The authors found that the average soil carbon stock in European grasslands and forest topsoil ranges between 46 and 84 Mg C ha−1, with the highest values in coniferous or mixed forest soils.
They show that additional carbon storage capacity depends on the relative distribution of carbon between the types of soil organic matter (particulate organic and mineral-associated organic matter), and recommend that strategies be informed by clear science-based guidelines that consider site-specific soil and ecosystem properties.
Mitigating climate change through carbon sequestration
The sequestration of carbon in soil organic matter is one of the main cost-effective climate mitigation strategies for removing global-warming carbon dioxide (CO2) from the atmosphere.
Soil carbon sequestration is a process whereby CO2 is removed from the atmosphere by vegetation, and stored in the soil’s pool of organic carbon.
As well as reducing the amount of carbon in the atmosphere, soil carbon sequestration can also improve soil health and its ability to provide vital ecosystem services, such as providing a medium for plant growth, recycling organic wastes and nutrients, modifying the atmosphere, providing a habitat for soil organisms, and regulating the supply and purification of water.
To effectively help mitigate climate change, land-based solutions should maximise soil carbon storage without generating a surplus of nitrogen (which can lead to water pollution and greenhouse gas emissions).
However, most land management strategies for carbon sequestration do not consider the form in which carbon is stored, its capacity, persistency and nitrogen demand.
The authors set out to rectify this by measuring the European topsoil carbon and nitrogen stocks in forests and grasslands, and showing their distribution between mineral-associated and particulate organic matter pools.
They analysed 186 soil samples from the Land Use/Land Cover Area Frame Survey (LUCAS) database, separating the soil organic matter into particulate and mineral-associated organic matter. They then coupled the data with a high-resolution pan-European tree occurrence dataset to determine how mycorrhizal association could influence the pathway of soil organic carbon formation and stabilisation.
Forest and grassland soils are crucial to soil carbon management
The soils of temperate forests and grasslands can play a critical role in soil carbon management, as they occupy a vast land surface area, are often managed, and can store large amounts of carbon.
The authors found that grasslands and arbuscular mycorrhizal forests (whose roots form an intracellular symbiosis with fungi) store more soil carbon in mineral-associated organic carbon, which is long lasting but has a high demand for nitrogen and can quickly become carbon-saturated.
Ectomycorrhizal forests (whose roots form an extracellular symbiosis with fungi) store more carbon in particulate organic matter, which is more vulnerable to disturbance but has a lower nitrogen demand and can potentially accumulate indefinitely.
Particulate organic matter vs mineral-associated organic matter
The total organic carbon can be separated into two types of soil organic matter: particulate and mineral-associated organic matter. The conceptualisation of organic carbon into those fractions provides a feasible, well‐supported, and useful framework that will allow scientists to gain knowledge about the mechanisms that drive the carbon transformation in soils.
Particulate organic matter is predominantly made up of plant-based particulates of 0.053 mm to 2 mm in size, while mineral-associated organic matter consists of single molecules or microscopic pieces of organic material that have either leached directly from plants or been chemically transformed by the soil biota.
Particulate organic matter is often used as an indicator of soil quality as it is a readily available source of soil nutrients, contributes to soil structure, is highly sensitive to soil management and has a low nitrogen content.
Mineral-associated organic matter is largely made of microbial products that are richer in nitrogen, and more persistent in time.
Their highly contrasting physical and chemical properties and mean residence times in soil determine their different responses to global pressures, such as land use change, warming and atmospheric CO2 enrichment.
Clear science-based guidelines for carbon sequestration strategies
The authors recommend that the implementation of strategies for soil carbon sequestration be informed by clear science-based guidelines that consider site-specific soil and ecosystem properties, including the relative distribution of soil organic matter as particulate and mineral-associated fractions.
They show that grasslands (which are dominated by mineral-associated organic matter) are a good long-term sink for carbon, but require a lot of nitrogen and quickly become carbon-saturated. Therefore, management strategies to sequester carbon in grasslands should target soils that are not yet saturated.
This points to a need for geographical estimates of soil carbon saturation deficits.
Forests, on the other hand, have a greater plasticity in how soil carbon may be accrued because they can store more carbon in the particulate organic matter fraction.
Afforestation strategies designed to sequester carbon in soil should therefore consider the soil properties, carbon deficit and nitrogen availability, and should use appropriate tree species to maximise carbon accrual.
The importance of collaborative research
This ground-breaking study is the result of collaborative research involving the JRC, Colorado State University, University of Perugia and ETH. It is also based on extended soil analyses from the LUCAS-Soil survey, the largest harmonised open‐access dataset of topsoil properties, available for the European Union and at the global scale. As an expandable database, LUCAS-Soil has a wide scope, from increasing knowledge of biogeochemical processes to supporting policymakers by mapping and modelling the impacts of different soil policies.