Nutrient Cycles

SPEC 3.5.4 - NUTRIENT CYCLES

Nitrogen cycle, phosphorus cycle, saprobionts, mycorrhizae, fertilisers and eutrophication

Why Nutrients Must Be Recycled

Unlike energy, which flows through an ecosystem and is lost as heat at each trophic level, chemical elements such as nitrogen and phosphorus are recycled within ecosystems. They pass between organisms and the abiotic environment in continuous cycles.

Key principle

Nutrients removed from the soil by plant uptake or harvesting must be replaced. If they are not returned, soil fertility falls and productivity declines. This is why decomposers, natural fertilisers, and artificial fertilisers are all important.

Saprobiont

An organism (usually a fungus or bacterium) that feeds on dead organic matter by secreting extracellular enzymes and absorbing the soluble products. Also called decomposers or saprotrophic organisms. They are essential for releasing inorganic nutrients from organic molecules back into the soil.

Mycorrhizae

A mutualistic association between fungi and plant roots. The fungal hyphae greatly increase the root surface area for absorption of water and inorganic ions — particularly phosphate. The plant benefits from enhanced mineral uptake; the fungus receives organic nutrients (sugars) from the plant.

Plant benefit

Fungal hyphae extend far beyond the root hair zone, accessing phosphate ions that roots alone could not reach. This dramatically increases the plant's phosphate uptake — critical for ATP, DNA, and phospholipid synthesis.

Fungus benefit

The fungus receives sugars (glucose/sucrose) produced by the plant through photosynthesis. This is a true mutualism — both partners benefit.

Exam Tip

When asked about mycorrhizae, always state: (1) mutualistic association; (2) fungal hyphae increase surface area; (3) mainly improves uptake of phosphate (and water). The plant provides the fungus with organic nutrients.

The Nitrogen Cycle — Overview

Nitrogen makes up ~78% of the atmosphere as N₂ gas, but most organisms cannot use it directly. Bacteria carry out the key transformations that make nitrogen available.

ProcessOrganismTransformationConditions

Nitrogen fixationRhizobium (root nodules); Azotobacter (free-living)N₂ → NH₄⁺Any

AmmonificationSaprobionts (bacteria & fungi)Organic N → NH₄⁺Any

NitrificationNitrosomonas: NH₄⁺ → NO₂⁻; Nitrobacter: NO₂⁻ → NO₃⁻NH₄⁺ → NO₃⁻Aerobic

DenitrificationDenitrifying bacteriaNO₃⁻ → N₂Anaerobic

AssimilationPlantsNO₃⁻ → amino acids, nucleotides—

Examiner Alert

The spec states that individual species names of bacteria are not required — but Rhizobium, Azotobacter, Nitrosomonas, and Nitrobacter are so commonly used in mark schemes that knowing them is strongly recommended. Never confuse nitrification (NH₄⁺ → NO₃⁻) with nitrogen fixation (N₂ → NH₄⁺).

Nitrogen Fixation & Ammonification

Nitrogen Fixation

The conversion of atmospheric nitrogen gas (N₂) to ammonium ions (NH₄⁺) by nitrogen-fixing bacteria. This is the only biological route by which N₂ enters the nitrogen cycle.

Rhizobium — lives in root nodules of leguminous plants (peas, beans, clover) in a mutualistic relationship

Azotobacter — free-living in soil

Ammonification

The conversion of organic nitrogen (proteins, amino acids, nucleotides in dead organisms and animal waste) to ammonium ions (NH₄⁺) by saprobionts. Saprobionts secrete proteases and other enzymes extracellularly, then absorb the soluble products — releasing NH₄⁺ into the soil.

Why ammonification matters

Dead organisms and excretory products contain organic nitrogen (in amino acids, urea, etc.). Without saprobionts, this nitrogen would remain locked up in organic form and unavailable to plants. Ammonification converts it to NH₄⁺, which can then be nitrified or taken up directly by some plants.

Nitrification & Denitrification

Nitrification

The two-stage aerobic oxidation of ammonium ions to nitrate by nitrifying bacteria:

Stage 1: Nitrosomonas oxidises NH₄⁺ → NO₂⁻ (nitrite)

Stage 2: Nitrobacter oxidises NO₂⁻ → NO₃⁻ (nitrate)

Both stages require aerobic conditions. Waterlogged, anaerobic soils inhibit nitrification.

Denitrification

The conversion of nitrate (NO₃⁻) back to nitrogen gas (N₂) by denitrifying bacteria in anaerobic conditions (e.g. waterlogged soil). This removes fixed nitrogen from the soil, reducing fertility.

Nitrification vs Denitrification

Nitrification: NH₄⁺ → NO₃⁻ (aerobic; increases plant-available nitrogen). Denitrification: NO₃⁻ → N₂ (anaerobic; decreases plant-available nitrogen). Waterlogged fields promote denitrification — one reason why drainage improves soil fertility.

