How does nitrogen cycle through ecosystems and how do humans alter it?
Explain the nitrogen cycle and how human activity disrupts it
A focused answer to the WACE Year 12 Earth and Environmental Science dot point on the nitrogen cycle. Covers nitrogen fixation, nitrification, assimilation, ammonification and denitrification, the role of bacteria, and human disruption through fertiliser use and eutrophication, with Australian context.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
What this dot point is asking
SCSA wants you to explain the steps of the nitrogen cycle, the central role of microorganisms, and how human activity overloads the system. The key tension is that nitrogen is abundant in the air as unreactive nitrogen gas, yet often limits plant growth because most organisms cannot use that form directly.
Why nitrogen needs converting
The atmosphere is about 78 percent nitrogen gas, but the strong triple bond in the molecule makes it inert. Plants and animals cannot use nitrogen gas directly; they need it in reactive forms such as ammonium or nitrate. Converting nitrogen between these forms is what the cycle does, and bacteria do most of the work.
The steps of the cycle
- Nitrogen fixation. Specialised bacteria, including those in legume root nodules, and lightning convert nitrogen gas into ammonia or ammonium.
- Nitrification. Soil bacteria oxidise ammonium to nitrite and then nitrate, the form plants take up most readily.
- Assimilation. Plants absorb nitrate and ammonium and build them into proteins and nucleic acids; animals get nitrogen by eating plants.
- Ammonification. Decomposers break down dead organisms and waste, returning nitrogen to the soil as ammonium.
- Denitrification. Other bacteria convert nitrate back to nitrogen gas, completing the cycle by returning nitrogen to the atmosphere.
How humans disrupt the cycle
The biggest human change is the industrial production of nitrogen fertiliser, which fixes atmospheric nitrogen on a vast scale, roughly doubling the natural input of reactive nitrogen to ecosystems.
- Fertiliser runoff. Excess nitrate washes from farmland into rivers, wetlands and the sea.
- Eutrophication. The added nutrients trigger algal blooms; when the algae die and decompose, oxygen is consumed, killing fish and other life. Australian waterways and estuaries, including parts of the Swan-Canning system, have suffered nutrient-driven algal blooms.
- Other effects include nitrous oxide emissions, a potent greenhouse gas, and the leaching of nitrate into groundwater.