Organic and Inorganic Carbon Stores
- Carbon exists in both organic and inorganic forms across different environmental reservoirs.
- These carbon stores play a crucial role in the carbon cycle, regulating atmospheric CO₂ levels and maintaining ecosystem balance.
Organic Carbon Stores:
- Organic carbon is stored in living organisms (plants, animals, microorganisms) and in fossilized remains such as crude oil, coal, and natural gas.
- These stores represent carbon that was once part of biological material and can be released back into the system through decomposition, combustion, or consumption.
- Living organisms (plants, animals, microbes) store carbon in their biomass through photosynthesis.
- Dead organic matter (like decaying plants and animals) contributes to soil carbon as decomposers break down organic material.
- Fossil fuels (such as crude oil, natural gas, and coal) are long-term organic carbon stores, formed over millions of years from buried organic material under high pressure and heat.
Inorganic Carbon Stores:
- Inorganic carbon exists as dissolved CO₂, carbonates, and bicarbonates in water, air, and soil.
- It does not directly form part of living organisms.
- Atmosphere: Carbon dioxide (CO₂) and methane (CH₄) are key greenhouse gases regulating Earth's climate.
- Soils: Carbonates in soil minerals help store carbon over long periods.
- Oceans: Oceans absorb CO₂ from the atmosphere, where it reacts with water to form carbonic acid (H₂CO₃), bicarbonates, and calcium carbonates (which marine organisms use to build shells).
- Sedimentary Rocks: Limestone (CaCO₃) and dolomite act as massive carbon reservoirs formed from marine organisms over geological timescales.
Equilibrium in Carbon Stores
- A carbon store is in equilibrium when the rate of carbon absorption equals the rate of carbon release.
- This dynamic balance ensures that the carbon content of the store remains relatively stable over time.
- Absorption occurs when carbon moves into a store (e.g., photosynthesis removes CO₂ from the atmosphere, oceans absorb CO₂).
- Release occurs when carbon exits a store (e.g., respiration, decomposition, volcanic activity, fossil fuel combustion).
- Equilibrium ensures that atmospheric CO₂ remains stable, supporting a balanced climate and ecosystem function.
Human activities (such as burning fossil fuels and deforestation) are disrupting this equilibrium by releasing more carbon into the atmosphere than natural systems can absorb, leading to climate change.
Residence Time of Carbon
Definition
Residence time in carbon cycle
Residence time is the average duration that a carbon atom remains in a particular store before moving to another part of the carbon cycle.
Different stores have vastly different residence times, depending on how quickly carbon moves in and out of them.
| Carbon store | Approximate residence time |
|---|---|
| Atmosphere | Few years |
| Plants & soil | Decades to centuries |
| Fossil fuels | Millions of years |
| Limestone & sedimentary rocks | Ten to hundreds of millions of years |
| Oceans (deep) | Centuries to millenia |
Carbon Flows in Ecosystems
- Carbon moves between organic and inorganic stores through various biological and physical processes.
- These carbon flows include both transfers (where carbon changes location but not form) and transformations (where carbon changes its chemical form).
Transfers
- Feeding: carbon moves up trophic levels (e.g., herbivores eating plants).
- Defecation: carbon is returned to the soil as organic waste.
- Death and decomposition: carbon moves from organisms to decomposers and soil.
- Think of transfers as moving money between bank accounts without changing currency.
- The carbon stays chemically the same.
Transformations
- Photosynthesis: converts inorganic CO₂ + H₂O into organic glucose using light energy.
- Cellular respiration: releases CO₂ and H₂O from organic molecules to provide energy.
- Dissolution: atmospheric CO₂ dissolves into ocean water.
- Combustion: converts organic carbon in biomass/fossil fuels into CO₂.
- Fossilization: partially decomposed organic matter is converted to fossil fuels over geological time.
When asked to “identify” or “explain” flows, specify whether each is a transfer or transformation, and mention the direction (e.g., atmosphere → plant via photosynthesis).
NoteTransformation: Organic carbon (fossil fuels) → Inorganic carbon (CO₂).
Creating a Systems Diagram of the Carbon Cycle
A typical systems diagram of the carbon cycle should include arrows between the atmosphere, producers, consumers, decomposers, soils, and fossil fuel stores, labeled with these key processes.
- Identify the Stores like the Atmosphere:
- Biosphere: Plants, animals, and decomposers.
- Lithosphere: Fossil fuels and sedimentary rocks.
- Hydrosphere: Dissolved carbon in oceans.
- Add the Flows:
- Photosynthesis: $CO_2$ → Plants.
