Forest soils store almost half of the total organic carbon in terrestrial ecosystems, and forest management practices can influence the rates of input or release of carbon from soils (Mayer et al. 2020, Mäkipää et al. 2023, Ontl et al. 2020). An important factor for soil C stocks, for Europe and globally, is to maintain existing forest cover and avoid its removal or degradation. Forest management can have various objectives, such as timber production, biodiversity conservation, recreation, carbon sequestration, and other ecosystem services. It is, however, likely that, in many forest situations, the main societal goal will be habitat for wildlife with managements being tailored for different species in different situations. Forest management require thus a holistic approach serving several ecosystem services other than simply exploring its potential in storing soil carbon. Consideration of C stocks will thus be a secondary factor. Many factors influence the interactions between forest management and SOC stocks, such as forest type, disturbance, soil type, climate, time (Ahmed et al. 2012, Jandl et al. 2021) and the carbon use efficiency (CUE; Qiao et al. 2019, Tao et al. 2023).

Several studies underscore the need for sustainable management practices and innovative solutions to meet the growing demand for timber and forest waste as bioenergy in the context of climate change. The demand for wood-based energy is expected to increase, but the carbon impacts of forest bioenergy are uncertain (Giuntoli et al. 2020). Forest residues can also be used for biochar production, with substantial climate benefits even after all environmental costs associated with production and application are discounted through life cycle analysis (Tisserant et al. 2022). This is further complicated by the potential effects of climate change and air pollution on forest productivity and carbon sequestration (Matyssek et al. 2012). The removal of forest residues for bioenergy could also have negative consequences for how forest systems provide and sustain their ecosystem services (Clark 2012).

Sustainable food production requires increasing the productivity and efficiency of land, water, and other inputs, while reducing the environmental impact and greenhouse gas emissions of agriculture. Overall, the adoption of recommended agricultural practices, such as reduced tillage, crop rotation, cover crops, including agroecology and intercropping can lead to enhanced SOC storage and restoration of soil quality, and by that strengthen long-term food security. But it must be recognized that the production benefits may not be apparent in the short or medium-term. Soil carbon stocks and quality are influenced by climate, soil minerals and aggregation (Lehmann and Kleber 2015), the rate of plant primary production, plant root interaction with soil and soil biology (Bai and Cotrufo 2022, Kätterer et al. 2011), and various management factors, such as land use, soil management and crop rotation (Cui et al. 2022, Fornara and Higgins 2022, Haddaway et al. 2017). The review of recent studies by Bai and Cotrufo (2022) highlight the essential role of management improvements, restoration and the capacity of plants and soil biology in controlling the formation of mineral associated organic material (MAOM) and particulate organic material (POM) promoting SOC storage, mediating the impacts of climate change. The biogeochemistry of SOC is a dynamic continuum from intact plant residues to highly oxidized carbon in carboxylic acids which requires a mechanistic understanding of the SOC interaction with minerals, the mediation of microbial activity balancing both the stocks and flows of organic matter (Lehmann and Kleber 2015). Agronomic practices vary, and potential impact on soil organic carbon stocks is shortly discussed for some of the emerging once.

Growing cover crops where soil would otherwise be bare, has many benefits including decreased nitrate leaching over winter and decreased soil erosion compared to bare soil. However, their role in increasing SOC may have distinct limitations in many European situations. For instance, in regions of NW Europe with temperate climate, autumn-sown crops are common, which leaves limited scope to include cover crops unless under-sown at an early stage in the development of the main crop. By contrast, in regions with a more continental climate such as much of North America, spring-sown crops are more commonly grown, leaving large areas of soil bare over winter. However, where cover crops can be grown, it is likely that they will lead to some increase in SOC, though the magnitude of increase may well be less than often assumed. For example, a recent review calls into question the often-quoted view that cover crops can increase SOC by about 0.3 tC/ha/yr – see Chaplot and Smith (2023). Depending on climatic region and plant species, cover crops may contribute to increased nitrous oxide emissions in the long term due to accumulation of organic N in the increased stock of soil organic matter. (Guenet et al. 2021, Lugato et al. 2018).

