The Recycling of Nutrients
A biogeochemical cycle is the pathway by which a chemical substance moves through both the biotic (living) and abiotic (non-living) components of Earth. While energy flows one-way through an ecosystem, matter is recycled.
The Nitrogen Cycle
Nitrogen is an essential component of proteins and nucleic acids, but atmospheric nitrogen (N₂) is unusable by most organisms.
1.Nitrogen Fixation: The conversion of atmospheric nitrogen (N₂) into ammonia (NH₃) or ammonium (NH₄⁺). This is primarily done by nitrogen-fixing bacteria, some of which live in the root nodules of legume plants.
2.Nitrification: The conversion of ammonia/ammonium into nitrites (NO₂⁻) and then to nitrates (NO₃⁻) by other types of bacteria. Nitrates are the form of nitrogen most readily absorbed by plants.
3.Assimilation: Plants absorb nitrates from the soil and use them to build proteins and nucleic acids. Animals get nitrogen by eating plants or other animals.
4.Ammonification: Decomposers break down dead organic matter and waste products, returning nitrogen to the soil in the form of ammonia.
5.Denitrification: The conversion of nitrates back into atmospheric nitrogen gas (N₂) by denitrifying bacteria, completing the cycle.
The Phosphorus Cycle
Phosphorus is a key component of ATP, nucleic acids, and cell membranes.
Key Difference: Unlike the carbon and nitrogen cycles, the phosphorus cycle has no major atmospheric component.
Reservoir: The largest reservoir of phosphorus is in sedimentary rocks.
Process:
1.Weathering of rocks slowly releases phosphate (PO₄³⁻) into soils and water.
2.Plants absorb the phosphate (assimilation).
3.Animals get phosphorus by eating plants.
4.Decomposition of dead organic matter returns phosphate to the soil and water.
5.Phosphates can eventually be incorporated into new sediments, which over geologic time become rock, completing the cycle. Because the geological part of the cycle is so slow, phosphorus is often a limiting nutrient in ecosystems.