Describe the carbon, nitrogen, and water cycles and the role of organisms in moving matter.

What this dot point is asking

Why matter cycles but energy does not

Earth is essentially a closed system for matter: the same atoms are used over and over. Energy, by contrast, flows in as sunlight and leaves as heat. So while ecosystems need a constant energy supply, they can recycle their carbon, nitrogen, and water indefinitely. Biogeochemical cycles describe how each element moves between living organisms (the biotic part) and the air, water, and rocks (the abiotic part).

The carbon cycle

Carbon is the backbone of all organic molecules and moves through several key processes:

• Photosynthesis removes carbon dioxide from the atmosphere and fixes it into glucose in producers.
• Respiration by all organisms returns carbon dioxide to the atmosphere.
• Feeding transfers carbon compounds from one organism to the next along food chains.
• Decomposition releases carbon from dead organisms and waste as decomposers respire.
• Combustion of wood and fossil fuels releases stored carbon as carbon dioxide.

Carbon can be stored for long periods in fossil fuels, limestone, and the deep ocean. Burning fossil fuels moves this long-stored carbon into the atmosphere faster than natural processes remove it, which is the main driver of the rise in atmospheric carbon dioxide and climate change.

The nitrogen cycle

Nitrogen is needed for proteins and nucleic acids. Although the air is about seventy-eight percent nitrogen gas, most organisms cannot use it directly. Bacteria do most of the conversion work:

• Nitrogen fixation: certain bacteria (free-living or in legume root nodules) convert nitrogen gas into ammonia or ammonium that plants can use. Lightning also fixes a small amount.
• Nitrification: soil bacteria convert ammonium into nitrites and then nitrates, the form plants absorb most readily.
• Assimilation: plants take up nitrates and build them into proteins, which pass to animals through feeding.
• Ammonification: decomposers break down dead organisms and waste, returning nitrogen to the soil as ammonium.
• Denitrification: other bacteria convert nitrates back into nitrogen gas, returning it to the atmosphere.

The water cycle

Water moves through the environment by physical processes, with organisms playing a part:

• Evaporation turns liquid water from oceans, lakes, and soil into vapour.
• Transpiration releases water vapour from plant leaves, adding significantly to atmospheric moisture.
• Condensation forms clouds as vapour cools.
• Precipitation returns water to the surface as rain, snow, or hail.
• Runoff and infiltration move water across and into the land, eventually back to the sea.

Plants link the water cycle to the rest of the ecosystem by drawing water up from the soil and releasing it through transpiration, a process explored further in the plant transport notes.

The role of decomposers and microbes

Across all three cycles, decomposers and microbes are essential. Decomposers release carbon and nitrogen from dead matter back into circulation, and specialised bacteria carry out the nitrogen conversions that no other organisms can. Without them, nutrients would stay locked in dead bodies and waste, and the cycles would grind to a halt.

Human impact on cycles

Human activity disturbs all three cycles. Burning fossil fuels and clearing forests add carbon dioxide to the air. Fertiliser use adds large amounts of fixed nitrogen, which can run off into waterways and cause algal blooms. Land clearing and dams alter the water cycle. Understanding the natural cycles helps explain why these disturbances cause problems and how they might be managed.