Nutrient cycles - AL only (3.5.4)

Ecosystems recycle essential nutrients because they are finite and must be reused for life to continue; they cycle between the biotic and abiotic components.

Saprobionts (bacteria and fungi) break down dead organisms and waste. They release inorganic ions (e.g., ammonium, phosphate) back into soil or water.

Mycorrhizae are a mutualistic relationship between certain fungi and the roots of plants.
The fungal hyphae greatly increase the surface area for absorption, allowing the plant to take up more water and mineral ions, particularly phosphate ions, from the soil.

In return, the plant supplies the fungus with organic compounds, such as sugars (produced during photosynthesis) and amino acids.

Ammonification involves saprobiontic organisms (mostly bacteria and fungi) breaking down organic nitrogen in dead organisms and waste into ammonia. This releases ammonium ions into the soil.

Nitrification occurs when ammonium ions are oxidised to nitrite ions (), which are then further oxidised to nitrate ions () by nitrifying bacteria.

This process requires oxygen, so soil must be well aerated.

There are two main types of bacteria that carry out nitrogen fixation:

• Free-living nitrogen-fixing bacteria reduce gaseous nitrogen to ammonia, which is used to produce amino acids.
• Mutualistic nitrogen-fixing bacteria live in nodules on plants (such as legumes), producing amino acids from nitrogen in the air and the soil.

Denitrification involves denitrifying bacteria that convert nitrate ions back into nitrogen gas under anaerobic conditions, returning to the atmosphere.

Phosphorus is recycled as phosphate ions () and unlike carbon and nitrogen, does not have a gaseous phase. Phosphate enters ecosystems through the weathering of phosphate-containing sedimentary rocks.

Plants absorb phosphate ions from the soil or water and use them to produce biomass.

Phosphorus is transferred into the food chain when plants are eaten. Phosphate is returned to the soil or water via excretion, egestion and decomposition of dead organisms by saprobionts.

Some phosphate is washed into rivers and oceans, where it becomes part of sediments and may form rocks over geological timescales.

When crops are harvested or livestock are removed from land, nitrates and phosphates stored in their biomass are taken away from the ecosystem from which they came.

Soil nutrient levels therefore reduce over time which limits plant growth. Fertilisers replace lost nutrients to maintain productivity.

There are two types of fertiliser:

• Natural fertilisers are made from dead / decaying plant and animal matter (e.g., manure, compost, bone meal). Nutrients are released slowly as microorganisms decompose the material.
• Artificial fertilisers are manufactured chemical fertilisers containing specific, concentrated mineral ions, such as nitrates, phosphates, and potassium (NPK fertilisers). They provide nutrients rapidly, in precise amounts.

There are environmental issues associated with the use of fertilisers:

Leaching occurs when excess nitrates / phosphates are washed out of the soil by rainfall. They enter rivers and lakes, reducing water quality. Nitrates in drinking water can harm human health.

Eutrophication is when leached mineral ions cause rapid algal growth on the surface of bodies of water (algal bloom). Dense algae block light, causing plants below to die.

The dead plant matter is decomposed by aerobic bacteria, which use up oxygen from the water. Oxygen concentration falls, causing fish and aquatic animals to die, meaning a collapse in biodiversity.