5.1D Soil as a carbon sink Notes

1. Soils contain carbon in both living organisms and dead organic matter, making them one of the most important global carbon reservoirs.
2. Soils globally store approximately 1325 GtC in topsoils and up to 2300 GtC when vegetation inputs are included.
3. An additional 1600 GtC is stored in permafrost, representing ancient carbon that has remained frozen and protected from decomposition.
4. 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

1. Carbon enters soil through:
   1. Photosynthesis, where plants convert atmospheric CO₂ into biomass.
   2. Root exudates, which release organic compounds into surrounding soil.
   3. Dead plant and animal material, which is mixed into soil by decomposers and detritivores.
   4. Soil organisms, which contribute carbon through their biomass and waste products.
2. Carbon leaves soil through:
   1. Microbial respiration, which converts soil organic carbon back to CO₂.
   2. Mineralization, where humus breaks down into simple compounds released as CO₂.
   3. Disturbances, such as land clearing, ploughing, erosion, and drainage, which expose soil carbon to oxygen and speed up decomposition.

Global Soil Carbon Storage

1. Soils hold approximately 1325 GtC in the upper soil horizons alone.
2. When both soil and vegetation are included, global storage reaches approximately 2300 GtC.
3. An additional 1600 GtC is stored in permafrost, where frozen soils protect organic matter from decomposition.
4. Forests store about 90% of global terrestrial biomass carbon, representing 400 GtC in vegetation and soil.

Why Ecosystems Differ in Soil Carbon Storage

1. Tropical Rainforests – Low Soil Carbon

1. Tropical forest soils contain very little carbon, despite high levels of biomass above ground.
2. High temperatures and high humidity accelerate decomposition, causing carbon to be rapidly returned to the atmosphere.
3. Intense microbial activity ensures that organic matter decomposes quickly.
4. Heavy rainfall causes nutrient leaching, preventing long-term accumulation of carbon in soil.
5. Most carbon in tropical forests is stored in biomass, not in the soil.

2. Tundra – Very High Soil Carbon

1. Tundra soils store large amounts of carbon because low temperatures greatly reduce decomposition.
2. Permafrost prevents microbes from breaking down organic matter, causing organic matter to accumulate over centuries.
3. When permafrost melts due to climate warming, stored carbon is released as CO₂ and methane.
4. Tundra ecosystems therefore act as large carbon stores, though they may shift to carbon sources as temperatures rise.

3. Wetlands and Peatlands – Extremely High Soil Carbon

1. Waterlogged soils reduce oxygen availability, limiting aerobic decomposition.
2. Under anaerobic conditions, decomposition is very slow, causing carbon-rich plant material to accumulate.
3. Peat forms when layers of partially decomposed organic matter build up over time.
4. Some peatlands contain metres-thick layers of carbon-rich peat, storing carbon for thousands of years.
5. Wetlands often remain carbon sinks as long as waterlogged, cool conditions persist.

4. Temperate Grasslands – High Soil Carbon

1. Grasslands have deep root systems that deposit carbon-rich organic material into the soil.
2. Seasonal climate patterns create moderate decomposition rates, allowing more carbon to accumulate than in tropical forests.
3. Frequent root turnover increases below-ground carbon inputs.
4. Soil carbon storage can exceed that of forests because most biomass is below ground, not above.

Factors Determining Soil as Sink, Store, or Source

1. Low temperatures slow microbial respiration, making soils more likely to act as carbon sinks.
2. High temperatures increase decomposition, causing soils to act as carbon sources.
3. Waterlogged conditions promote anaerobic decomposition, which is slower and results in carbon accumulation.
4. Disturbance, such as agriculture, drainage, or deforestation, increases oxygen exposure and accelerates decomposition, causing soils to become carbon sources.
5. Vegetation types influence the amount, depth, and quality of organic material entering the soil.

Wetlands as Long-Term Carbon Stores

1. Wetlands accumulate thick layers of undecomposed organic matter.
2. Methane may form under anaerobic conditions, but long-term accumulation still makes wetlands major carbon sinks.
3. Drainage of wetlands for agriculture releases large amounts of stored carbon into the atmosphere.

Human Impacts on Soil Carbon Storage

1. Agriculture, especially ploughing, increases oxygen exposure and speeds decomposition, releasing carbon.
2. Deforestation exposes soil to sunlight and increases decomposition.
3. Drainage of wetlands converts long-term carbon stores into carbon sources.
4. Reforestation, restoration of wetlands, and sustainable grazing can restore soils as carbon sinks.