How are nitrogen and phosphorus recycled through an ecosystem?
The nitrogen cycle and the roles of saprobionts, nitrogen-fixing, nitrifying and denitrifying bacteria; the phosphorus cycle and the role of mycorrhizae in phosphorus uptake; the role of microorganisms in recycling nutrients; the use of natural and artificial fertilisers and the environmental consequences of using nitrogen-containing and phosphorus-containing fertilisers, including leaching and eutrophication.
A focused answer to the AQA 3.5 dot point on nutrient cycles. Sets out the nitrogen cycle and its four bacterial groups, the phosphorus cycle and mycorrhizae, the role of microorganisms in recycling, and how fertilisers cause leaching and eutrophication.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
AQA wants you to describe the nitrogen and phosphorus cycles with the correct microorganisms named at each step, explain how decomposers recycle nutrients, and set out the eutrophication sequence caused by fertiliser leaching.
The answer
Unlike energy, nutrients are recycled. Microorganisms convert nutrients between forms so that elements locked in dead organisms become available again to plants.
The nitrogen cycle
Plants need nitrogen for amino acids, proteins and nucleic acids, but cannot use nitrogen gas directly. Four groups of bacteria drive the cycle.
- Nitrogen fixation. Nitrogen-fixing bacteria (for example Rhizobium in the root nodules of legumes, and free-living Azotobacter) convert atmospheric nitrogen gas into ammonia / ammonium compounds. Lightning and the industrial Haber process also fix nitrogen.
- Ammonification. Saprobionts decompose proteins and other nitrogen compounds in dead organisms and waste, releasing ammonium ions.
- Nitrification. Nitrifying bacteria oxidise ammonium ions in two stages: first to nitrite (by Nitrosomonas), then to nitrate (by Nitrobacter). This is an aerobic process, so it needs well-aerated soil. Plants absorb nitrate through their roots.
- Denitrification. Denitrifying bacteria convert nitrate back to nitrogen gas, which returns to the atmosphere. This occurs in anaerobic (waterlogged) soils, which is why good drainage and ploughing improve soil fertility.
The phosphorus cycle
Phosphorus is needed for phospholipids, ATP, DNA and RNA. The phosphorus cycle has no gaseous phase, so it is slower than the nitrogen cycle.
- Phosphate ions are released from rock by weathering and dissolve in water and soil.
- Plants absorb phosphate ions through their roots; the phosphate then passes along food chains to consumers.
- Saprobionts decompose dead organisms and waste (including animal droppings and bird guano), returning phosphate to the soil.
- Phosphate may be washed into seas and deposited in sediment, where over geological time it forms new rock.
Mycorrhizae are mutualistic associations between fungi and plant roots. The fungal hyphae act like an extension of the root system, hugely increasing the surface area for absorbing water and mineral ions, especially phosphate. In return the plant supplies the fungus with organic compounds. Mycorrhizae make phosphorus uptake far more efficient, particularly in poor soils.
Fertilisers and eutrophication
Farmers replace nitrogen and phosphorus removed at harvest using fertilisers.
- Natural fertilisers (manure, slurry, compost) release nutrients slowly as decomposers break them down.
- Artificial (inorganic) fertilisers supply soluble nitrate and phosphate in precise, fast-acting amounts.
If more fertiliser is applied than the crop can absorb, the excess soluble ions are leached into rivers and lakes, causing eutrophication:
- Leached nitrate (and phosphate) enriches the water, removing the limiting factor on algal growth.
- Algae multiply rapidly, forming an algal bloom at the surface.
- The bloom blocks light, so submerged plants cannot photosynthesise and die.
- Saprobiotic bacteria decompose the dead plants and algae, multiplying and respiring aerobically.
- Their respiration depletes the dissolved oxygen, so fish and other aerobic organisms suffocate and die.
Examples in context
Example 1. Legume crop rotation. Farmers grow legumes such as clover or beans, whose root nodules house nitrogen-fixing Rhizobium, then plough them in. This naturally enriches the soil with nitrogen compounds, reducing the need for artificial fertiliser and lowering the risk of nitrate leaching, a direct application of the nitrogen cycle.
Example 2. Eutrophication of the Norfolk Broads. Decades of fertiliser runoff and sewage raised nitrate and phosphate levels in these lakes, triggering algal blooms that shaded out the diverse submerged plants. As decomposers consumed the dead material, dissolved oxygen crashed and fish populations collapsed, illustrating the full eutrophication sequence at landscape scale.
