The nitrogen cycle and the phosphorus cycleAQA A-Level Biology: Revision notes

Nutrients are recycled within natural ecosystems; the nitrogen and phosphorus cycles are examples. Microorganisms play a vital role in recycling elements such as nitrogen and phosphorus.

Saprobionts (many bacteria and fungi) feed on dead organic matter and waste by saprobiotic nutrition: they secrete enzymes onto the material (extracellular digestion) and absorb the soluble products. This decomposition releases mineral ions such as ammonium and phosphate that can be absorbed by plants, so the nutrients are recycled.

Mycorrhizae are mutualistic associations between fungi and the roots of plants. The fungal hyphae form a network that extends much further into the soil than the roots, increasing the surface area for absorption. They facilitate uptake of water and inorganic ions (especially phosphate) by the plant. In return the fungus receives organic compounds, such as sugars, from the plant. Mycorrhizae are particularly important in soils that are poor in phosphate.

Plants absorb nitrogen as nitrate ions and use it to make amino acids, proteins and nucleotides. Bacteria carry out the main conversions (species need not be named):

• Nitrogen fixation: $\text{N}_2 \rightarrow \text{NH}_3/\text{NH}_4^+$, by free-living bacteria in the soil and by bacteria in the root nodules of legumes, such as clover.
• Ammonification: saprobionts decompose proteins and urea in dead material and waste, releasing ammonia and ammonium ions.
• Nitrification: nitrifying bacteria oxidise ammonium ions to nitrite ($\text{NO}_2^-$) and then to nitrate ($\text{NO}_3^-$). This needs oxygen.
• Denitrification: in anaerobic (waterlogged) soil, denitrifying bacteria convert nitrate to nitrogen gas, which is lost to the atmosphere.

The main store of phosphorus is rock. Weathering releases phosphate ions into the soil. Plants absorb them, with the help of mycorrhizae, and use them to make ATP, nucleic acids and phospholipids. Phosphate passes to consumers when they feed, and returns to the soil in dead remains, faeces and urine. Saprobionts decompose this material and release phosphate ions, which can be absorbed again.

Unlike the nitrogen cycle, there is no gaseous stage, so there is no equivalent of nitrogen fixation or denitrification, and the phosphate is cycled again and again between the soil, the trees and the decomposers.