5.11: Soil organic matter and global change
Soil, land and climate change
Soil contains significant amounts of carbon and nitrogen, which can be released into the atmosphere depending on how we use the land. Clearing or planting forests, the melting of permafrost can tilt the greenhouse gas emission balance one way or the other. Climate change can also substantially alter what farmers can produce and where. The Food and Agriculture Organization of the United Nations (FAO) map below shows that the top 30 cm of the world’s soil contains about twice as much carbon as the entire atmosphere. After oceans, soil is the second largest natural carbon sink , surpassing forests and other vegetation in its capacity to capture carbon dioxide from air. These facts remind us how important healthy soils are, not just for our food production but also for our efforts to prevent the worst effects of climate change.
Soil organic carbon (SOC) is the carbon that remains in the soil after partial decomposition of any material produced by living organisms. It constitutes a key element of the global carbon cycle through atmosphere, vegetation, soil, rivers and the ocean is the main component of soil organic matter (SOM) and as such constitutes the fuel of any soil. SOM supports key soil functions as it is critical for the stabilization of soil structure, retention and release of plant nutrients, and allowing water infiltration and storage in soil. It is therefore essential to ensuring soil health, fertility and food production. The loss of SOC indicates a certain degree of soil degradation. Soils represent the largest terrestrial organic carbon reservoir. Depending on local geology, climatic conditions and land use and management (amongst other environmental factors), soils hold different amounts of SOC.
The largest amounts of SOC have been estimated to be stored in the northern permafrost region with around 190 Pg C in the first 30 cm of the soil (0-30cm) [1] , mostly in peat soils. There, carbon accumulates in soils in huge quantities due to the low temperatures leading to low biological activity and slow SOM decomposition. The corresponding soil type is called Histosol and is characterized by a SOC content of 12 to 18% [2] . In contrast, in dry and hot regions such as the Sahara Desert, plant growth is naturally scarce and only very little carbon enters the soil. Arenosols, the typical soils of these areas, have mostly less than 0.6% SOC [3]. Black soils, such as Chernozems, are inherently fertile because of their relatively high SOC content (over 1% [2] ) and optimal plant growth conditions in terms of nutrient exchange capacity and a well-developed structure enabling sufficient water provision.
Unsustainable management practices such as excessive irrigation or leaving the soil bare endanger these soils, causing SOC loss and massive erosion. Caring for these soils and preserving the SOC they contain can be achieved through sustainable soil management, including mulching, planting cover crops, judicious fertilization and moderate irrigation.
Loss of SOC negatively affects not only soil health and food production, but also exacerbates climate change. When SOM is decomposed, carbon-based greenhouse gases are emitted to the atmosphere. If this occurs at too high rates, soils can contribute to warming our planet. On the flip side, many soils have the potential to increase their SOC stocks, thus mitigating climate change by reducing the atmospheric CO2 concentration.
The Global Soil Organic Carbon Map (GSOCmap), a country driven endeavour, allows the estimation of SOC stock from 0 to 30 cm. The GSOCmap represents the first ever global soil organic carbon assessment produced through a participatory approach in which countries developed their capacities and stepped up efforts to compile all the available soil information at national level. In many cases, this is paving the way to establishing national soil information systems and represents the first step toward introducing a soil monitoring program.
Changes in seasonal temperatures can also shift the annual cycles of plants and animals, resulting in lower yields. For example, spring can arrive earlier and trees can blossom before their pollinators have hatched. With the expected population growth, world food production needs to increase rather than decrease. This hinges largely on maintaining healthy soil and managing agricultural areas sustainably. At the same time, there is a growing demand for biofuels and other plant-based products, driven by the urgent need to replace fossil fuels and prevent greenhouse gas emissions.
