Decomposition and the Nitrogen Cycle Revision Notes | A Level AQA Biology

1. Why Decomposition Keeps Life Going

Every organism that has ever lived contained nitrogen — locked inside proteins, DNA, and ATP. When it died, that nitrogen could have been lost forever. The reason it wasn't comes down to a group of organisms most people overlook entirely: the decomposers.

Without decomposition, nutrients would accumulate in dead organic matter, soils would become depleted, and plant growth would grind to a halt. The entire productivity of an ecosystem depends on matter being broken down and recycled — and saprobionts are the organisms that make this happen.

2. Saprobionts and Saprobiotic Nutrition

Picture a fallen leaf on the forest floor. Bacteria and fungi colonise its surface and secrete digestive enzymes directly onto the organic material — this is extracellular digestion. The enzymes break large, insoluble molecules (proteins, polysaccharides, lipids) into small, soluble ones, which the saprobiont then absorbs across its cell surface membrane.

This process releases inorganic ions — including ammonium ions — back into the soil, making them available to other organisms. Saprobionts therefore act as the critical link between dead organic matter and the inorganic nutrient pool that plants and other producers depend on.

Saprobionts are not selective — they break down proteins, nucleic acids, and other nitrogen-containing compounds in dead organisms and animal waste. This releases nitrogen in the form of ammonium ions, which is the starting point for the nitrogen cycle's soil-based stages.

3. The Nitrogen Cycle: An Overview

Nitrogen makes up approximately 78% of the atmosphere, yet most organisms cannot use atmospheric nitrogen ($N_2$) directly. The nitrogen cycle is the set of processes that convert nitrogen between its various forms — atmospheric $N_2$, ammonium ions ($NH_4^+$), nitrite ions ($NO_2^-$), nitrate ions ($NO_3^-$), and organic nitrogen compounds — making it accessible to living organisms.

• Saprobiotic nutrition: decomposers break down organic nitrogen in dead matter, releasing ammonium ions
• Ammonification: the specific release of ammonium ions (NH4+) from organic nitrogen compounds
• Nitrification: bacteria oxidise ammonium ions to nitrite, then nitrite to nitrate
• Nitrogen fixation: bacteria convert atmospheric N2 into ammonium ions
• Denitrification: bacteria reduce nitrate back to N2, returning it to the atmosphere

Each stage is carried out primarily by bacteria, and each depends on specific environmental conditions. Understanding which conditions favour which process is where exam questions get their teeth — so let's work through each stage carefully.

4. Ammonification

When organisms die, their proteins and nucleic acids contain nitrogen bonded into organic molecules. Saprobiotic bacteria and fungi secrete proteases and other enzymes that hydrolyse these compounds, ultimately releasing ammonium ions ($NH_4^+$) into the soil. This process is ammonification.

Urea and uric acid in animal excretory products are also substrates for ammonification — so the process is not limited to dead organisms. Any nitrogen-containing organic waste feeds into this stage. The ammonium ions produced either remain in the soil for uptake by plants or become the substrate for the next stage: nitrification.

5. Nitrification

Ammonium ions in the soil are a resource, but plants preferentially absorb nitrate ions ($NO_3^-$). Nitrifying bacteria bridge this gap by oxidising ammonium ions in two sequential steps, each carried out by a different group of bacteria.

First, one group of nitrifying bacteria oxidises ammonium ions to nitrite ions ($NO_2^-$). A second group then oxidises nitrite ions to nitrate ions ($NO_3^-$). Both steps release energy, which the bacteria use for their own metabolism — these are chemoautotrophic organisms, obtaining energy from chemical oxidation rather than light.

Worked example: Tracing Nitrification: Step by Step

Describe the two-stage process of nitrification, identifying the substrate, product, and conditions required at each stage.

1. Ammonium ions ($NH_4^+$) are present in the soil following ammonification of dead organic matter. — These ions come from saprobiotic breakdown of proteins and nucleic acids — linking back to the previous stage of the cycle.
2. $NH_4^+$ is oxidised to $NO_2^-$ (nitrite ions) by nitrifying bacteria. — This oxidation releases energy used by the bacteria. Aerobic conditions are essential — oxygen acts as the terminal electron acceptor.
3. $NO_2^-$ is oxidised to $NO_3^-$ (nitrate ions) by a second group of nitrifying bacteria. — Again, aerobic conditions are required. The nitrate ions produced are the form most readily absorbed by plant roots via active transport.
4. Nitrification requires aerobic (well-oxygenated) soil conditions. — Waterlogged or compacted soils become anaerobic, inhibiting nitrifying bacteria and reducing nitrate availability for plants.

