Nitrogen Cycle (AQA A-Level Biology): Revision Notes

Nitrogen Cycle

Why nitrogen matters for life

Nitrogen is essential for all living organisms because it forms a key component of proteins, nucleic acids, and other vital nitrogen-containing compounds. Although nitrogen gas makes up 78% of the atmosphere, most organisms cannot use this form directly. Instead, plants absorb nitrogen in the form of nitrate ions ($NO\_3^-$) from soil through active transport via their roots. This is how nitrogen enters the living components of ecosystems, with animals obtaining their nitrogen by consuming plants.

In natural ecosystems, nitrate concentrations are maintained through the recycling of nitrogen-containing compounds. In agricultural systems, farmers add fertilisers to increase soil nitrate levels.

Overview of the nitrogen cycle

The nitrogen cycle involves four main stages that work together to recycle nitrogen between living organisms and the environment. Each stage is carried out by different types of saprobiotic microorganisms that break down organic matter and obtain nutrients from it.

The four stages are:

• Nitrogen fixation
• Ammonification
• Nitrification
• Denitrification

Nitrogen fixation

Nitrogen fixation converts atmospheric nitrogen gas ($N\_2$) into nitrogen-containing compounds that organisms can use. This process occurs both industrially and naturally, but biological nitrogen fixation by microorganisms is most important for ecosystems.

There are two main types of nitrogen-fixing bacteria:

1. Free-living nitrogen-fixing bacteria exist independently in soil. These bacteria convert gaseous nitrogen into ammonia, which they use to manufacture amino acids. When these bacteria die and decay, their nitrogen-rich compounds are released into the soil for other organisms to use.
2. Mutualistic nitrogen-fixing bacteria live in root nodules of plants such as peas and beans (legumes). This creates a beneficial relationship where the bacteria obtain carbohydrates from the plant, while the plant receives amino acids from the bacteria.

Ammonification

Ammonification produces ammonia from organic nitrogen-containing compounds found in dead organisms and waste products. Saprobiotic microorganisms, mainly fungi and bacteria, feed on materials such as:

• Urea (from breakdown of excess amino acids)
• Proteins, nucleic acids and vitamins in faeces
• Dead organism tissues

These decomposers break down the organic matter, releasing ammonia which forms ammonium ions ($NH\_4^+$) in soil. This process returns nitrogen from the living component back to the non-living environment, making it available for recycling through the rest of the cycle.

Nitrification

Nitrification converts ammonium ions into nitrate ions through a two-stage oxidation reaction that releases energy. This process is carried out by free-living soil microorganisms called nitrifying bacteria.

The conversion occurs in two stages:

1. Oxidation of ammonium ions to nitrite ions ($NO\_2^-$)
2. Oxidation of nitrite ions to nitrate ions ($NO\_3^-$)

Farmers maintain soil productivity by keeping soil structure light and well-aerated through ploughing. Good drainage prevents air spaces from becoming waterlogged, which would force oxygen out of the soil.

Denitrification

Denitrification occurs when soils become waterlogged and have low oxygen concentrations. Under these anaerobic conditions, the types of microorganisms present change. Fewer aerobic nitrifying and nitrogen-fixing bacteria survive, while anaerobic denitrifying bacteria increase in number.

These denitrifying bacteria convert soil nitrates into gaseous nitrogen, which returns to the atmosphere. This process reduces the availability of nitrogen-containing compounds for plants, making waterlogged land less productive for agriculture.

Agricultural applications

The nitrogen cycle has important implications for farming. Since plants are primary producers, nitrate availability affects entire ecosystems. Farmers can influence the cycle through:

• Adding fertilisers to increase soil nitrate levels
• Using crop rotation with legumes to naturally increase soil nitrogen
• Maintaining soil aeration to promote beneficial bacterial activity
• Ensuring good drainage to prevent denitrification

Remember!