Introduction to the Carbon Cycle

Introduction to the Carbon Cycle

• It captures the sun's energy, transforming it into chemical energy to sustain life.
• Key chemical forms include carbon dioxide (CO2), organic matter/carbohydrates (CH2O), and calcium carbonate/limestone (CaCO3).
• The main processes are photosynthesis and respiration.

Photosynthesis (Carbon Uptake by Plants)

• The equation is CO2 + H2O → CH2O + O2.
• Sunlight (electromagnetic energy) is converted into stored chemical energy in organic matter.
• Primary production and carbon fixation create the base of the food web.
• Sunlight (energy), nutrients (Nitrogen: NO3- & NH4+; Phosphorus: PO43-), and water are required.
• Remineralization is the breakdown of organic matter, recycling nitrogen, ammonium, and phosphate.

Fixation Processes

• Carbon Fixation converts CO2 into organic biomass
• Nitrogen Fixation transforms atmospheric N2 into organic nitrogen in biomass

Respiration (Carbon Release)

• The equation is CH2O + O2 → CO2 + H2O
• Autotrophic respiration occurs in plants, in the dark.
• Heterotrophic respiration involves microbial decomposition and animal respiration.

Life Forms & Carbon Cycling

• Autotrophs are self-feeding primary producers. Examples:
  • Phototrophs use sunlight, like plants.
  • Chemotrophs use chemical energy.
• Heterotrophs are consumers. Examples:
  • Herbivores eat plants.
  • Carnivores eat other animals.
  • Omnivores eat both.
• Food Web: 90% of energy is lost, with a 10% energy transfer.

Terrestrial vs. Aquatic Ecosystems

• Terrestrial ecosystems are land-based.
• Aquatic ecosystems are water-based, including marine, freshwater, and brackish environments.

Measuring Carbon Flux

• Flux Chambers measure CO2 exchange.
  • Flux is measured as AMT/ M^2 x time.
  • Biomass is measured as Amt/m^2
  • CO2 decreases in full sun due to photosynthesis.
  • CO2 increases in dark conditions due to respiration.

Above & Belowground Carbon Storage

• Aboveground biomass contributes organic matter through leaf litter.
• Belowground root systems release organic matter, storing carbon.
• Biomass is the standing stock of living matter
• Production is new growth of living matter.
• Biomass is at a steady state if production equals death.
• Steady-state inputs equal outputs, with no change in the amount.
• In a steady state, input equals output in the reservoir.
• Residence time is the amount of biomass (Mass/area) /Input mass/(area *time) in units of time; also called turnover time.

Carbon Storage in Different Ecosystems

• Prairies have mostly belowground biomass due to water limitation.
• Forests feature significant aboveground biomass.
• Soil Organic Matter (Dead Carbon Pool) is decomposed by microbes (releasing CO2) and heterotrophic organisms like earthworms.
  • The decomposed carbon is respired back to CO2 and H2O: soil organic matter heterotrophic respiration

Net Carbon Production

• Describes the interaction of the earth's surface and the atmosphere
• GPP (Gross Primary Production) is total CO2 uptake by plants.
• NPP (Net Primary Production) is GPP minus Autotrophic Respiration (biomass growth).

NEP (Net Ecosystem Production)

• Is GPP - Autotrophic Respiration - soil (Heterotrophic) Respiration.
  • Positive NEP means carbon accumulates in biomass and soils.

Biogeochemical Cycles

• Earth's Reservoirs:
  • Lithosphere (Land/Crust)
  • Hydrosphere (Oceans)
  • Atmosphere (Air, least dense & most easily changed)
  • Biosphere (Living organisms)
• Carbon cycles through these reservoirs via photosynthesis, respiration, and sedimentation.

Residence Time & Carbon Turnover

• Residence Time is Biomass / Input Rate.
• The Mean Carbon Residence Time:
  • Plant Biomass: ~4.6 years
  • Fossil Carbon (coal, kerogen): Much longer

Oceanic Carbon Cycle & Ocean Acidification

• CO2 dissolves in water, forming Carbonic Acid (H2CO3):
  • CO2 + H2O → H2CO3 → H+ + HCO3− → H+ + CO32-
  • Ca2+ + CO32− → CaCO3 (Calcium Carbonate - shells, corals, limestone)
• Ocean Buffering resists rapid pH changes.
• Ocean Acidification increases CO2 levels, reducing pH and harming shelled organisms.

Ocean Carbon Zones

• Coastal/Photic Zone (Sunlit Upper 500m):
  • Dominated by photosynthesis, which fixes carbon and produces oxygen.
• Deep Ocean (>500m):
  • Dominated by respiration, with organic matter decomposing (remineralization).
• Particle Flux: ~2% of fixed carbon sinks to the seafloor as sediment.

Limitations on Plant Growth

• Terrestrial limitations: light, water, and nutrients (N, P, K)
• Aquatic limitations: light, nutrients (N, P, Si), and buoyancy adaptations (flotation, oil globules).

Organic Matter Composition

• Includes CH2O (Carbohydrates), and C6H12O6 (Glucose).
• Redfield Ratio (C:N:P = 106:16:1):
  • Marine organic matter follows this ratio.
  • Terrestrial organic matter has a higher C/N ratio (~25).

Summary

• The carbon cycle connects living and non-living systems.
• Photosynthesis captures carbon; respiration releases it.
• Oceans, soils, and vegetation are major carbon reservoirs.
• Human activities impact carbon flux and cause ocean acidification.

An overview of the carbon cycle, including key chemical forms like carbon dioxide and organic matter. It focuses on photosynthesis, where sunlight's energy is captured and converted into chemical energy. Also covered are fixation processes and respiration, detailing how carbon is released back into the environment.