Nutrient Cycles, Mycorrhizae and Fertilisers

Plants absorb nitrogen-containing ions and use nitrogen to synthesise compounds such as amino acids and nucleotides. Most plants cannot directly use atmospheric nitrogen gas. Nitrogen-fixing microorganisms convert nitrogen gas into biologically usable nitrogen compounds, including in associations with some plant roots.

Saprobionts secrete enzymes onto organic material and absorb soluble digestion products. Ammonification releases ammonium from organic nitrogen. Nitrifying bacteria oxidise ammonium to nitrite and then nitrate under suitable aerobic conditions. Denitrifying bacteria can convert nitrate to nitrogen gas in low-oxygen conditions, returning nitrogen to the atmosphere. These are different reactions, not interchangeable names for decay.

Weathering releases phosphate from rocks into soil or water. Plants absorb phosphate and incorporate it into nucleotides, ATP and phospholipids; feeding transfers it through food webs. Decomposition and excretion return phosphorus-containing compounds to the environment, while sediment formation can place it into longer-term stores.

Mycorrhizal fungi associate with plant roots. Their hyphae explore a large soil volume and can improve water and mineral-ion uptake, including phosphate, while receiving organic compounds from the plant. Explain the exchange of benefits rather than claiming the fungus performs photosynthesis for the plant.

Harvesting crops and removing livestock exports nutrients, so natural or artificial fertilisers can replace nitrogen and phosphorus to support growth. The effect depends on the nutrient actually limiting growth and other conditions. Adding unlimited fertiliser does not produce unlimited photosynthesis or biomass.

Leaching or runoff can transfer nutrients to water, stimulating algal growth where nutrients were limiting. Increased algal biomass can shade submerged plants; subsequent death and microbial decomposition increase oxygen demand. Lower dissolved oxygen can harm aquatic organisms. Explain that respiration by decomposers contributes to oxygen depletion rather than saying nitrate directly consumes all oxygen.

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.