Nitrogen cycle (Edexcel GCSE Biology): Revision Notes

Nitrogen cycle

Why is nitrogen important?

Nitrogen is absolutely essential for all living things because it forms a key part of amino acids, which are the building blocks of proteins. Although nitrogen makes up roughly 78% of our atmosphere, plants face a major problem - they cannot directly absorb nitrogen gas ($N_2$) from the air.

This is where the nitrogen cycle becomes crucial. It's nature's way of recycling nitrogen and transforming it into forms that living organisms can use. Without this cycle, plants couldn't grow properly, and entire food webs would collapse.

The nitrogen cycle overview

The nitrogen cycle involves several interconnected processes that move nitrogen between the atmosphere, soil, plants, and animals. Think of it as a continuous recycling system where nitrogen changes from one chemical form to another as it moves through different parts of the ecosystem.

Key processes in the nitrogen cycle

Nitrogen fixation

This process converts nitrogen gas ($N_2$) from the atmosphere into compounds that plants can actually use. There are three main ways this happens:

Biological fixation occurs when special bacteria called nitrogen-fixing bacteria convert atmospheric nitrogen into ammonia ($NH_4^+$). These bacteria live either freely in the soil or in special bumps called root nodules on leguminous plants like peas, beans, and clover. The bacteria and plants have a mutually beneficial relationship - the bacteria get nutrients from the plant, while the plant receives nitrogen compounds in return.

Lightning fixation happens when the enormous energy in lightning bolts provides enough power to make nitrogen react with oxygen in the air, forming nitrates that fall to earth in rain.

Industrial fixation through the Haber process allows humans to manufacture ammonia from nitrogen and hydrogen gases. This ammonia is then used to create fertilisers that farmers spread on their crops.

Nitrification

This two-step process is carried out by nitrifying bacteria in the soil. First, they convert ammonia ($NH_4^+$) into nitrites ($NO_2^-$), and then other nitrifying bacteria convert these nitrites into nitrates ($NO_3^-$).

Assimilation

Plants absorb nitrates from the soil through their root systems and use them to build proteins and other nitrogen-containing molecules. When animals eat plants, they break down the plant proteins and rebuild them into their own body proteins. This transfers nitrogen up through the food chain.

Decomposition (ammonification)

When plants and animals die, or when animals produce waste, decomposer organisms like bacteria and fungi break down the nitrogen-containing proteins. This process releases ammonia back into the soil, where it can be converted into nitrates by nitrifying bacteria, continuing the cycle.

Denitrification

In waterlogged soils where oxygen is scarce, denitrifying bacteria convert nitrates back into nitrogen gas, which escapes into the atmosphere. While this completes the nitrogen cycle, it can reduce soil fertility because it removes nitrogen that plants could otherwise use.

Types of bacteria in the nitrogen cycle

Different types of bacteria play specific and vital roles in keeping the nitrogen cycle functioning properly.

How farmers increase nitrogen availability

Farmers use several strategies to ensure their crops have access to sufficient nitrogen for healthy growth.

Crop rotation

Farmers often use crop rotation to naturally boost soil nitrogen levels. They grow leguminous crops (like peas or beans) one year, which have nitrogen-fixing bacteria in their root nodules that enrich the soil. The following year, they plant other crops (like wheat or corn) that benefit from this increased nitrogen content. This reduces the need for expensive artificial fertilisers.

Fertilisers

Farmers can add nitrogen to their fields in two main ways:

Natural fertilizers like manure or compost provide organic matter that decomposes to release nitrogen into the soil. These are environmentally friendly but work more slowly.

Artificial fertilizers such as ammonium nitrate provide nitrogen directly to plants and work quickly to boost crop yields. However, they're expensive and can cause environmental problems like eutrophication if overused.