The general structure of biogeochemical cycles with their stores and fluxes, the phosphorus and sulfur cycles, the role of decomposers, and how human activity alters these cycles.

What this dot point is asking

AQA wants you to describe the general structure of biogeochemical cycles in terms of stores and fluxes, describe the phosphorus and sulfur cycles, explain the role of decomposers, and explain how human activity alters these cycles. The strongest answers use the stores-and-fluxes framework precisely and contrast the two named cycles on the key point of whether they have a gas phase.

Structure of a biogeochemical cycle

Stores differ in size and in how long the element stays in them. Short-term stores (organisms, soil water) cycle the element quickly; long-term stores (rocks, ocean sediments, fossil fuels) hold it for thousands to millions of years. The size of a store divided by the flux out of it gives the residence time. A cycle is in balance when inputs to a store equal outputs; human activity often unbalances cycles by speeding one flux (for example combustion) far beyond the rate at which a slow flux (weathering, sedimentation) can compensate. Whether an element has a gas phase determines how fast it can move globally: gas-phase cycles (carbon, nitrogen, sulfur) can equilibrate worldwide through the atmosphere, while sedimentary cycles (phosphorus) move only as fast as rock and water allow.

The phosphorus cycle

In detail: weathering of phosphate-bearing rock releases dissolved phosphate; plants take it up through roots and build it into DNA, ATP, phospholipids and bone; it transfers through food chains; decomposers release it back to soil from dead matter and waste; and a one-way leak carries phosphate down rivers to the sea, where it settles into sediment. Because the only major input is slow weathering and there is a steady loss to sediment with no atmospheric reservoir to buffer it, available phosphorus is scarce. Phosphate rock is mined for fertiliser, and because reserves are finite and concentrated in a few countries, phosphorus supply is a long-term sustainability concern.

The sulfur cycle

Sulfur moves between rocks, soil, water, organisms and, importantly, the atmosphere as a gas. Natural inputs to the atmosphere include volcanic emissions and the breakdown of organic matter, which releases sulfur compounds. Plants and microbes take up sulfate from soil and water and build it into proteins (sulfur is in some amino acids); decomposition recycles it. The defining human impact is burning fossil fuels, especially coal, which releases large amounts of sulfur dioxide ($SO_2$) into the atmosphere. This dissolves in rainwater to form acid rain (sulfuric and sulfurous acids), which acidifies soils and freshwater, damages forests and corrodes buildings. The presence of a gas phase is exactly why sulfur pollution travels through the air and can fall hundreds of kilometres downwind.

The role of decomposers

Decomposers (bacteria and fungi) are the key flux returning elements from dead matter to circulation. They break down dead organisms and waste, releasing the elements they contain back into the soil and water as soluble inorganic compounds (phosphate, sulfate, ammonium) that plants can reabsorb. Without decomposers, nutrients would stay locked in dead biomass, the soluble nutrient supply would dwindle, and the cycles would slow or stall. Decomposition rate depends on temperature, moisture and oxygen, so cold, waterlogged or very dry conditions slow nutrient release and can cause nutrients to accumulate as peat or litter.

Human disruption

• Fertiliser use adds reactive phosphorus (and nitrogen) to soils; runoff carries it to rivers and lakes, causing eutrophication, algal blooms and oxygen depletion.
• Burning fossil fuels releases sulfur dioxide, driving acid rain and soil and water acidification.
• Mining phosphate depletes a finite long-term store much faster than weathering replaces it.
• Detergents historically added phosphate to waterways, worsening eutrophication before low-phosphate formulations were introduced.

Try this

Q1. Explain why the phosphorus cycle is described as having no significant gas phase, and state one consequence of this. [3 marks]

• Cue. Phosphorus moves through rocks, soil, water and organisms, not the atmosphere; consequently it cycles slowly and is often a limiting nutrient.

Q2. Describe the role of decomposers in a biogeochemical cycle and name two factors that affect their rate of work. [3 marks]

• Cue. They break down dead matter and release soluble nutrients for reuse; rate depends on temperature, moisture and oxygen availability.

Q3. A reservoir holds 8000 kg of phosphorus with a through-flux of 400 kg per year. Calculate the residence time. [2 marks]

• Cue. $8000 / 400 = 20$ years.