Explain the nitrogen cycle and how human activity disrupts it

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.

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.