Nutrient Cycles: Nitrogen and Phosphorus

The Importance of Nutrient Cycling

In ecosystems, matter is recycled but energy flows through (entering as light, leaving as heat). Essential elements such as nitrogen, phosphorus, and carbon are continuously cycled between living organisms and the abiotic environment. Without recycling, nutrients would become locked in dead organic matter, and new growth would be impossible.

The Nitrogen Cycle

Nitrogen is essential for life — it is a component of amino acids (and therefore proteins), nucleotides (DNA, RNA, ATP), and other biological molecules. Although nitrogen gas (N₂) makes up 78% of the atmosphere, most organisms cannot use it directly because the triple covalent bond (N≡N) is extremely strong and unreactive. Nitrogen must be converted to reactive forms (fixed) before organisms can use it.

Key Processes in the Nitrogen Cycle

1. Nitrogen Fixation

The conversion of atmospheric N₂ into ammonia (NH₃) or ammonium (NH₄⁺):

Biological nitrogen fixation:

• Carried out by nitrogen-fixing bacteria:
• Rhizobium — lives in root nodules of leguminous plants (peas, beans, clover) in a mutualistic relationship. The plant provides organic molecules (photosynthate) and an anaerobic environment; the bacteria fix N₂ and supply the plant with ammonium
• Azotobacter — free-living soil bacteria that fix N₂
• Some cyanobacteria (blue-green algae) in aquatic environments
• These bacteria use the enzyme nitrogenase to catalyse: N₂ + 8H⁺ + 8e⁻ + 16ATP → 2NH₃ + H₂ + 16ADP + 16Pᵢ
• Nitrogenase is inhibited by oxygen, which is why root nodules contain leghaemoglobin to bind O₂ and maintain anaerobic conditions

Industrial nitrogen fixation (Haber process):

• N₂ + 3H₂ → 2NH₃ (high temperature, high pressure, iron catalyst)
• Used to manufacture fertilisers (ammonium nitrate, urea)

Lightning can also fix small amounts of N₂ by providing energy to combine N₂ with O₂ to form nitrogen oxides, which dissolve in rain to form nitrates.

2. Ammonification (Decomposition)

Saprobiotic microorganisms (decomposers — fungi and bacteria) break down organic nitrogen compounds in dead organisms and excretory products (urea, uric acid, faeces) into ammonium ions (NH₄⁺):

• Organic N (proteins, nucleic acids) → amino acids → NH₄⁺
• This returns nitrogen to the soil in an inorganic form

3. Nitrification

Nitrifying bacteria (chemoautotrophs) in the soil oxidise ammonium ions to nitrates in two steps:

• Nitrosomonas: NH₄⁺ → NO₂⁻ (nitrite) — oxidation provides energy for the bacteria
• Nitrobacter: NO₂⁻ → NO₃⁻ (nitrate) — further oxidation

Nitrification requires aerobic conditions (oxygen as the electron acceptor). This is why waterlogged, anaerobic soils are poor for plant growth — nitrification is inhibited.

Nitrate (NO₃⁻) is the form most readily absorbed by plant roots (via active transport) and is used to synthesise amino acids, nucleotides, and chlorophyll.

4. Denitrification

Denitrifying bacteria (e.g. Pseudomonas) convert nitrates back to nitrogen gas (N₂), which is released into the atmosphere:

• NO₃⁻ → NO₂⁻ → N₂O → N₂
• Occurs in anaerobic (waterlogged) conditions — the bacteria use nitrate as an alternative electron acceptor in their respiration
• Denitrification removes nitrogen from the soil, reducing its availability for plant growth
• This is why farmers drain fields and plough to aerate the soil — maintaining aerobic conditions minimises denitrification

5. Assimilation

Plants absorb NH₄⁺ and NO₃⁻ from the soil through their roots and use them to synthesise amino acids and other nitrogen-containing organic molecules. Animals obtain nitrogen by eating plants (or other animals). The nitrogen is then incorporated into the animal's own proteins and nucleic acids.

