Nutrient Cycles: How Nitrogen and Phosphorus Are Reused

Why nutrients must cycle: the role of saprobionts

Earth does not get new atoms of nitrogen or phosphorus. The same atoms are used again and again. This is a nutrient cycle.

Saprobionts (saprotrophs) are decomposer fungi and bacteria. They:

• release enzymes onto dead leaves, dead animals and waste;
• absorb the small soluble molecules (this is extracellular digestion);
• release mineral ions such as ammonium (NH₄⁺) and phosphate (PO₄³⁻) back into the soil.

Without saprobionts, nutrients would stay locked in dead bodies and plants would run short.

The nitrogen cycle

Plants and animals need nitrogen to make amino acids, proteins, nucleic acids (DNA, RNA) and ATP. About 78% of air is N₂ gas, but most organisms cannot use it directly. Bacteria do the key steps.

1. Nitrogen fixation (N₂ → ammonia / ammonium)

• Mutualistic bacteria such as Rhizobium live in root nodules of legumes (peas, beans, pulses). They give the plant nitrogen compounds; the plant gives them sugars.
• Free-living nitrogen-fixing bacteria in soil (e.g. Azotobacter).
• Lightning, and the industrial Haber process that makes fertiliser.

2. Ammonification

Saprobionts turn nitrogen in dead organisms, urea and faeces into ammonia, which forms ammonium ions in soil.

3. Nitrification (needs oxygen)

Nitrifying bacteria oxidise ammonium to nitrite (NO₂⁻) (e.g. Nitrosomonas) and then nitrite to nitrate (NO₃⁻) (e.g. Nitrobacter). They are aerobic, so ploughing and good drainage help. Plants absorb nitrate by active transport.

4. Denitrification (no oxygen)

In waterlogged, anaerobic soil, denitrifying bacteria turn nitrate back into N₂ gas. The soil loses fertility.

The phosphorus cycle

Phosphorus is needed for DNA, RNA, ATP, NADP and phospholipids in membranes. Unlike nitrogen, it has no gas stage, so the cycle is much slower.

1. Weathering and erosion of rocks release phosphate ions into soil and water.
2. Plants absorb phosphate ions (helped by mycorrhizae).
3. Animals get phosphate by eating plants or other animals.
4. Saprobionts release phosphate from dead matter and waste. Guano (seabird droppings) is very rich in phosphate.
5. Phosphate washed into rivers and seas settles as sediment. Over millions of years it becomes rock again, and geological uplift can bring it back to land.

Mycorrhizae

Mycorrhizae are partnerships between fungi and plant roots. Thin fungal threads called hyphae grow into the soil.

• They hugely increase the surface area for absorbing water and mineral ions, especially phosphate (which moves poorly in soil).
• They hold water like a sponge, helping the plant in dry spells.
• The plant gives the fungus sugars (carbohydrates) from photosynthesis.

Both partners benefit, so this is mutualism. Most land plants have mycorrhizae.

Fertilisers, leaching and eutrophication

In a natural ecosystem, ions return to the same soil when things die. On a farm, crops are harvested and taken away, so ions are lost. Farmers add fertilisers to replace them and raise productivity.

• Natural (organic): manure, compost, crop residues. Release ions slowly as they decompose; also improve soil structure.
• Artificial (inorganic): mined or made chemicals such as ammonium nitrate or urea. Exact amounts, fast acting, but more easily washed away.

Leaching

Leaching is when rain dissolves soluble ions (especially nitrate) and carries them below the roots or into streams and lakes. It is worse with artificial fertilisers, heavy rain and adding more than plants can use.

Eutrophication, step by step

1. Leached nitrate or phosphate enters a lake or river.
2. Algae grow fast at the surface (an algal bloom).
3. The bloom blocks light; plants below cannot photosynthesise and die.
4. Saprobiotic bacteria feed on the dead matter, multiply and use up dissolved oxygen in aerobic respiration.
5. Fish and other aerobic animals die.

Ways to reduce it: apply fertiliser in the right amount and season, leave grass strips near rivers, use manure carefully and avoid spreading before heavy rain.

Try it at home

Put pond water in two clear jars on a sunny window. Add a pinch of plant fertiliser to one. Check after one week: which turns greener?

Key formulas and definitions

• Nitrogen fixation: N₂ → NH₃ / NH₄⁺ (Rhizobium, Azotobacter, lightning, Haber process).
• Ammonification: organic N in dead matter → NH₄⁺ (saprobionts).
• Nitrification (aerobic): NH₄⁺ → NO₂⁻ → NO₃⁻.
• Denitrification (anaerobic): NO₃⁻ → N₂.
• Phosphorus cycle: rock → PO₄³⁻ → plants → animals → decomposers → sediment → rock (no gas stage).
• Mycorrhizae = fungus + root mutualism; ↑ surface area for water and phosphate.
• Eutrophication: leaching → algal bloom → light blocked → plants die → bacteria use O₂ → fish die.

Worked examples

Common mistakes

• Saying plants absorb N₂ gas from the air. Most plants absorb nitrate (or ammonium) ions from soil.
• Mixing up nitrification (ammonium → nitrate, needs oxygen) and denitrification (nitrate → N₂, no oxygen).
• Saying algae use up the oxygen that kills fish. It is mainly the decomposing bacteria that use it.
• Giving the phosphorus cycle a gas stage. Phosphorus has no common gas form; it cycles through rock, soil, water and living things.