Need to monitor and measure soil carbon

Soil has become a vulnerable resource. A new FAO report shows which areas have the best carbon storage potential and suggests improvements in monitoring and measurement.

Soil works as a carbon reservoir and contains more carbon than the atmosphere and terrestrial vegetation combined. The report “Soil organic carbon – the hidden potential” from the Food and Agriculture Organization (FAO), highlights the importance of shifting focus on how to best capture the carbon dioxide from the atmosphere and instead focus on how to find ways to keep carbon in the soil. The targets that have been set in the Kyoto protocol and Paris agreement include regular reporting of anthropogenic greenhouse gas emissions, which among other things include making inventories of emissions caused by soil organic carbon.

The removal of carbon dioxide from the air takes place through plant photosynthesis, in which plant biomass converts carbon dioxide into carbon in the soil in the form of soil organic carbon. The soil organic carbon reservoir constantly moves between different global carbon pools, in different molecular forms. Various components in the soil, such as the macrofauna which include fly larvae, snails and spiders etc., have the potential to move soil organic carbon to greater depths, where the carbon has the best potential to be stored.

Soil organic carbon is the main component of soil organic matter, which describes the soil in its various stages of decomposition. Soil organic carbon is important for the structure and stability of the soil, and therefore its capacity for aeration and water filtration. Decomposition affects the microbial activity in the soil that controls the soil’s potential for carbon storage. Soil organic carbon is dynamic and different anthropogenic impacts on the soil turn it into either a net sink or a net source of greenhouse gases. The carbon-based greenhouse gases emitted by soil are mainly carbon dioxide CO₂ and methane CH₄. Nitrous oxide N₂O can also be emitted and has become increasingly anthropogenically driven, largely from agricultural soils and livestock facilities.

Soil organic carbon plays an important role in food security by increasing soil productivity and its contribution to high yields. Soil organic carbon increases the water and nutrient retaining capacity that contributes to improved soil structure, which in turn is good for plant growth. If this retaining capacity is not supported, the soil organic carbon may be emitted back into the atmosphere, the soil may become eroded, or dissolved organic carbon may be washed into rivers and oceans.

Storage of soil organic carbon is more effective in some places than others, and these so-called hot spots are areas such as peatlands and temperature-vulnerable permafrost zones. These areas act as a major carbon sink but they can become a big problem in the future if they are managed in an unsustainable way. Hot spots like these need more attention in the measurement and management of soils. Large land-areas, called bright spots, that currently have low soil organic carbon content, can also become long-term carbon sinks in the future.

Environmental change and unsustainable agricultural management, such as monocultures and the use of chemicals, damage the soil’s biodiversity and affect several ecosystem functions, including the decomposition of soil organic carbon. Rising temperatures and more frequent occurrence of extreme weather events will lead to increased losses of soil organic carbon to the atmosphere, although the overall impact varies depending on factors such as soil type, climate conditions and region. It is difficult to predict the impact of climate change on the activity of soil due to those varying influences. Estimates still indicate that the carbon response of soil can go from small losses to moderate profits and it is therefore important to find a way of understanding the relationship between the soil’s biodiversity and the carbon cycle.

To model carbon dynamics, soil organic carbon is divided into three pools depending on its physical and chemical stability: the fast pool, intermediate pool and slow pool. The fast pool is the most labile and sensitive one and the process of preserving carbon can take time, from days to several years. At deeper depth, where the soil has a higher capacity to store carbon, is the slow pool. The global stock of soil organic carbon has been estimated at 1,500 gigatonnes of carbon in the top one-metre layer. The data that was used in previous measurements was collected over long periods using different calculations and methods, which makes the estimates inaccurate.

Each country that has signed the Paris Agreement must regularly report its greenhouse gas emissions. All countries follow the same estimation methods for soil organic matter and soil organic carbon changes, provided by the International panel on climate change (IPCC) guidelines for national greenhouse gases.

The FAO report raises several challenges in soil organic carbon sequestration and its preservation. Some are affected by human practices, such as shortcomings in sustainable soil management and lack of knowledge. Other factors that are mentioned are beyond human control, such as soil structure, and are also important to consider. Some scientific progress has been achieved in understanding and explaining soil organic carbon dynamics but there is much more to do.

Malin Larsson

Source: FAO report: Soil organic carbon – the hidden potential.