Describe the cycling of matter through biogeochemical cycles, including the carbon, nitrogen and water cycles, and evaluate the impact of human activities on these cycles

What this dot point is asking

QCAA wants you to describe the cycling of carbon, nitrogen and water between living organisms and the physical environment, name the biological and physical processes that move atoms between pools, and evaluate human impacts. Diagrams are common stimulus material.

The answer

Energy flows one way through ecosystems, but matter cycles. Atoms of carbon, nitrogen, hydrogen and oxygen move between four reservoirs (atmosphere, hydrosphere, lithosphere and biosphere) through biogeochemical cycles. Human activities have measurably altered all three of the cycles QCAA asks about.

The carbon cycle

Carbon moves between an atmospheric pool (CO2 and methane), an oceanic pool (dissolved CO2, bicarbonate, carbonate), a lithospheric pool (carbonate rocks, fossil fuels) and a biospheric pool (living and dead organic matter).

Key processes.

• Photosynthesis. Plants, algae and cyanobacteria convert atmospheric CO2 and water to glucose using light energy. 6CO2 plus 6H2O yields C6H12O6 plus 6O2.
• Cellular respiration. All living organisms oxidise glucose, releasing CO2 and water and producing ATP.
• Decomposition. Bacteria and fungi break down dead organic matter, releasing CO2 in aerobic conditions and methane (CH4) in anaerobic conditions (wetlands, ruminant guts, rice paddies).
• Ocean exchange. CO2 dissolves into surface ocean water, forming carbonic acid, bicarbonate and carbonate. The ocean stores about 50 times more carbon than the atmosphere.
• Carbonate sedimentation. Calcifying organisms (corals, foraminifera, molluscs) precipitate calcium carbonate. On their death, shells sink and accumulate as limestone over geological time.
• Fossilisation. Buried plant and plankton biomass, transformed by heat and pressure over millions of years, forms coal, oil and natural gas in the lithosphere.
• Combustion. Wildfires and the burning of fossil fuels return carbon directly to the atmosphere as CO2.
• Volcanic outgassing. Slow release of CO2 from the lithosphere through volcanic and metamorphic processes.

Residence times vary enormously: years to decades in the biosphere, centuries in surface ocean water, millennia in deep ocean water, millions of years in fossil fuels and limestone.

Human impacts.

• Fossil fuel combustion has moved roughly 700 billion tonnes of carbon from the lithosphere to the atmosphere since 1750, faster than natural fluxes can absorb. Atmospheric CO2 has risen from about 280 to over 420 ppm.
• Deforestation removes photosynthetic biomass and releases stored carbon. In Queensland alone, land clearing through the 2010s released several tens of millions of tonnes per year.
• Cement production releases CO2 from limestone during calcination.
• Ocean acidification. Excess atmospheric CO2 dissolves in seawater, lowering pH and reducing carbonate availability for shell-building organisms.
• Net effect. A net transfer from the lithospheric and biospheric pools to the atmospheric and oceanic pools, driving global warming and acidification.

The nitrogen cycle

Nitrogen is the most abundant atmospheric gas (78 per cent of air as N2) but is unusable to most organisms in that form because of the strong triple bond. Specialist microorganisms perform the key transformations.

Key processes.

• Nitrogen fixation. N2 to ammonia (NH3) or ammonium (NH4 plus).
  • Biological fixation by free-living soil bacteria (Azotobacter, cyanobacteria) and symbiotic bacteria (Rhizobium in legume root nodules, Frankia in casuarinas).
  • Abiotic fixation by lightning, which oxidises atmospheric N2 to nitrogen oxides that dissolve in rain.
• Nitrification. Aerobic soil bacteria oxidise ammonium to nitrite then nitrate.
  • Nitrosomonas converts NH4 plus to NO2 minus.
  • Nitrobacter converts NO2 minus to NO3 minus.
• Assimilation. Plants absorb nitrate (and some ammonium) from soil and incorporate it into amino acids, proteins and nucleotides. Animals obtain nitrogen by eating plants or other animals.
• Ammonification (decomposition). Decomposer bacteria and fungi break down organic nitrogen in dead matter and excretory waste back to ammonium.
• Denitrification. Anaerobic bacteria (Pseudomonas) reduce nitrate to N2 gas, returning it to the atmosphere. Occurs in waterlogged soils and aquatic sediments.

Australian soils are naturally low in nitrogen, and native ecosystems rely on biological fixation (legumes, cyanobacteria in soil crusts) and slow ammonification.

Human impacts.

• Haber to Bosch industrial fixation produces ammonia from atmospheric N2 for fertilisers, roughly doubling the global rate of reactive nitrogen production.
• Fertiliser runoff transports nitrate into waterways, driving eutrophication in catchments such as the Great Barrier Reef lagoon, where algal blooms reduce light to seagrass and contribute to coral decline.
• Combustion of fossil fuels releases nitrogen oxides (NOx) into the atmosphere, contributing to smog and acid rain.
• Land clearing removes deep-rooted vegetation, allowing nitrate to leach below the root zone and into groundwater.
• Nitrous oxide (N2O) released by denitrifiers acting on excess fertiliser is a greenhouse gas about 300 times stronger than CO2 per molecule.