- Feeding: Plants → Animals.
- Respiration: Plants/Animals → $CO_2$.
- Decomposition: Dead matter → Soil/Atmosphere.
- Fossilization: Organic matter → Fossil fuels.
- Combustion: Fossil fuels → $CO_2$.
Use different colors or symbols to distinguish between transfers (e.g., arrows) and transformations (e.g., labeled processes).
Common Mistake- Avoid confusing stores with processes.
- For example, the atmosphere is a store, while photosynthesis is a process.
Carbon sequestration
Definition
Carbon sequestration
Carbon sequestration is the process of capturing atmospheric carbon dioxide (CO₂) and storing it in solid or liquid form.
It plays a critical role in mitigating climate change by reducing the amount of CO₂ in the atmosphere.
Natural Sequestration
- Photosynthetic organisms (e.g., trees, seagrasses, algae) absorb atmospheric CO₂ and convert it into biomass, locking carbon in living tissues.
- Over geological timescales, organic matter is buried and transformed into coal, oil, and natural gas, representing long-term geological sequestration.
A mature mangrove forest sequesters carbon both in above-ground biomass and in deep, anoxic soils, making it one of the most efficient natural carbon sinks.
Artificial Sequestration
- Carbon Capture and Storage (CCS) technologies capture CO₂ from industrial sources and store it underground in geological formations.
- These are increasingly being explored as part of climate change mitigation strategies.
- Be prepared to explain sequestration in the context of climate regulation.
- E.g., “Carbon sequestration removes CO₂ from the atmosphere, reducing greenhouse gas concentrations and mitigating climate change".
The Role of Carbon Sequestration in Climate Mitigation
- Carbon sequestration is a critical tool for addressing climate change, but it is not a standalone solution.
- It must be combined with efforts to reduce emissions, such as transitioning to renewable energy, improving energy efficiency, and adopting sustainable land management practices.
Ecosystems as Carbon Stores, Sinks, and Sources
- Ecosystems play a crucial role in the carbon cycle by acting as stores, sinks, or sources of carbon.
- The balance between carbon inputs (such as photosynthesis) and outputs (such as respiration, decomposition, or combustion) determines whether an ecosystem absorbs, stores, or releases carbon.
1. Carbon Sink
- A carbon sink absorbs more carbon than it releases, leading to a net uptake of carbon dioxide (CO₂) from the atmosphere.
- This happens when photosynthesis exceeds cellular respiration.
- Over time, this stored carbon helps reduce atmospheric CO₂ levels, mitigating climate change.
- A young forest is an active carbon sink because growing trees require large amounts of CO₂ for photosynthesis.
- They take in more CO₂ than they release through respiration, storing carbon in their biomass (trunks, branches, leaves, and roots).
2. Carbon Store
- A carbon store holds carbon but does not significantly increase or decrease its total amount over time.
- It has a near balance between photosynthesis and respiration.
- A mature forest is a carbon store because the rate of photosynthesis and respiration is roughly equal.
- Trees still absorb CO₂, but older trees also respire more and decompose when they die, releasing some carbon back into the atmosphere.
Although mature forests no longer act as major carbon sinks, they serve as long-term carbon reservoirs, helping to stabilize global carbon levels.
3. Carbon Source
- A carbon source releases more carbon than it absorbs, leading to an increase in atmospheric CO₂.
- This occurs when respiration, decomposition, or combustion surpasses photosynthesis.
- In a wildfire, trees burn and release CO₂ almost instantly.
- In deforestation, trees are cut down and either burned or left to decay, both of which release stored carbon into the atmosphere.
A forest destroyed by fire or deforestation becomes a carbon source because large amounts of stored carbon are suddenly released.
Note- The net carbon status of an ecosystem depends on the relative magnitude of photosynthesis vs. respiration and decomposition:
- Sink → when photosynthesis > respiration, net carbon is absorbed from the atmosphere.
- Store → when photosynthesis ≈ respiration, carbon levels remain stable.
- Source → when photosynthesis < respiration, net carbon is released.
Fossil Fuels as Carbon Stores and Sources
- Fossil fuels represent vast, long-term carbon stores formed when past ecosystems acted as sinks millions of years ago.
- However, when extracted and burned, they become major carbon sources, disrupting the natural carbon balance.
1. Formation as Carbon Stores
- In past geological eras, ecosystems acted as carbon sinks, absorbing large amounts of atmospheric carbon through photosynthesis.
- Instead of being released back into the atmosphere, much of this carbon became trapped in sediments and gradually transformed into fossil fuels under pressure and heat over millions of years.
