- Soils contain carbon in both living organisms and dead organic matter, making them one of the most important global carbon reservoirs.
- Soils globally store approximately 1325 GtC in topsoils and up to 2300 GtC when vegetation inputs are included.
- An additional 1600 GtC is stored in permafrost, representing ancient carbon that has remained frozen and protected from decomposition.
- The role of soil in the global carbon cycle depends on whether carbon inputs exceed outputs or vice versa.
How Carbon Enters and Leaves the Soil
- Carbon enters soil through:
- Photosynthesis, where plants convert atmospheric CO₂ into biomass.
- Root exudates, which release organic compounds into surrounding soil.
- Dead plant and animal material, which is mixed into soil by decomposers and detritivores.
- Soil organisms, which contribute carbon through their biomass and waste products.
- Carbon leaves soil through:
- Microbial respiration, which converts soil organic carbon back to CO₂.
- Mineralization, where humus breaks down into simple compounds released as CO₂.
- Disturbances, such as land clearing, ploughing, erosion, and drainage, which expose soil carbon to oxygen and speed up decomposition.
The rate of decomposition is the single most important factor determining whether soil becomes a sink or a source.
Global Soil Carbon Storage
- Soils hold approximately 1325 GtC in the upper soil horizons alone.
- When both soil and vegetation are included, global storage reaches approximately 2300 GtC.
- An additional 1600 GtC is stored in permafrost, where frozen soils protect organic matter from decomposition.
- Forests store about 90% of global terrestrial biomass carbon, representing 400 GtC in vegetation and soil.
Soil carbon storage far exceeds atmospheric carbon storage, making soils a critical part of climate regulation.
| Biome | Soil carbon storage | Reason |
|---|---|---|
| Tropical forests | Low | Warm temperatures and high microbial activity lead to rapid decomposition of organic matter. Most carbon is stored in vegetation, not soil. |
| Temperate grasslands | High | Deep-rooted grasses contribute to large organic matter input, and moderate temperatures slow decomposition, allowing carbon accumulation. |
| Wetlands | Very high | Waterlogged, anaerobic conditions slow decomposition, leading to the accumulation of peat and organic carbon. |
| Tundra | Very high | Cold temperatures and permafrost prevent decomposition, trapping carbon in frozen organic material. |
Why Ecosystems Differ in Soil Carbon Storage
1. Tropical Rainforests – Low Soil Carbon
- Tropical forest soils contain very little carbon, despite high levels of biomass above ground.
- High temperatures and high humidity accelerate decomposition, causing carbon to be rapidly returned to the atmosphere.
- Intense microbial activity ensures that organic matter decomposes quickly.
- Heavy rainfall causes nutrient leaching, preventing long-term accumulation of carbon in soil.
- Most carbon in tropical forests is stored in biomass, not in the soil.
Tropical soils behave like a high-speed recycling system, where organic matter is broken down and reused almost immediately.
Common Mistake- It is incorrect to assume that high plant productivity means high soil carbon.
- Tropical rainforests have high productivity but low soil carbon.
2. Tundra – Very High Soil Carbon
- Tundra soils store large amounts of carbon because low temperatures greatly reduce decomposition.
- Permafrost prevents microbes from breaking down organic matter, causing organic matter to accumulate over centuries.