Summary Table

Process	Organisms	Conversion	Conditions
Nitrogen fixation	Rhizobium, Azotobacter	N₂ → NH₄⁺	Anaerobic (for nitrogenase)
Ammonification	Saprobiotic bacteria/fungi	Organic N → NH₄⁺	Aerobic (mainly)
Nitrification	Nitrosomonas, Nitrobacter	NH₄⁺ → NO₂⁻ → NO₃⁻	Aerobic
Denitrification	Pseudomonas	NO₃⁻ → N₂	Anaerobic
Assimilation	Plants (and animals via diet)	NO₃⁻/NH₄⁺ → organic N	—

The Phosphorus Cycle

Phosphorus is essential as a component of ATP, DNA and RNA (phosphodiester bonds in the sugar-phosphate backbone), phospholipids (cell membranes), and bones/teeth (as calcium phosphate).

Unlike nitrogen, phosphorus does not have a gaseous phase — it cycles primarily between rock/soil and living organisms via water.

Key Processes

1. Weathering and Release

Phosphorus is found in rocks as phosphate minerals (e.g. apatite). Weathering (physical and chemical breakdown of rock) and erosion release phosphate ions (PO₄³⁻) into the soil and waterways.

2. Absorption by Plants

Plants absorb inorganic phosphate (PO₄³⁻ / HPO₄²⁻) from the soil solution through their roots, often assisted by mycorrhizal fungi (mutualistic associations between fungi and plant roots that greatly increase the absorptive surface area). Phosphate is incorporated into organic molecules (ATP, DNA, phospholipids).

3. Transfer Through Food Chains

Animals obtain phosphorus by consuming plants or other animals. Phosphorus is recycled within ecosystems through:

• Decomposition — saprobiotic organisms break down dead organic matter, releasing phosphate back into the soil
• Excretion — animal waste products contain phosphorus

4. Sedimentation

Phosphate that enters waterways eventually settles in marine sediments, where it becomes incorporated into new rock over geological time scales. This sedimentary phosphorus is only released back into the cycle through tectonic uplift and weathering — making the phosphorus cycle very slow compared to the nitrogen or carbon cycles.

5. Human Impact

• Fertiliser application adds phosphate to agricultural soils, but excess can run off into waterways
• Eutrophication — excess phosphate (and nitrate) in water bodies causes algal blooms → decomposition of algae depletes O₂ → fish and other organisms die (see Succession & Conservation notes)
• Mining of phosphate rock for fertiliser is depleting finite reserves — phosphorus is a non-renewable resource on human timescales

Comparison of Nitrogen and Phosphorus Cycles

Feature	Nitrogen cycle	Phosphorus cycle
Gaseous phase?	Yes (N₂ in atmosphere)	No
Main reservoir	Atmosphere (78% N₂)	Rocks and sediments
Main form absorbed by plants	NO₃⁻ (and NH₄⁺)	PO₄³⁻
Specialist bacteria involved?	Yes (many types)	Decomposers only (no specialist fixers)
Speed of cycle	Relatively fast	Very slow (geological timescales for sedimentary return)
Human disruption	Fertilisers, fossil fuel combustion	Fertilisers, mining

Exam Tips

• AQA expects you to name specific bacteria for each stage of the nitrogen cycle — Rhizobium, Nitrosomonas, Nitrobacter, Pseudomonas are essential
• Always specify the conditions: nitrification needs aerobic conditions; denitrification occurs in anaerobic conditions
• When explaining the importance of nitrogen fixation, state that N₂ has a triple bond that most organisms cannot break
• For phosphorus, emphasise that there is no gaseous phase — this is the key difference from nitrogen and carbon cycles
• Link nutrient cycles to agriculture — explain why farmers use fertilisers, plough fields, rotate crops with legumes, and drain waterlogged soil