Nutrient Cycles — Nitrogen & Phosphorus

Nutrient Cycles in Natural Ecosystems

Natural ecosystems (i.e. systems that have not been substantially altered by human activity) rely on closed-loop recycling of chemical elements. Two headline examples required by the A-Level AQA specification are the nitrogen and phosphorus cycles.

Ecological significance:

  • Prevents long-term nutrient depletion of soils and waters.

  • Supports primary productivity → supports entire food webs.

  • Human interference (fertilisers, deforestation, urban runoff) can unbalance cycles leading to eutrophication, soil acidification, climate feedbacks etc.

Key idea: Energy flows, nutrients cycle – energy ultimately leaves as heat (see energy pyramid at end of transcript) whereas atoms are reused.

Role of Microorganisms in Cycling Elements

Microorganisms (mainly bacteria & fungi) underpin nutrient recycling. Their activities convert elements between organic and inorganic states, allowing uptake by plants and re-entry into food webs.

Saprobionts

  • Definition: Heterotrophic microorganisms that feed on dead organic matter & waste.

  • Twin activities:

    1. Decomposition – breakdown of complex organic molecules.

    2. Extracellular digestion – hydrolytic enzymes are secreted outside the cell; soluble products (glucose, amino acids, phosphate, nitrate etc.) are absorbed.

  • Termed saprobiotic nutrition.

  • Ecological result: release of inorganic ions back into soil/water for primary producers.

Mycorrhizae

  • A symbiotic (mutualistic) association between specific fungi and plant roots.

  • Fungal hyphae form an extensive network → huge surface area.

  • Benefits:

    • To plant – increased uptake of scarce inorganic ions (notably \text{PO}_4^{3-}) & water; improved drought tolerance.

    • To fungus – receives organic carbon (glucose, sucrose) produced by plant photosynthesis.

  • Practical application: commercial inoculation of crops (e.g. wheat with Glomus intraradices) to enhance yield, especially under water-limited conditions (see greenhouse study; statistical support given by ±2 SD error bars).

Terminology:

  • Symbiosis – two species living in close physical association.

  • Mutualism – both partners benefit (cf. parasitism, commensalism).

  • Cultivated land – land purposely used for agriculture; generally higher anthropogenic N-inputs than uncultivated land.

The Nitrogen Cycle

Plants & animals require nitrogen for amino acids, proteins, ATP, nucleic acids (DNA/RNA). Atmospheric \text{N}_2 (≈78 %) is inert; biologically useful forms are ammonium, nitrite, nitrate and organic N.

Four core bacterial processes (+ two abiotic routes):

1. Nitrogen Fixation

  • Conversion:The process in which nitrogen gas ( N) is converted into ammonia (NH₃) or related compounds, enabling plants to utilize nitrogen.

  • Key agents:

    • Rhizobium (inside legume root nodules) – mutualistic; receives carbohydrates.

    • Free-living soil cyanobacteria & Azotobacter.

  • Abiotic fixation: lightning (≈5.4 Tg N yr⁻¹) produces \text{NO}_x; Haber-Bosch fertiliser manufacture (≈86 Tg N yr⁻¹ early 1990s).

2. Ammonification

  • Deamination of amino acids / nucleic acids from detritus & excreta (urea) by saprobionts.

  • Explains high ammonium levels in anaerobic, waterlogged marshes where further oxidation is inhibited.

3. Nitrification (aerobic)

oxidation of ammonium → nitrite → nitrate by chemoautotrophic bacteria.

  1. Nitrosomonas:a genus of bacteria that plays a critical role in the nitrification process by oxidizing ammonium to nitrite.

  2. Nitrobacter: another genus of bacteria that further oxidizes nitrite to nitrate, completing the nitrification process and making nitrogen available for plant uptake.

4. Denitrification (anaerobic)

  • Facultative anaerobes (e.g. Pseudomonas, Paracoccus) use nitrate as terminal electron acceptor → \text{N}_2 gas escapes.

  • Favoured in waterlogged soils; reduces soil fertility unless managed (drainage, crop rotation).

Net cycle diagram (simplified)
Atmospheric N2
   |  (fixation)                     Lightning / Haber
NH3 / NH4+ (soil)  --(Nitrosomonas)-->  NO2-  --(Nitrobacter)-->  NO3-
   ^                 |                              |
   |                 |                              | (denitrification)
   |  (ammonification)              Plants --> Animals --> Waste --> Saprobionts

The Phosphorus Cycle

Phosphorus is essential for

  • Phospholipids (membranes),

  • ATP/ADP energy currency,

  • DNA/RNA backbone.
    Unlike nitrogen, there is no gaseous P phase; main reservoir is rock phosphate.

Key Steps

  1. Weathering releases \text{PO}_4^{3-} from rock → soil or aquatic environments.

  2. Assimilation by plants (aided by mycorrhizae). \text{PO}_4^{3-} incorporated into biomass.

  3. Trophic transfer up food chains; some P lost as waste.

  4. Decomposition of detritus & excreta by saprobionts → returns \text{PO}_4^{3-} to soil/water.

  5. Run-off transports phosphate to rivers, lakes, oceans.

  6. Marine food webs move P into fish, birds.

  7. Guano (seabird excrement; high in phosphate) deposits on coastal land – historically mined as natural fertiliser.

Equation example – release from calcium phosphate rock:

Human impact

  • Detergents & artificial P-fertilisers elevate aquatic phosphate → algal blooms → hypoxia → fish kills (e.g. Lake Windermere case study: 84 % drop in plant biomass 1985→1995; linked to rising \text{PO}_4^{3-} concentrations).

  • Ethical dimension: balancing food security with ecosystem health; EU regulations now limit P content in detergents.

Environmental Scenarios

• Waterlogging → anaerobic → denitrification ↑, nitrification ↓ → less \text{NO}_3^- for plants → reduced growth.
• Crop rotation with legumes enriches soil N; reduces need for synthetic fertiliser (economic & environmental benefit).
• Mycorrhizal inoculation under drought (greenhouse study):

  • Water-stressed treated plants produced higher tomato mass than untreated.

  • Under normal water, little difference → cost-benefit analysis needed.

Connections to Core Principles

  1. Biochemistry – ATP, DNA structure, amino acid composition depend on N & P.

  2. Energy transfer – nutrient availability limits primary production which dictates energy pyramid magnitudes (producers ≈ 20\,000\,\text{kcal m}^{-2}\text{yr}^{-1}, tertiary consumers only 20\,\text{kcal m}^{-2}\text{yr}^{-1} etc.).

  3. Microbiology – metabolism (aerobic vs anaerobic), enzyme secretion, mutualistic interactions.

  4. Human biology – N balance in diet, phosphate additives.

  5. Sustainability – circular economy mimics natural nutrient cycles.

Key Terms

• Natural ecosystem, nutrient cycle, saprobiont, saprobiotic nutrition, extracellular digestion, mycorrhizae, hyphae, symbiosis, mutualism, nitrifying bacteria, denitrifying bacteria, nitrogen fixation, ammonification, nitrification, denitrification, assimilation, weathering, guano, eutrophication.

End-Point Checklist (Specification 3.5.4)

✔ Nutrients recycled in natural ecosystems.
✔ Detailed mechanisms of nitrogen & phosphorus cycles.
✔ Roles of saprobionts, mycorrhizae, bacteria (all named processes).
✔ Ability to perform quantitative reasoning on cycling data.
✔ Understanding of anthropogenic impacts & management strategies.