The Nitrogen Cycle — Diagram

The Phosphorus Cycle

Unlike nitrogen, phosphorus has no significant atmospheric pool. It cycles between rocks, soil, organisms, and water.

1

Weathering of phosphate-containing rocks releases inorganic phosphate (PO₄³⁻) into the soil.

2

Plants absorb phosphate from soil solution through roots (assisted by mycorrhizae). Used to make ATP, DNA, RNA, and phospholipids.

3

Consumers obtain phosphorus by eating plants or other animals. Passes up the food chain.

4

Saprobionts decompose dead organic matter, releasing inorganic phosphate back into the soil.

5

Leaching washes phosphate from soil into waterways — contributing to eutrophication.

Role of mycorrhizae in phosphorus cycling

In many ecosystems, mycorrhizal fungi are the primary route by which plants access phosphate. Fungal hyphae have a far greater surface area than plant roots, and can access phosphate from soil pores too small for root hairs. This is especially important in low-phosphate soils.

Fertilisers: Natural & Artificial

Harvesting crops removes nitrogen and phosphorus from the land. Fertilisers are added to replace these nutrients and maintain soil fertility.

Natural Fertilisers

Include manure (animal faeces and urine), compost, and slurry. They contain organic nitrogen and phosphorus, which saprobionts decompose to release inorganic ions. Also improve soil structure and water-holding capacity.

Artificial Fertilisers

Manufactured inorganic compounds containing nitrates (NO₃⁻) and/or phosphates (PO₄³⁻). Immediately available to plants. Examples: ammonium nitrate (NH₄NO₃), superphosphate. Provide concentrated, fast-acting nutrients but are associated with environmental issues.

Exam Tip

When comparing natural and artificial fertilisers: natural fertilisers release nutrients slowly (need decomposition first); artificial fertilisers release nutrients immediately but are more prone to leaching. Both replace nitrates and phosphates lost by crop harvesting.

Leaching & Eutrophication

Leaching

The washing of soluble nitrates and phosphates from soil into waterways (rivers, lakes) by rainwater. Leaching is more severe with heavy rainfall, porous soils, and when fertilisers are applied before rain.

Eutrophication

The process by which excess nutrients (especially nitrates and phosphates) in water stimulate algal and plant growth, leading to reduced oxygen levels and death of aquatic organisms.

Eutrophication sequence — must learn in order

Nitrates/phosphates leach from farmland into rivers/lakes

Algae rapidly multiply → algal bloom on the water surface

Dense algae block light from reaching submerged aquatic plants

Aquatic plants die (cannot photosynthesise)

Massive increase in dead organic matter → saprobiont populations increase

Saprobionts respire aerobically → consume dissolved oxygen

Dissolved oxygen falls below threshold → aerobic organisms die (fish, invertebrates)

Eutrophication: Critical Exam Points

Most Common Error

Students write: "Algae block light → algae use up all the oxygen → fish die." This is wrong. Algae actually produce oxygen during photosynthesis. The oxygen depletion is caused by decomposing bacteria (saprobionts) respiring aerobically to break down the dead algae and other organic matter. The sequence must be: dead organic matter → saprobiont population increase → aerobic respiration by saprobionts → O₂ depletion.

Second Common Error

Students write "algal bloom blocks light → fish die." The mechanism requires: algal bloom → aquatic plants die (no light) → increase in dead organic matter → saprobiont respiration → O₂ depletion → fish die. All steps must be included for full marks.

Biological Oxygen Demand (BOD)

BOD is the amount of dissolved oxygen consumed by biological activity in water. High BOD = lots of decomposer activity = low dissolved oxygen = poor water quality. Eutrophication dramatically increases BOD.

DEEP Framework: Extended Answer

A common AQA extended response: "Explain how the application of fertilisers to farmland can lead to the death of fish in a nearby river."

DEEP Chain — Fertilisers → Eutrophication → Fish Death

D

Define: Leaching is the washing of soluble nitrates and phosphates from soil into waterways. Eutrophication is the process by which excess nutrients stimulate algal growth, leading to reduced oxygen and death of aquatic organisms.

E

Explain mechanism: Nitrates/phosphates leach from farmland into the river. Algae multiply rapidly (algal bloom) on the surface. Algae block light reaching submerged aquatic plants — those plants die. The large quantity of dead organic matter causes saprobiont populations to increase rapidly. Saprobionts respire aerobically, consuming dissolved oxygen. Dissolved oxygen concentration falls below the threshold needed by fish and invertebrates — they die.

E

Evidence: Dissolved oxygen measurements in eutrophic water fall significantly below the levels required by aerobic organisms. Biological oxygen demand (BOD) increases dramatically, indicating intensive saprobiont aerobic respiration consuming dissolved oxygen faster than it can be replenished.