There are other impacts on soil related to climate change, including erosion , which can be accelerated by extreme climate events, such as intense rain, drought, heat waves and storms. In addition to causing the loss of areas of land, rising sea levels may change soil in coastal areas or bring contaminants, including salt, from the sea. In relation to land use, climate change may make some agricultural areas, mainly in the south, unusable or less productive while possibly opening up new possibilities further north. In forestry, the decline in economically valuable tree species might cut the value of forest land in Europe by between 14 and 50 % by 2100. The overall impacts of climate change could produce a significant loss for the agricultural sector with large regional variations.
Yet perhaps the biggest climate concern linked to soil is the carbon dioxide and methane stored in permafrost in boreal regions, mainly in Siberia. As the global temperatures increase, the permafrost melts. This thawing causes the organic material trapped in the frozen soil to disintegrate, which can lead to the release of massive amounts of greenhouse gases into the atmosphere, which could hence lead to the accelerating of global warming far beyond people’s control.
Tackling the climate crisis with soil
In April 2019, a group of highly influential scientists and activists called for ‘defending, restoring and re-establishing forests, peatlands, mangroves, salt marshes, natural seabeds and other crucial ecosystems’ to let nature remove carbon dioxide from the atmosphere and store it. Restoring ecosystems would also support biodiversity and enhance a wide range of ecosystem services, including cleaning air and water, and providing people with enjoyable spaces for recreation.
Despite the uncertainties, restoring ecosystems and improving soil quality could be a very cost-efficient measure in terms of climate action with a triple impact. First, growing plants remove carbon dioxide from the atmosphere. According to the FAO , restoring currently degraded soils could remove up to 63 billion tonnes of carbon, which would offset a small but important share of global greenhouse gas emissions. Second, healthy soils keep the carbon underground. Third, many natural and semi-natural areas act as powerful defences against the impacts of climate change.
The examples of benefits are many. For example, areas next to rivers (riparian zones) and green spaces in cities can act as cost-effective protection against floods and heat waves . Healthy land and soil can absorb and store excess water and alleviate floods. Parks and other natural areas in cities can also help with cooling down during heat waves, partly because of the water present in their soil. During dry seasons, healthy ecosystems can slowly release the water they have stored underground, mitigating the worst impacts of droughts.
Capturing the carbon in the air
There are also various methods for increasing land’s capacity to capture carbon dioxide from air. A recent European research project ( Caprese study ) found that the conversion of arable land to grassland is the most rapid way of increasing the amount of carbon in soil. For arable land, the use of cover crops — plants such as clover grown in between harvest and sowing the next crop mainly to increase soil fertility and avoid erosion — was the most effective way of increasing carbon stocks in soil.
In contrast, decisions to use land differently can also change areas, making them sources of emissions. Notable examples of this are draining peatlands , burning peat from bogs for heating, ploughing up grassland and cropland, which releases previously stored carbon. For forests , the dynamic is the same but with a different timescale. Like soil, forests are both carbon stocks and carbon sinks, meaning that they both store carbon and capture it from the air. In many cases, young, growing forests capture carbon more rapidly than old forests but harvesting old forests removes the carbon stock from the forest. Depending on how the wood is used, the carbon may be released sooner, such as when the wood is burned for heating, or much later, when the wood is used for building houses, for example.
Healthier soils and land ecosystems could capture and store more carbon dioxide from the atmosphere than they currently do. Green spaces and natural areas could also help people and nature to adapt to the inevitable changes in our climate. Soil alone cannot fix climate change but it needs to be factored in and could be a powerful partner in our efforts.
Excerpted from:
Food and Agricultural Organization, "World’s most comprehensive map showing the amount of carbon stocks in the soil launched" November 2023, Accessed November 2023
https://web.archive.org/web/20180128061450/http://www.fao.org/3/a-i8195e.pdf
European Environment Agency, 2019. Accessed November 2023
https://www.eea.europa.eu/signals-archived/signals-2019-content-list/articles/soil-land-and-climate-change#:~:text=Climate%20change%20affects%20soil&text=Continuing%20declines%20in%20soil%20moisture,dramatic%20impacts%20on%20food%20production.