The nitrogen cycle

In a nutshell

Nitrogen is an important element required by plants and animals. They use the nitrogen to produce proteins and nucleic acids (DNA and RNA). Nitrogen makes up around $78\%$ of the Earth's atmosphere but it is too unreactive for plants and animals to use it directly from the air. It must first be converted into soluble ions. The nitrogen compounds will be passed along the food chain until it is returned back to the atmosphere during decay.

The nitrogen cycle

There is a limited amount of nitrogen in the air so it must be recycled through the following cycle.

Biology; Ecosystems and material cycles; KS4 Year 10; The nitrogen cycle

There are four main types of bacteria in the nitrogen cycle that carry out four main processes.

	Bacteria	Description
1.	Nitrogen-fixing bacteria	Nitrogen gas ( $N_2$ ) makes up around $78\%$ of the Earth's atmosphere but it is too unreactive for plants and animals to use it directly from the air. It must first be converted into soluble ions. Lightning bolts contain enough energy to make nitrogen react with oxygen in the air to produce nitrate ions.
2.	Nitrogen-fixing bacteria	Nitrogen fixation can also occur in the soil by nitrogen-fixating bacteria. These are found free in the soil and in the root nodules of leguminous plants. These bacteria can convert nitrogen gas into nitrates.
3.	Decomposers	Decomposers convert nitrogen-containing compounds like proteins and urea into ammonia ( $NH_3$ ). Ammonia forms ammonium ions ( $NH_4\ ^+$ ) in the soil that plants can use.
4.	Nitrifying bacteria	Nitrifying bacteria turn ammonium ions in the soil into nitrites ( $NO_2\ ^-$ ). Then, different nitrifying bacteria will turn the nitrites into nitrates ( $NO_3\ ^-$ ).
5.	Denitrifying bacteria	Denitrifying bacteria turns nitrates in the soil back into $N_2$ gas during respiration. This happens in anaerobic conditions, like in waterlogged soils, where there is very little oxygen available.

Increasing nitrate concentration

An important part of the nitrogen cycle is the nitrates are returned to the soil as the plants decompose. However, crops are usually harvested which halts this cycle and means the nitrogen they contain isn't returned to the soil.

This causes the nitrate concentration of the soil to decrease which limits the growth of the other plants. Farmers can take certain measures to increase the concentration of nitrates in their soil, whilst still harvesting their crops.

Method

Description

Crop rotation

Farmers can change the type of crop they grow each year. They may choose to grow a leguminous plant, like peas, one year as these will increase the nitrate concentration in the soil for the following year.

Fertilisers

Farmers may put compost or manure on their fields as there are nitrates in the plant and animal waste that can be decomposed. This will increase the concentration of nitrates in the soil.

Farmers can also buy artificial fertilisers that contain other mineral ions as well as nitrates, but these are expensive.