The nitrogen cycle and its disruption: WACE Year 12 Earth and Environmental Science
“Explain the nitrogen cycle and how human activity disrupts it”
Nitrogen cycles between the atmosphere, soils, living things and water through steps driven mostly by bacteria: fixation converts inert atmospheric nitrogen into usable forms, nitrification and assimilation move it into living tissue, ammonification recycles it from dead matter, and denitrification returns it to the air. Humans disrupt the cycle by manufacturing fertiliser, which floods ecosystems with reactive nitrogen and causes problems such as eutrophication.
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.
Most steps of the nitrogen cycle depend on specific microorganisms. Nitrogen-fixing bacteria, nitrifying bacteria, decomposers and denitrifying bacteria each handle one conversion, so soil microbial communities effectively control nitrogen availability for whole ecosystems.
Nitrogen fixation: converting nitrogen gas into usable ammonia or ammonium. Nitrification: oxidising ammonium to nitrate. Assimilation: incorporating nitrogen into living tissue. Ammonification: releasing ammonium from dead matter. Denitrification: returning nitrogen gas 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.
Start with the cause: fertiliser runoff adds nitrate (and phosphate) to a water body. Then the chain of effects: excess nutrients cause rapid algal growth; the bloom blocks light and then dies; decomposers consume oxygen as they break it down; low oxygen kills fish and invertebrates. Linking the human nitrogen input to oxygen depletion is the marking point.
The Haber process and the scale of human change
The single biggest disruption to the global nitrogen cycle is industrial nitrogen fixation through the Haber process, which combines atmospheric nitrogen with hydrogen under high temperature and pressure to make ammonia for fertiliser. This one technology now fixes a quantity of reactive nitrogen comparable to all natural fixation combined, so humans have roughly doubled the rate at which nitrogen enters the biosphere in reactive form. The benefit is enormous (the food supply for billions depends on it), but the consequence is a flood of reactive nitrogen that natural denitrification cannot keep pace with, so reactive nitrogen accumulates in soils, water and the atmosphere. This is why SCSA frames the nitrogen cycle as a system humans have fundamentally altered, not merely nudged, and why the impacts (eutrophication, nitrate in groundwater, nitrous oxide emissions) appear at local, regional and global scales at once.
- Saying plants use nitrogen gas directly
- They need reactive forms such as nitrate or ammonium, produced by fixation and nitrification.
- Leaving out bacteria
- Microorganisms drive most steps; omitting them loses marks.
- Confusing nitrification and nitrogen fixation
- Fixation makes nitrogen gas usable; nitrification converts ammonium to nitrate.
- Stopping at runoff for eutrophication
- You must explain the oxygen depletion that follows the algal die-off.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2022-style6 marksA flow diagram of the nitrogen cycle shows arrows labelled W, X, Y and Z connecting atmospheric nitrogen gas, soil ammonium, soil nitrate and plant protein. Describe the process represented by each of nitrogen fixation, nitrification, assimilation and denitrification, and state the type of organism responsible for each where relevant.Show worked answer →
A 6 mark answer rewards a correct process description and the organism for each step.
- Nitrogen fixation
- Converts inert atmospheric nitrogen gas into ammonia or ammonium, carried out by nitrogen-fixing bacteria (for example in legume root nodules); a small amount is fixed by lightning.
- Nitrification
- Soil bacteria oxidise ammonium to nitrite and then nitrate, the form plants take up most readily; carried out by nitrifying bacteria.
- Assimilation
- Plants absorb nitrate and ammonium and build them into proteins and nucleic acids; animals assimilate nitrogen by eating plants. No bacteria needed.
- Denitrification
- Bacteria convert nitrate back to nitrogen gas under low-oxygen conditions, returning nitrogen to the atmosphere; carried out by denitrifying bacteria.
Markers reward an accurate one-line process for each step and naming the responsible bacteria for fixation, nitrification and denitrification.
WACE 2020-style7 marksExplain how the industrial production and use of nitrogen fertiliser disrupts the nitrogen cycle, and evaluate one strategy farmers could use to reduce this impact.Show worked answer →
A 7 mark answer needs the disruption explained plus an evaluated strategy.
Disruption. Industrial fixation manufactures reactive nitrogen on a vast scale, roughly doubling the natural input to ecosystems. Applied as fertiliser beyond crop uptake, the surplus nitrate leaches into groundwater and runs off into rivers and estuaries, driving eutrophication (algal blooms, then oxygen depletion and fish kills). Denitrification of the excess also releases nitrous oxide, a potent greenhouse gas.
Strategy and evaluation. Matching fertiliser application to crop demand (precision application, split timing, soil testing) reduces the surplus available to leach. Strength: it directly cuts runoff and nitrous oxide while saving cost. Limitation: it requires testing, equipment and good rainfall prediction, and weather can still wash applied nitrogen off before uptake. Buffer strips of vegetation along waterways are a complementary measure. The judgement: demand-matched application is effective and economically attractive but cannot fully eliminate losses.
Markers reward the scale-of-fixation and runoff-to-eutrophication chain plus a strategy with a balanced evaluation.