The Role of Carbon in Promoting Healthy Soils
- Soils have become one of the most vulnerable resources in the world.
- The huge carbon reservoir found in soil is not static but is constantly cycling between the different global carbon pools in various forms.
- Soils are a key reservoir of global biodiversity (the variety of plant and animal life in a specific habitat) which has a fundamental role in supporting soil functions.
- Water stored in soil serves as the source for 90 per cent of the world’s agricultural production and represents about 65 per cent of global fresh water.
- Soil organic carbon plays an essentially important role in ensuring global food security.
As the effects of land degradation, biodiversity loss and climate change become increasingly severe, soils have become one of the most vulnerable resources in the world. Soils are a major carbon reservoir containing more carbon than the atmosphere and terrestrial vegetation combined. Soil organic carbon (SOC) is dynamic, however, and human impacts on soil can turn it into either a net sink or a net source of atmospheric greenhouse gases (GHG). Enormous scientific progress has been achieved in understanding and explaining SOC dynamics. Protection and monitoring of SOC stocks at national and global levels, however, still face complicated challenges impeding effective on-the-ground policy design and regionally adapted implementation.
After carbon enters the soil in the form of organic material from soil flora and fauna, it can persist in the soil for decades, centuries or even millennia. Eventually, SOC can be lost as carbon dioxide (CO2) or methane emitted back into the atmosphere, becoming eroded soil material, or by being dissolved as organic carbon and washed into rivers and oceans. The dynamics of these processes highlight the importance of quantifying global carbon fluctuations to ensure maximum benefits of SOC to human well-being, food production and water and climate regulation.
SOC is the main component of soil organic matter (SOM). As an indicator for soil health, SOC is important for its contribution to food production, as mitigation and adaptation to climate change and the achievement of the sustainable development goals. A high SOM content provides nutrients to plants and improves water availability, both of which enhance soil fertility and ultimately improve food productivity. Moreover, SOC improves soil structural stability by promoting aggregate formation which, together with porosity, ensure sufficient aeration and water infiltration to support plant growth. With an optimal amount of SOC, the water filtration capacity of soils further supports the supply of clean water. Through accelerated SOC mineralisation, soils can be a substantial source of GHG emissions into the atmosphere. Although the overall impact of climate change on SOC stocks is very variable according to the region and soil type, rising temperatures and increased frequency of extreme events are likely to lead to increased SOC losses.
Globally, SOC stocks are estimated at an average of 1500 peta-grams of carbon (PgC) (one peta-gram of Carbon equals one billion metric tonnes) in the first meter of soil, although their distribution is spatially and temporally variable. SOC hot-spots and bright spots, which are respectively areas of high SOC content (e.g. peatlands or black soils) and large surface areas of low SOC content (e.g. drylands) constitute major zones of concern. With climate change and unsustainable management, these areas are likely to become net sources of GHG emissions. When managed wisely, however, they have the potential to sequester large amounts of carbon in their soils, thus contributing to climate change mitigation and adaptation