Nutrient Cycles (A Level only) — AQA A Level Biology 7402 Revision Notes & Topical Questions

In a functioning ecosystem the elements that organisms need are constantly recycled. They are incorporated into biological molecules in tissues, and are released back into the environment when decomposers break down dead and waste matter. Nitrogen is present as N₂ gas in the atmosphere and within biological molecules such as proteins.

The nitrogen cycle involves four processes: nitrogen fixation (N₂ converted to ammonia or ammonium by nitrogen-fixing bacteria, free-living or in root nodules), ammonification (nitrogen compounds in dead matter and waste converted to ammonia by saprobionts), nitrification (ammonium oxidised to nitrite and then nitrate by nitrifying bacteria) and denitrification (nitrate converted back to N₂ by denitrifying bacteria in anaerobic, waterlogged soil). Names of bacterial species need not be learned, but the roles must be described.

Plants and animals need phosphorus for making phospholipids, nucleic acids and ATP.

Many biological molecules in plants cannot be made from the products of photosynthesis alone but need extra elements, e.g. proteins and nucleic acids require nitrogen.

Microorganisms recycle elements such as phosphorus and nitrogen. Saprobionts (saprotrophs) decompose dead and waste matter by extracellular digestion, releasing mineral ions such as nitrates and phosphates to the environment; they carry out ammonification. In an exam it is not enough to say 'decomposition' — say that they release mineral ions. Mycorrhizal fungi form symbiotic relationships with plant roots.

In natural ecosystems, decomposition by saprobionts recycles nutrients from waste and dead organisms back into the soil.

Fertilisers are often applied in larger quantities than crops require, so excess mineral ions remain dissolved in soil water. These ions can be carried by rainwater into nearby rivers, lakes or streams; this is called leaching. The risk is greater with artificial fertilisers because the inorganic ions are highly soluble.

Use the precise term leaching; avoid vague statements such as 'fertilisers wash away'.

Link cause and effect at each stage in an answer.

Natural (organic) fertilisers such as manure and compost release minerals slowly as saprobionts decompose them, so less is leached; artificial (inorganic) fertilisers contain soluble ions that are available immediately but are more easily leached. Applying fertiliser only in the amounts crops need, avoiding application before heavy rain, and leaving uncropped margins beside watercourses reduce leaching and so eutrophication.