes such as nitrogen-fixing bacteria or lightning.

Absorption of nitrates into plants

Plants absorb the nitrates from the soil and convert them into proteins and other essential compounds.

Movement of nitrogen along the food chain

When animals eat plants or other animals, they take in nitrogen from the consumed proteins. These proteins are then used to build their tissues.

Waste and death

Decomposers break down animal waste (urine and faeces), and dead remains. This returns nitrogen to the soil as ammonium ions.

Nitrifying bacteria can convert the ammonium ions into nitrates, which plants can absorb.

Release of nitrogen back into the atmosphere

Anaerobic bacteria, known as denitrifying bacteria, are found in low-oxygen regions of soil. The bacteria break down nitrates and convert them back to nitrogen gas, which returns to the air.

Farmers can reduce the number of denitrifying bacteria by turning over or ploughing the soil.

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