The effects of tillage practices on SOC at different soil depths are not uniform and depend on various factors, such as soil type, climate, crop type, tillage practices (e.g. no tillage to high intensity, Haddaway et al. 2017), tillage frequency and bulk density (Fornara and Higgins 2022). For example, a review of 351 studies from warm temperate and snow climate zones, found that SOC was significantly higher in no tillage soils compared to high intensity systems in the upper 30 cm soil layers, but no effect was found in the full soil profile. The higher SOC in top soil layers in no tillage systems, may provide resilience to extreme weather conditions though (Haddaway et al. 2017). A recent study from a mediterranean climate, showed among other findings, that tilled wheat had greatest soil C stabilization at intermediate depts (30-60 cm), whereas no-tilled wheat had highest carbon stabilization and microbial biomass at top soil layers (0-30 cm) (Taylor et al. 2024). The increased soil C stabilization in top soil was connected to better plant growth at no-tillage in mediterranean (rather dry) climates. The Fornara and Higgins (2022) study of 500 grassland fields in Northern Ireland, UK, showed that C and N stocks (mg/ha) in the top 30 cm depts were not affected by frequency of tillage + reseeding, as differences in bulk density levels out the stock variation. They report, however, that the concentrations of C (%) and N (%) decreased while bulk density increased by the tillage + reseeding frequency. Thus can soil compaction affect soil C storage more than the frequency of tillage.

Crop rotation is normally practiced in several farming systems, such as organic farming, regenerative agriculture and conservation agriculture, but according to Land et al. (2017) there are not many comprehensive studies designed to unravel the effect of crop rotation on soil organic carbon stocks. Calculation indicate that perennial forages can increase below-ground SOC more than the common crops, especially if crop residues are not returned, or if the perennial forages are discontinued (Bolinder et al. 2007, Bolinder et al. 2012, Land et al. 2017). Perennial crop seems to increase the C storage and flux, more strongly in shallow soil (0-15) compared to deeper soil layers (15-30) (Means et al. 2022) in comparison to annual monoculture crop.

Several of the emerging farming system described under, practice the use of minimal disturbance and reduces tillage, cover crops, crop rotation, plant or animal residue return. Hence does the overlapping practices have shades of the same impacts on soil organic carbon storage. They are however discussed separately for the purpose of the terminologies in use.

Organic farming have the potential to increase soil organic carbons stocks and sequestration rates (Clark and Tilman 2017), offering larger environmental benefits in comparison to conventional agricultural systems (Gattinger et al. 2012). Organic farming generally produce slightly less biomass though, and the effect on soil carbon stock from organic farming is complex and content performance dependent (Seufert and Ramankutty 2017). Organic farming may involve reduced tillage, and reduced tillage have a potential to increase total SOC stocks, if crop management is optimized. Krauss et al. (2022) reported the effect of reduced tillage on SOC stocks in organic farming systems in temperate Europe. They found slight increas in top 10-15 cm, slight decrease in intermediate dept (down to 50 cm), followed by a slight increas again in 70-100 cm depth. The investigation reported in Gaudaré et al. (2023) indicate, though, that unless appropriate farming practices are implemented, expanding organic farming might reduce the potential for soil carbon sequestration. According to Lorenz et al. (2019), the demand for organic products will continue to grow driven by food safety concerns. Due to lower yields, however, natural ecosystems may be increasingly converted to agroecosystems to meet the demand with less well-known consequences for the environment.

Agroforestry and intercropping have shown to significantly impact soil organic carbon stocks in Europe. Mayer et al. (2022) conducted a meta study on temperate climate zones worldwide and found that agroforestry systems sequester significant amounts of SOC in top soils and subsoils. Zuazo et al. (2014) reported that forest, shrubland, and grassland in a Mediterranean agroforestry landscape had higher soil organic carbon stocks compared to abandoned farmland. Kay et al. (2019) further emphasized the potential of agroforestry in sequestering carbon and mitigating environmental pressures in European farmland. It has generally been reported positive effects of diversified arable cropping systems and environmentally friendly farming management on soil organic carbon content in European agroecosystems (Francaviglia et al. 2019).

