In-depth Notes on Plants and the Carbon Cycle

Plants and the Carbon Cycle

• Carbon Dioxide (CO2) in the Atmosphere

  • CO2 is a key player in the global carbon cycle.
  • Plant processes:
  • Photosynthesis: Plants absorb CO2, converting it into organic matter (sugar) using sunlight.
  • Respiration: Plants and other living organisms release CO2 back into the atmosphere after using the organic matter for energy.
  • Natural carbon sinks: Trees, phytoplankton, water bodies, and soil retain carbon.

• Human Impact:

  • The burning of fossil fuels (coal, oil, natural gas) and wood releases additional CO2 into the atmosphere.

Simplified Model of a Nutrient (Biogeochemical) Cycle

• Components:

  • Abiotic Pools/Sinks: Atmosphere, ocean, soil are examples where carbon is stored.
  • Biotic Component: Living organisms (plants, animals, microbes) that partake in the carbon cycle.

• Flux Rate: Refers to the movement of carbon among these pools.

The Global Carbon Cycle

• Total Global Carbon: Estimated at 10^23 g.
  • Inactive Pool: 8.1 x 10^22 g (Includes fossil fuels and other non-cycling carbon).
  • Fossil Fuels: 4000 x 10^15 g.
  • Active Surface Pools: 40,000 x 10^15 g.

Distribution of Global Biomass

• Biomass Composition:
  • Plants dominate Earth’s biomass (over 82%).
  • Micro-organisms (bacteria) are the second largest (13%).
  • Animals constitute only 0.4% of global biomass; humans represent 0.01%.
  • Livestock biomass is significantly higher than that of wildlife.

Measures of Atmospheric CO2 Changes

• Trends:
  • CO2 concentration in the atmosphere has been closely monitored since 1960.
  • Current concentration is around 400 ppm (parts per million), with an increase rate of 0.4%/yr.

CO2 Emissions: Carbon Sources

• Main Sources:
  • Fossil Fuel Combustion: Releases approximately 6 x 10^15 g/yr (e.g., 1 gallon gasoline = 8600 g CO2).
  • Deforestation: Contributes about 0.9 x 10^15 g/yr of CO2.
  • Total Emissions: Approximately 6.9 x 10^15 g/yr.

Carbon Sinks & Budget

• Carbon Balance:
  • Emissions: 6.9 x 10^15 g/yr.
  • Uptake (absorption): 5.2 x 10^15 g/yr.
  • Remaining carbon in the atmosphere: 3.2 x 10^15 g/yr, oceans: 2.0 x 10^15 g/yr, and an unknown sink of 1.7 x 10^15 g.

Consequences of an Altered Carbon Cycle

• Direct Effects: Elevated CO2 levels directly impact ecosystems.
• Indirect Effects: They can enhance the greenhouse effect, contributing to climate change.

Experimental Approaches to Study Effects of Elevated CO2

• Techniques:
  • Use of open-top chambers and Free-Air Carbon Dioxide Enrichment (FACE) methods to monitor ecosystem changes under increased CO2 conditions.

Increased CO2 Effects on Photosynthesis and Growth

• CO2 Fertilization Effect: Higher CO2 levels (e.g., 800 ppm vs 400 ppm) may stimulate photosynthesis and plant growth.
• Nutrient Interaction: Effective growth under increased CO2 often requires sufficient nutrients, as nutrient-poor conditions limit growth.

Variation in Plant Responses to Elevated CO2

• Differential Responses:
  • Different plant species (C3 vs C4) show varied growth responses to increases in CO2 levels.

Plant Community Composition Changes

• Impact on Diversity: Increasing CO2 levels could shift the composition of plant communities, impacting ecosystem dynamics.

Effects on Water Use and Plant Quality

• Stomatal Conductance: Elevated CO2 can alter stomatal density and water use efficiency (WUE) of plants.
• Nutritional Quality: Increased CO2 can modify plant nutritional profiles, affecting food quality and insect herbivory.

Conclusion on Ecosystem Impact

• Cascading Effects: Direct effects on plants will create repercussions for higher trophic levels.
• Ecosystem Process Alteration: Fundamental processes, such as productivity and nutrient cycling, will be influenced by increased CO2.
• Future Implications: CO2 interactions with other global environmental changes will challenge biodiversity and ecosystem health.