Answer: Nitrification is a two-stage aerobic process: NH4+ is oxidised to NO2-, then NO2- is oxidised to NO3-, by two different groups of nitrifying bacteria. Nitrate ions are the form absorbed by plant roots.

The dependence of nitrification on aerobic conditions has a direct practical consequence: waterlogged soils become anaerobic, nitrifying bacteria cannot function, and nitrate levels fall. This is one reason why poorly drained agricultural soils have reduced fertility.

6. Nitrogen Fixation

Atmospheric nitrogen ($N_2$) is an extraordinarily stable molecule — the triple bond between the two nitrogen atoms requires a great deal of energy to break. Most organisms cannot do this at all. Nitrogen-fixing bacteria, however, possess the enzyme nitrogenase, which catalyses the conversion of $N_2$ into ammonium ions ($NH_4^+$).

Nitrogen fixation occurs in two main contexts. Free-living nitrogen-fixing bacteria live independently in the soil and fix nitrogen directly into the soil solution. Mutualistic nitrogen-fixing bacteria live in root nodules of leguminous plants (such as peas, beans, and clover), where they fix nitrogen in exchange for carbohydrates from the plant. Both routes add fixed nitrogen to the soil, increasing its fertility.

The mutualistic relationship is particularly significant in agriculture. Farmers have long used crop rotation with legumes to restore soil nitrogen levels — a practice that makes biological sense now that the mechanism is understood. The bacteria receive a carbon source; the plant receives a supply of ammonium ions that can be incorporated into amino acids and proteins.

7. Denitrification

Denitrification is, in a sense, the nitrogen cycle's release valve. Denitrifying bacteria reduce nitrate ions ($NO_3^-$) back to nitrogen gas ($N_2$), which returns to the atmosphere. This removes fixed nitrogen from the soil and is generally detrimental to soil fertility.

The critical condition here is anaerobic soil. Denitrifying bacteria use nitrate as an alternative electron acceptor when oxygen is unavailable — so waterlogged, compacted, or poorly drained soils actively promote denitrification. This is the opposite of what favours nitrification, and the contrast is a favourite exam comparison.

Notice the symmetry: nitrification builds up the soil's nitrate supply under aerobic conditions, while denitrification depletes it under anaerobic conditions. Soil management practices — drainage, aeration, avoiding compaction — are therefore directly linked to maintaining the balance between these two processes.

9. Evaluating the Importance of Nitrogen Cycle Stages

Earlier, we established that saprobionts release ammonium ions from dead organic matter. Now the full picture emerges: those ammonium ions feed into nitrification (producing nitrate for plants). Separately, atmospheric nitrogen is introduced into the soil system via nitrogen fixation by specific bacteria, which convert N₂ into ammonium ions. If conditions turn anaerobic, nitrate produced from these processes can be lost from the soil via denitrification.

Each stage of the nitrogen cycle plays a different but vital role in ecosystem function. Biological nitrogen fixation is especially important for introducing new, usable nitrogen into ecosystems, forming the initial supply of nitrogen compounds required for life. Ammonification and nitrification ensure that nitrogen from organic matter and waste is recycled efficiently, maintaining soil fertility and supporting plant growth. Denitrification, although a natural process that prevents excess nitrate accumulation and leaching, can reduce soil fertility when it dominates—such as in waterlogged or anaerobic conditions. Ecosystems lacking nitrogen fixation would quickly become nitrogen-deficient, severely limiting plant and animal productivity. In contrast, unchecked denitrification would similarly lower soil nitrate and reduce growth. Thus, the balance between these processes is crucial: nitrogen fixation is a major input, and denitrification is a key output, with other stages recycling nitrogen internally. Disruption of any of these stages can have substantial impacts on the viability and productivity of ecosystems.

9. Exam Technique and Common Mistakes

Nitrogen cycle questions reward precision. Examiners distinguish between students who know the names of processes and those who can explain the conditions, substrates, and products with accuracy. The following guidance targets the most common sources of mark loss.

For six-mark questions on the nitrogen cycle, structure your answer as a sequence: start with dead organic matter, move through ammonification, then nitrification, then plant uptake, and address nitrogen fixation and denitrification as inputs and outputs to the cycle. Each stage should name the type of organism responsible and the condition required.