How Do Nitrogen and Phosphorus Cycle Through Ecosystems?
Learn how nitrogen and phosphorus cycle through ecosystems, including decomposition, bacterial processes, rock weathering and a common AQA Biology exam mistake.
Nutrients cycle through ecosystems because organisms take them up, pass them along food chains and return them to the environment through waste and decomposition. The nitrogen cycle includes transfers between the atmosphere, soil and living organisms. The phosphorus cycle mainly involves rocks, soil, water and living organisms.
Nitrogen cycle
Most organisms cannot use nitrogen gas directly from the air. Nitrogen-fixing bacteria convert it into ammonium compounds that plants can use. Nitrifying bacteria convert ammonium into nitrites and then nitrates. Plants absorb nitrates through their roots and use the nitrogen to make amino acids and proteins. Animals obtain nitrogen by eating plants or other animals.
When organisms produce waste or die, saprobionts (decomposers that feed on dead organic matter) break down nitrogen-containing compounds. This releases ammonium into the soil, a process called ammonification. Some ammonium is nitrified again. In waterlogged, low-oxygen soil, denitrifying bacteria convert nitrates back into nitrogen gas, returning it to the atmosphere.
Fertilisers can increase the availability of nitrate ions and improve plant growth. However, nitrate ions may leach into waterways. This can cause eutrophication: rapid algal growth reduces light, plants die, and decomposition uses oxygen, harming aquatic organisms.
Phosphorus cycle
Weathering of phosphate-containing rocks releases phosphate ions into soil and water. Plants absorb these ions and use phosphorus to make molecules such as DNA and ATP. Animals obtain phosphorus through feeding. Saprobionts break down waste and dead organisms, releasing phosphate ions for plants to absorb again. Some phosphate is carried into water and becomes part of sediments, which can eventually form rock. Unlike nitrogen, phosphorus has no major atmospheric stage in its cycle.
Phosphorus may therefore remain locked in rocks or sediments for long periods. This makes weathering an important source of phosphate for ecosystems.
Exam mistake to avoid: decomposition does not return nitrogen gas directly to the air. Ammonification releases ammonium; denitrification returns nitrogen gas to the atmosphere.
