Nutrient Cycles (Carbon & Water)

Carbon Cycle

The carbon cycle describes how carbon atoms move through the atmosphere, oceans, living organisms (biosphere), and the Earth's crust. Carbon is stored in three main places:

• Atmosphere: Carbon dioxide (CO₂) gas.
• Oceans: Dissolved CO₂ and marine organisms.
• Biosphere: Carbon in plants, animals, and soil organic matter.

Key processes in the carbon cycle include:

• Photosynthesis: Plants absorb CO₂ from the atmosphere and convert it into glucose and oxygen using sunlight. This stores carbon in plant biomass.
• Respiration: Plants, animals, and microbes release CO₂ back into the atmosphere by breaking down glucose for energy.
• Combustion: Burning fossil fuels or biomass releases stored carbon as CO₂ into the atmosphere.
• Decomposition: When plants and animals die, decomposers (microbes and fungi) break down organic matter, releasing CO₂ into the atmosphere or soil.

Plants act as carbon sinks by absorbing CO₂ during photosynthesis. Animals contribute by consuming plants and respiring CO₂. Microbes are essential in decomposition, recycling carbon back to the atmosphere or soil.

Human activities have disrupted the carbon cycle:

• Deforestation: Reduces the number of trees absorbing CO₂, increasing atmospheric carbon.
• Fossil fuel burning: Releases large amounts of stored carbon rapidly, increasing greenhouse gases and contributing to climate change.

For instance, tropical rainforests store vast amounts of carbon. When cleared, not only is this carbon storage lost, but the burning or decay of trees releases CO₂.

Example: If a tree absorbs 10 $\mathrm{kg}$ of CO₂ per year through photosynthesis and respires 4 $\mathrm{kg}$ back, the net carbon stored is 6 $\mathrm{kg}$ annually.

Water Cycle

The water cycle describes the continuous movement of water through the atmosphere, oceans, land, and living organisms. Water is stored mainly in:

• Oceans: The largest water store, holding about 97% of Earth's water.
• Atmosphere: Water vapour and clouds.
• Groundwater: Water stored underground in soil and rock.

Key processes in the water cycle include:

• Evaporation: Water changes from liquid to vapour, mainly from oceans, lakes, and rivers.
• Condensation: Water vapour cools and forms clouds.
• Precipitation: Water falls as rain, snow, sleet, or hail.
• Infiltration: Water soaks into the soil and replenishes groundwater.
• Runoff: Water flows over the land surface into rivers, lakes, and oceans.

The water cycle supports ecosystems by providing fresh water essential for plants and animals. Plants absorb water through roots, which is then transpired into the atmosphere, linking the water and carbon cycles.

Human activities impact the water cycle:

• Water extraction: Excessive removal of groundwater or surface water can reduce water availability for ecosystems.
• Pollution: Contaminants from agriculture, industry, and urban areas pollute water sources, harming aquatic life and reducing water quality.

Example: If $1000\,\mathrm{L}$ of water evaporate from a lake, $800\,\mathrm{L}$ fall back as precipitation locally, and $200\,\mathrm{L}$ flow out as runoff, the water balance is maintained.

Interdependence of Carbon and Water Cycles

The carbon and water cycles are closely linked and influence climate and ecosystems:

• Plants: Use water in photosynthesis to absorb carbon dioxide, linking the two cycles.
• Transpiration: Plants release water vapour, affecting humidity and cloud formation, which influences carbon storage by affecting plant growth.
• Climate regulation: Carbon dioxide is a greenhouse gas that affects temperature and rainfall patterns, which in turn influence the water cycle.
• Feedback mechanisms: For example, increased CO₂ can lead to higher temperatures, increasing evaporation and possibly drought, which reduces plant growth and carbon absorption.

Ecosystems depend on the balance of both cycles. Disruption in one cycle can affect the other, impacting biodiversity and climate stability.

Example: In a drought, reduced water availability limits photosynthesis, lowering carbon uptake by plants and increasing atmospheric CO₂, which can worsen climate change.