The carbon cycle and land cover for QCE Geography Unit 3

What this dot point is asking

QCAA wants you to explain how land cover sits inside the global carbon cycle: where carbon is stored, how it moves (fluxes), and how transforming land cover turns a carbon sink into a carbon source. This is the mechanism that links land cover change to climate change, so you need to handle it precisely. The command word "explain" means you trace the cause and effect from a land cover change (clearing a forest) through to a carbon flux (release of stored carbon to the atmosphere) and an outcome (more atmospheric carbon dioxide). Strong answers name the stores, distinguish a store from a flux, and quantify with real places.

The answer

Stores and fluxes

The carbon cycle is the continuous movement of carbon between stores. A store (or reservoir) holds carbon; a flux is a flow of carbon between stores. The four major stores are the atmosphere (as carbon dioxide and methane), the oceans (the largest active store), the land (living biomass plus dead organic matter and soil carbon) and geological reservoirs (fossil fuels, limestone). Land cover is the visible part of the land store, and it is the part humans transform most directly.

Photosynthesis, respiration and sequestration

Plants draw carbon dioxide from the atmosphere through photosynthesis and lock the carbon into leaves, wood, roots and, over time, soil organic matter. This removal is called sequestration. Respiration and decomposition return carbon to the atmosphere. When sequestration exceeds release, the land cover is a net sink. When release exceeds sequestration, it is a net source. A mature, undisturbed forest is roughly in balance but holds a vast accumulated store; a young, growing forest is an active sink.

Where land carbon is stored

• Forests. Tropical, temperate and boreal forests store carbon in trunks, branches, roots and litter. Boreal forests and their underlying soils are among the largest terrestrial stores.
• Soils. Soil organic carbon globally holds more carbon than the atmosphere and all vegetation combined. Ploughing and erosion expose and release it.
• Peatlands. Waterlogged peat accumulates partly decomposed plant matter over thousands of years, storing immense carbon per hectare.
• Blue carbon ecosystems. Mangroves, saltmarsh and seagrass meadows sequester carbon rapidly and bury it in waterlogged sediments. Australia holds globally significant blue carbon along its coasts.
• Grasslands and rangelands. Most of their carbon is below ground in roots and soil rather than in visible biomass.

How transformation creates a carbon source

Transforming land cover releases stored carbon. Clearing and burning forest converts living biomass directly to atmospheric carbon dioxide. Even where timber is removed rather than burnt, decomposition and soil disturbance release carbon over years. Draining peatland for agriculture exposes peat to oxygen, so it decomposes and oxidises, releasing carbon for decades. Converting forest to cropland or pasture replaces a high carbon cover with a low carbon one, so the landscape holds far less carbon permanently. This is why land use change is one of the largest sources of human carbon emissions after fossil fuel combustion.

The two-way link to climate

Land cover and climate form a feedback relationship. Clearing forest adds carbon dioxide, which warms the climate; a warmer, drier climate then stresses remaining forest, increasing fire and dieback that release still more carbon. The Amazon illustrates this: parts of the south-eastern Amazon have shifted from a net sink toward a net source as clearing, drought and fire combine.

Examples in context

Example 1. Queensland brigalow and regrowth. Clearing brigalow scrub for cropping and grazing released stored biomass and soil carbon across the twentieth century. Allowing regrowth and retaining remnant vegetation lets the land act as a sink again, which is why vegetation management policy has a direct carbon consequence.

Example 2. Tropical peatlands. Draining and burning peat swamp forest in South East Asia for plantations releases carbon that took millennia to accumulate, producing emissions out of all proportion to the area cleared.

Example 3. Australian blue carbon. Mangroves and seagrass along the Queensland coast sequester and bury carbon rapidly. Protecting and restoring these habitats is a recognised land cover response that keeps carbon in the store rather than the atmosphere.