Try this
Q1. Name the type of bacteria responsible for each of: converting nitrogen gas to ammonia, converting ammonium to nitrate, and converting nitrate to nitrogen gas. [3 marks]
- Cue. Nitrogen-fixing bacteria; nitrifying bacteria; denitrifying bacteria.
Q2. Explain the role of mycorrhizae in the phosphorus cycle. [2 marks]
- Cue. Mycorrhizae are fungus-root associations whose hyphae greatly increase the surface area for absorbing mineral ions, especially phosphate, improving phosphorus uptake, particularly in poor soils.
Q3. Describe how leaching of nitrate fertiliser can lead to the death of fish in a lake. [4 marks]
- Cue. Nitrate is leached into the lake; algae bloom and block light; submerged plants die; saprobiotic bacteria decompose the dead material and respire aerobically, depleting dissolved oxygen; fish cannot respire and die.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2017 AQA5 marksExplain how the use of nitrogen-containing fertilisers can lead to the death of fish in a nearby river.Show worked answer →
A 5-mark answer needs the eutrophication sequence in order.
- Excess nitrate is leached from the soil into the river.
- The extra nitrate removes the limiting factor, so algae grow rapidly (an algal bloom) at the surface.
- The bloom blocks light, so plants below it cannot photosynthesise and die.
- Saprobiotic bacteria decompose the dead plants and algae and respire aerobically, using up the dissolved oxygen.
- The fall in dissolved oxygen means fish and other aerobic organisms cannot respire and die.
Markers reward leaching, algal bloom blocking light, decomposers respiring, and oxygen depletion killing fish.
Related dot points
- Photosynthesis as a two-stage process: the light-dependent reactions in the thylakoid membranes (photoionisation of chlorophyll, photolysis of water, the production of ATP by photophosphorylation, the production of reduced NADP, and the role of the electron transport chain); the light-independent reactions in the stroma (the Calvin cycle: fixation of carbon dioxide by RuBP to form GP, reduction of GP to TP using reduced NADP and ATP, and regeneration of RuBP); the effect of light intensity, carbon dioxide concentration and temperature as limiting factors.
A focused answer to the AQA 3.5 dot point on photosynthesis. Covers the light-dependent reactions (photoionisation, photolysis, the electron transport chain and photophosphorylation), the Calvin cycle in the stroma, and how light, carbon dioxide and temperature act as limiting factors.
- Aerobic respiration as four stages: glycolysis in the cytoplasm (phosphorylation of glucose, oxidation to pyruvate, net yield of ATP and reduced NAD); the link reaction and the Krebs cycle in the mitochondrial matrix (decarboxylation, dehydrogenation, production of reduced NAD, reduced FAD, ATP and carbon dioxide); oxidative phosphorylation on the inner mitochondrial membrane (the electron transport chain, chemiosmosis, ATP synthase and the role of oxygen as the final electron acceptor); anaerobic respiration in animals (lactate) and in microorganisms and plants (ethanol).
A focused answer to the AQA 3.5 dot point on respiration. Covers glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation by chemiosmosis, the role of oxygen as the final electron acceptor, and anaerobic respiration producing lactate or ethanol.
- The transfer of biomass and energy through trophic levels in food chains and food webs; producers, primary, secondary and tertiary consumers, decomposers and saprobionts; the reasons why biomass and energy decrease at successive trophic levels; the calculation of the efficiency of energy transfer between trophic levels.
A focused answer to the AQA 3.5 dot point on energy and ecosystems. Explains trophic levels and food webs, why biomass and energy fall between levels, the role of decomposers, and how to calculate the percentage efficiency of energy transfer.
- Biomass as the mass of living material, measured as dry mass or as the chemical energy stored in dry biomass using calorimetry; gross primary production (GPP) as the chemical energy store in plant biomass; net primary production (NPP) as GPP minus respiratory losses; the calculation and units of GPP, NPP and net production of consumers; the ways in which farming practices increase the efficiency of energy transfer in food production.
A focused answer to the AQA 3.5 dot point on productivity and biomass. Defines biomass and dry mass, explains how calorimetry measures energy content, sets out GPP, NPP and net production of consumers with their units, and reviews farming practices that raise energy-transfer efficiency.