Double Award - The nitrogen cycle, minerals and eutrophication (CCEA)

Nitrogen is essential to life on Earth.

The nitrogen cycle continuously recycles nitrogen, changing it into different forms that plants and animals can use.

There are four key processes in the cycle: nitrogen fixation, nitrification, denitrification and decomposition.

In all these processes, microorganisms, like bacteria and fungi, play a vital role.

First comes nitrogen fixation.

Nitrogen gas makes up around 78% of the Earth’s atmosphere, but plants and animals can’t use it in this form.

So, nitrogen-fixing bacteria living in the soil, or in legume plants, convert the nitrogen from the atmosphere into ammonia or ammonium, which some plants can use for growth.

The second process is nitrification.

Here, nitrifying bacteria add oxygen to the ammonium, converting it into nitrites and then into nitrates – a form of nitrogen that plants can absorb more efficiently, helping them to grow.

Nitrifying bacteria are aerobic, which means they need oxygen to function.

The third process is denitrification.

Denitrifying bacteria take nitrates out of the soil, converting them back into nitrogen gas, which is released into the atmosphere.

This reduces the nitrogen in the soil, making it harder for plants to grow.

Denitrification takes place in low-oxygen environments, like waterlogged soil. These are known as anaerobic environments.

To increase levels of nitrifying bacteria in the soil that support plant growth, farmers drain and plough it, increasing the oxygen levels.

The final process is decomposition.

When plants and animals die, decomposers like bacteria break down their remains, converting proteins into ammonia or ammonium, and raising the nitrogen levels in the soil.

Understanding how the nitrogen cycle works is important to help us look after our ecosystems.

To grow and be healthy, plants need to absorb minerals.

They do this through specialised cells called root hair cells. This have an extended shape, which gives them a large surface area for more efficient absorption.

The minerals they absorb include nitrates, to make proteins for growth, calcium to create strong cell walls, and magnesium, a component of chlorophyll, which supports the absorption of light during photosynthesis.

Mineral ions are absorbed by the root hair cells through a process called active transport. This is how it works: respiration provides the energy to transport minerals against a concentration gradient, so they travel from a region of low concentration in the soil to a region of higher concentration in the root hair cell.

To help plants grow, farmers can increase the mineral levels of soil by adding slurry, manure, or artificial fertilisers. However, when fertilisers get leached or washed out of the soil into rivers or lakes, they cause eutrophication.

This means that the excess nitrates from the fertiliser initially causes rapid growth of aquatic plants and algae. This is known as an algal bloom. But then the plants die, due to the nitrate depletion and shading.

Next, aerobic microorganisms break down the dead plants, which consumes a lot of oxygen. This depletion of oxygen affects aquatic life, leading to the death of fish and other organisms that need oxygen to survive.

It's important we understand how mineral absorption in plants works, and the consequences of eutrophication, so we can maintain healthy ecosystems.