Regenerative agriculture (RA) may not have an exact definition, and the practice varies between regions and farmers and farming systems. It generally includes reduced tillage, crop residue management, and strong control of fertilization practice. These practices in combination have shown to increase soil organic carbon (SOC) stocks (Chahal and Singh 2020, Rhodes 2017). Regenerative agricultural practices do not only enhance carbon storage but reports also indicate improved soil fertility and crop yields in many situations (Rhodes 2017). In general, it seems likely that agroecological crop management and regenerative practices, particularly reduced or no-tillage and cover crops, have the potential to increase SOC content (Breil et al. 2021).

Conservation agriculture (CA) is based on many of the abovementioned principles. Conservation agriculture simply implies minimal soil disturbance, permanent soil cover, and crop rotation. The effects of CA on SOC stocks are not consistent and depend on various factors, such as soil type, climate, crop type, residue management, and duration of conservation agriculture. A global meta study showed that CA systems including legume residue retention in combination with manure and mineral N-admixing have considerable potential to increase SOC and total N in topsoil layers (Bohoussou et al. 2022). But, as with all the practices considered, research is required to identify opportunities, barriers, and trade-offs with other agronomic environmental goals in a range of environments.

The management of soil should focus on sustainability of food and fiber production and sustaining ecosystem services. This puts climate change adaptation as the primary aim for soil management rather than mitigation.

The impact of climate change on food and fiber production depends on the responses and adaptations of farmers, consumers, markets, and policies. These adaptations are the result of complex optimization decisions and general equilibrium dynamics, and thus difficult to measure and predict. Two recent approaches to studying climate change adaptation in agriculture are panel data methods and spatial general equilibrium models (Page et al. 2020). Increased SOC stocks generally favours both mitigation and adaptation as higher SOC in top soil layers in e.g. no tillage systems, provide resilience to extreme weather conditions though (Haddaway et al. 2017).

Experimental evidence drawn from biodiversity ecosystem functioning experiments has generally shown that higher plant biodiversity leads to both higher aboveground and belowground plant productivity and concordantly higher soil carbon. Already in 1994, Tilman and Downing reported that preservation of biodiversity is essential for the maintenance of stable productivity in ecosystems (Tilman and Downing 1994). It may be the case that in high clay soils where essential elements are limiting that the best yielding monoculture species may be superior to a mixture of plant species for producing biomass and storing soil carbon. However, there are also a host of what ecologists call niche differences that could explain why in some cases a higher number of species would yield greater soil carbon. For example, can plant species differentiate in hot and dry vs cold and wet seasons, exhibiting different rooting depths, producing different types of litter that are differentially processed by the microbial community (Furey and Tilman 2021, Kraychenko et al. 2019, Lange et al. 2015, Lange et al. 2021, Perry et al. 2023, Spohn et al. 2023, Yang et al. 2019).

The EUSO soil health dashboard reveals that 61% of EU soils are unhealthy (EU Science hub 2023), however gaps remain due to limited data various soil degradation issues. Soil health is closely linked to SOC, as soil organic matter affectes soil sturture, soil life and elemental cycles, which together sustaining essential ecosystem functions such as erosion protection, soil biodiversity, primary production, climate regulation and water quality (Hoffland et al. 2020). Soil carbon is vastly heterogeneous, encompassing everything from last hour’s root exudates to persistent humified material, millennia old (Amundson 2001). Soil organic matter is biologically most useful when it breaks down and releases plant nutrients, which is in direct contrast to the aim of storing more carbon in soils (Janzen 2006). Thus has the status of carbon quality, such as particulate and mineral associated fractions in relation to its stability and soil structure in agronomic and forests soils, has been a matter of intense research (Georgiou et al. 2022, Liang et al. 2017).

