6.2 Terrestrial and marine ecosystems in the carbon cycle

Carbon storage in ecosystems is a crucial part of the global carbon cycle. Terrestrial and marine ecosystems play different roles, with land storing more carbon in biomass and soils, while oceans have a larger active carbon pool due to CO2 solubility in seawater.

Photosynthesis and respiration are key processes that drive carbon exchange between ecosystems and the atmosphere. The balance between these processes determines whether an ecosystem acts as a carbon sink or source, influencing atmospheric CO2 levels and Earth's climate.

Carbon Storage and Cycling in Ecosystems

Carbon storage in ecosystems

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• Terrestrial ecosystems
  • Store carbon in living biomass including plants (forests, grasslands) and animals (insects, mammals) and in soils (organic matter, humus)
  • Photosynthesis by plants absorbs CO2 from the atmosphere converts it into organic compounds (glucose, cellulose)
  • Respiration by plants and animals breaks down organic compounds releases CO2 back to the atmosphere
  • Decomposition of dead organic matter by microbes releases CO2 and incorporates carbon into soils (peat, permafrost)
• Marine ecosystems
  • Store carbon in living biomass such as phytoplankton (diatoms, dinoflagellates), algae (kelp, seagrass), and marine animals (fish, whales) and dissolved in seawater as dissolved inorganic carbon (DIC)
  • Photosynthesis by phytoplankton and algae takes up CO2 from the atmosphere and surface waters converts it into organic compounds
  • Respiration by marine organisms breaks down organic compounds releases CO2 back to the water and atmosphere
  • Sinking of dead organisms and fecal pellets transports carbon to deep ocean sediments known as the biological pump (marine snow, whale falls)
• Comparison
  • Terrestrial ecosystems store more carbon in living biomass and soils (1,500 Gt C) than marine ecosystems (40 Gt C)
  • Marine ecosystems have a larger active carbon pool (38,000 Gt C) due to the high solubility of CO2 in seawater forming carbonic acid and bicarbonate ions
  • Turnover of carbon is faster in marine ecosystems (days to years) due to rapid cycling in the surface ocean compared to slower turnover in terrestrial ecosystems (decades to centuries)

Photosynthesis in carbon cycle

• Photosynthesis
  • Process by which plants and phytoplankton convert CO2 and water (H2O) into organic compounds (C6H12O6) using light energy captured by chlorophyll
  • Removes CO2 from the atmosphere and surface ocean waters stores it in living biomass
  • Produces oxygen (O2) as a byproduct released to the atmosphere and ocean
• Respiration
  • Process by which organisms break down organic compounds (glucose) to release energy in the form of ATP
  • Releases CO2 back into the atmosphere and ocean waters as a waste product
  • Consumes oxygen (O2) in the process of cellular respiration
• Balance between photosynthesis and respiration
  • Determines the net exchange of CO2 between ecosystems and the atmosphere on short timescales (diurnal, seasonal)
  • Influences the concentration of atmospheric CO2 and the Earth's greenhouse effect and climate
  • Excess photosynthesis over respiration leads to net CO2 uptake (carbon sink), while excess respiration leads to net CO2 release (carbon source)

Carbon Sequestration and Human Impacts

Carbon sequestration processes

• Soil carbon sequestration
  1. Accumulation of organic carbon in soils through plant growth (roots, litter) and incomplete decomposition
  2. Influenced by factors such as climate (temperature, moisture), vegetation type (grasses, trees), and soil properties (clay content, pH)
  3. Can be enhanced through land management practices that increase plant productivity and reduce soil disturbance (reforestation, cover crops, reduced tillage)
• Ocean carbon sequestration
  1. Removal of carbon from the surface ocean and atmosphere by physical and biological processes that transport it to the deep ocean
  2. Physical processes: dissolution of CO2 in cold, deep waters (solubility pump) and formation of calcium carbonate minerals (CaCO3) that sink to the seafloor
  3. Biological processes: photosynthesis by phytoplankton that converts CO2 into organic matter and the subsequent sinking of dead organisms and fecal pellets (biological pump)
  4. Can be enhanced through ocean fertilization techniques that stimulate phytoplankton growth (iron fertilization) and alkalinity enhancement that increases the ocean's capacity to absorb CO2 (olivine weathering)

Impacts on carbon cycle

• Land-use change
  • Deforestation and conversion of natural ecosystems to agriculture or urban areas releases stored carbon from living biomass and soils into the atmosphere (Amazon rainforest, Indonesian peatlands)
  • Reforestation and afforestation can increase carbon storage in terrestrial ecosystems by creating new carbon sinks (China's Grain for Green program, African Great Green Wall)
  • Changes in land use can alter the balance between photosynthesis and respiration shifting ecosystems from net carbon sinks to sources or vice versa
• Ocean acidification
  • Increased absorption of atmospheric CO2 by the ocean forms carbonic acid (H2CO3) lowers seawater pH and carbonate ion concentration
  • Affects the ability of calcifying marine organisms to build calcium carbonate shells and skeletons (corals, mollusks, some plankton)
  • Can reduce the efficiency of the biological pump by altering the composition and sinking rates of marine particles and organisms
  • Impacts the ocean's capacity to absorb and store carbon from the atmosphere potentially creating a positive feedback to climate change