P

Practical/Prevention: Reducing fertiliser application rates, using slow-release fertilisers, avoiding application before heavy rain, and creating buffer strips of vegetation along riverbanks can all reduce leaching and prevent eutrophication.

Practice Question

A farmer applies a large quantity of artificial fertiliser to a field adjacent to a river in spring. Three weeks later, the river shows a significant algal bloom and fish begin to die.

(a) Explain why the fish are dying. Include all steps in the process. [5 marks]

(b) Explain why nitrification in the soil is important for plant nutrition. [3 marks]

a:

Nitrates/phosphates leached from fertilised field into river → algal bloom (algae multiply rapidly) → algae on surface block light from reaching submerged aquatic plants → aquatic plants die → large amounts of dead organic matter → saprobiont populations increase → saprobionts respire aerobically → dissolved oxygen concentration falls → fish die from oxygen deprivation. (Note: oxygen is NOT used up by algae — it is consumed by saprobionts respiring.)

b:

Nitrogen-fixing bacteria and ammonification produce ammonium ions (NH₄⁺) in soil. Nitrification by Nitrosomonas converts NH₄⁺ to NO₂⁻, and Nitrobacter converts NO₂⁻ to NO₃⁻ (nitrate). Plants absorb nitrate ions from soil solution and incorporate nitrogen into amino acids and nucleotides (assimilation). Without nitrification, less nitrate would be available and plant growth would be limited.

Examiner Alerts: Common Errors

Error 1: Algae consume the oxygen

Algae produce oxygen through photosynthesis. The oxygen is consumed by decomposer bacteria (saprobionts) respiring aerobically. Always attribute O₂ depletion to saprobiont respiration, not algal growth.

Error 2: Confusing nitrification and nitrogen fixation

Nitrogen fixation: N₂ → NH₄⁺ (by Rhizobium/Azotobacter — atmospheric nitrogen enters the cycle). Nitrification: NH₄⁺ → NO₃⁻ (by Nitrosomonas/Nitrobacter — ammonium converted to plant-usable nitrate). These are completely different processes.

Error 3: Denitrification is beneficial

Denitrification reduces soil fertility — it converts NO₃⁻ back to N₂ gas, removing fixed nitrogen from the soil. It is not a beneficial process for farming. Waterlogged soils promote denitrification, which is why poor drainage leads to low crop yields.

Error 4: Mycorrhizae improve general mineral uptake

While mycorrhizae do improve uptake of water and minerals generally, the spec specifically emphasises their role in phosphate uptake. Always mention phosphate when asked about mycorrhizae.

Synoptic Links

Synoptic Link — Energy Transfers (3.5.3)

Biomass not consumed by the next trophic level passes to decomposers. Saprobionts break down this dead organic matter, releasing inorganic nutrients (N, P) back into the soil — completing the link between energy flow (Lesson 5.05) and nutrient cycling (this lesson). Energy is lost as heat; nutrients are recycled.

Synoptic Link — Biodiversity (3.4.6)

Eutrophication dramatically reduces aquatic biodiversity — a single dominant algal species outcompetes all others, and subsequent oxygen depletion kills most aerobic species. This connects to Simpson's index of diversity and the environmental threats covered in Lesson 4.07.

Synoptic Link — Respiration (3.5.2)

It is aerobic respiration by saprobionts that depletes dissolved oxygen in eutrophication. The process follows the same principles as Lesson 5.03–5.04: glucose + O₂ → CO₂ + H₂O + ATP. The high rate of organic matter decomposition creates extremely high oxygen demand, rapidly depleting the dissolved O₂.

Summary

Nitrogen Cycle — Five Processes

ProcessDirectionOrganismConditions

Nitrogen fixationN₂ → NH₄⁺Rhizobium; AzotobacterAny

AmmonificationOrganic N → NH₄⁺SaprobiontsAny

NitrificationNH₄⁺ → NO₃⁻Nitrosomonas; NitrobacterAerobic

DenitrificationNO₃⁻ → N₂Denitrifying bacteriaAnaerobic

AssimilationNO₃⁻ → organic NPlants—

Eutrophication sequence

Leaching → algal bloom → plants die (no light) → saprobionts respire → O₂ depleted → aerobic organisms die. O₂ consumed by saprobionts, not algae.

Mycorrhizae

Mutualistic fungi + plant roots. Fungal hyphae increase surface area for phosphate uptake. Plant provides organic nutrients to fungus.

Exam Tip

For eutrophication: always state that O₂ is depleted by saprobiont (decomposer) aerobic respiration — not by algae. For the nitrogen cycle: know all five processes, the direction of each transformation, and whether aerobic or anaerobic conditions are required.