The water cycle

Water moves between the atmosphere (water vapour), the hydrosphere (oceans, lakes, rivers, groundwater), the cryosphere (ice and snow) and the biosphere (water inside organisms).

Key processes.

• Evaporation. Liquid water becomes vapour, drawing energy from the surroundings. Highest from warm ocean surfaces.
• Transpiration. Water absorbed by plant roots is lost as vapour through stomata. Forests transpire enormous volumes; the Amazon basin recycles a large fraction of its rainfall this way.
• Evapotranspiration is the sum of evaporation and transpiration.
• Condensation. Water vapour cools and forms cloud droplets.
• Precipitation. Rain, snow, hail and dew return water to the surface.
• Infiltration and percolation. Water soaks into soil and downward into groundwater (aquifers).
• Surface runoff. Excess water flows over the surface to streams and rivers, returning to the ocean.
• Storage. Glaciers, ice caps and aquifers store water on timescales of years to millennia.

Human impacts.

• Land clearing removes deep-rooted vegetation, reducing transpiration and infiltration and increasing runoff. In the Murray to Darling Basin and the Western Australian wheatbelt, this has raised water tables and brought salt to the surface, causing dryland salinity.
• Dam construction and river regulation alter natural flow regimes, evaporative losses and groundwater recharge.
• Urbanisation replaces permeable soil with impermeable surfaces, reducing infiltration, increasing flash flooding and lowering groundwater recharge.
• Groundwater extraction for irrigation and town water depletes aquifers faster than they recharge in many parts of Australia.
• Climate change intensifies the hydrological cycle: more evaporation, more extreme rainfall events, longer droughts, and reduced snowpack feeding rivers.

How the three cycles connect

The cycles are not independent.

• Photosynthesis and respiration drive both carbon and water cycles (photosynthesis splits water; respiration produces water).
• Decomposition releases carbon (as CO2), nitrogen (as ammonium) and water back to their pools simultaneously.
• Climate change alters all three: warmer air holds more water vapour, oceans absorb more CO2, and warmer soils respire faster and release more N2O.

Examples in context

Example 1. Great Barrier Reef nitrogen cycle and Crown-of-Thorns outbreaks. Agricultural run-off from north Queensland sugarcane carries dissolved inorganic nitrogen (DIN) into Great Barrier Reef catchments. Nitrifying bacteria in soils convert ammonium to nitrate, which leaches via the Burdekin and Tully rivers onto the inner reef shelf. Excess nitrate fuels phytoplankton blooms that boost survival of Crown-of-Thorns starfish larvae. AIMS modelling links each major outbreak (2010s, 2020s) to wet-season DIN pulses above 1 million tonnes per year. The Reef 2050 Plan targets a 60 percent reduction in DIN by 2030 via fertiliser efficiency and wetland restoration, illustrating direct human impact on the nitrogen cycle and its ecological consequences.

Example 2. Daintree rainforest carbon cycle. Daintree rainforest north of Cairns sequesters carbon at roughly 6 to 10 tonnes per hectare per year via photosynthesis (CO2 to biomass), while decomposers and respiration return some via CO2 to the atmosphere. Tree death and litter feed the soil organic carbon pool. Indigenous fire management (cool burns) historically maintained carbon storage; uncontrolled hot fires after 2019 released stored carbon back to the atmosphere as CO2. Queensland Government carbon credit schemes value standing Daintree forest at roughly AUD 30 per tonne of CO2 equivalent stored, monetising the cycling pool and creating a financial incentive to keep the forest intact.

Try this

Q1. Describe the role of nitrogen-fixing bacteria in the nitrogen cycle and name two genera commonly found in Queensland legume root nodules. [3 marks]

• Cue. Convert N2 to ammonium for plant uptake; Rhizobium, Bradyrhizobium.

Q2. A bar chart shows annual carbon flux: photosynthesis 120, plant respiration 60, soil respiration 55, fossil-fuel emissions 10 gigatonnes/year. Calculate net atmospheric change and predict the trend over 50 years. [3 marks]

• Cue. Net +5 Gt/year; CO2 rises by 250 Gt over 50 years; atmospheric concentration climbs.

Q3. Refer to the water cycle in tropical north Queensland. (a) Identify three processes by which water enters the atmosphere. (b) Explain how deforestation affects evapotranspiration. (c) Justify whether dams reduce or increase regional rainfall. [2+2+2 marks]

• Cue. (a) Evaporation, transpiration, sublimation. (b) Less transpiration; lower humidity. (c) Reduces upstream flow; complex regional effects.