Many lists of indicators for soil quality and soil health include carbon content and microbial respiration together because they are positively correlated (EU commission 2023). Microbial biomass does provide 'early warning' of slow changes in total SOC (Powlson et al. 1987). But biomass is not the easiest method for routine use. Alternatives exists - see Bongiorno et al. (2019). As microbial activity and nutrient release increase with increasing carbon content, nutrient mining can occur, counteracting ideas of improving soil health. Additional biological indicators may also provide insight about C dynamics and microbial activity (Liptzin et al. 2022).

In a sustainable bioeconomy, recycling of nutrients from organic residues is imperative (Hellsmark et al. 2016, Sawatdeenarunat et al. 2016). There is a huge diversity in organic residues depending on their origin and/or the type of process involved in their production. Application of organic residues as soil amendment and fertilizer to agricultural land gives the opportunity of recovering the nutrients, primarily nitrogen and phosphorus, and of potentially improving soil quality by adding organic matter. However, such residues may also increase greenhouse gas production through the input of microbial substrates and increased mineralization of N.

Urbanization is the process of transforming rural areas into urban areas, which can have various effects on food production and soil organic carbon (SOC) stocks. In view of the need for housing increased populations in many European countries, some loss of agricultural land due to urbanization seems inevitable. Generally, there is a major conflict of interest between urbanization and the protection of productive soil. High quality soil for agriculture is a non-renewable resource since it takes centuries to build up few centimeters of productive soil. The conversion of agricultural land to urban land is de facto an irreversible process (Amundson et al. 2015), as new use may decrease the land’s ability and capacity to supply food and other vital ecosystem services (Tan et al. 2009). Historically, urbanization has occurred close to our most productive farmland (Ferrara et al. 2014), and most remaining farmland is located close to urban settlements. Thus, urban sprawl is consuming fertile agricultural land for urban use worldwide (Skog and Steinnes 2016). How to combine increased food production and soil organic matter conservation with increased urbanization and high pressure on productive agricultural land, i.e., multifunctional land use, is a challenge. The EU commission has onset several strategies, such as the biodiversity long-term plan to protect nature and reverse the degradation of ecosystems. The strategy aims to put Europe's biodiversity on a path to recovery by 2030 (EU commission 2020), the EU soil strategy EU commission 2021 to protect and restore soils, and ensure that they are used sustainably and finally the "no net land take " brief, to outline what measures can avoid, reduce or compensate for land take (Science for Environment Policy 2016).

Awareness of the importance of soil health has increased recently. For instance, will the PREPSOIL project support the implementation of the Soil Mission by creating awareness and knowledge on soil needs among stakeholders in regions across Europe. Considering the recognition of soil and its importance to sustain essential ecosystem services, there is a need to improve fellow citizens, land managers, politicians and policymakers common understanding of SOC dynamics and its central role in soil fertility and carbon storage. Soil carbon storage refers to an increase of soil carbon stocks, while soil carbon sequestration implies a net removal of atmospheric CO2. However, these terms are often used interchangeably or ambiguously, which can cause confusion and misunderstanding among different stakeholders and audiences.

Education and awareness on the importance of SOC management for global benefits, particularly in climate change adaptation and is vital role for sustainable food security is challenging to communicate though. In part this is because of the complexity of organic carbon composition and complex fate in soil, and the linkages to soil functions such as soil structure, soil life and elemental cycles (Chenu et al. 2019).

The import of food and fiber into Europe has a complex and varied impact on soil organic carbon (SOC) levels in soils outside of Europe. Frank et al. (2015) found that changes in SOC stocks depend on management regime and environmental factors, with a potential for carbon sequestration in European cropland. However, if C sequestration as opposed to food production is prioritized in Europe, this would lead to increased imports of food. Much being likely to be grown on recently cleared land elsewhere in the world with the resulting loss of SOC, and increased CO₂ emissions, in those regions. For instance, if organic farming increases, this may come at the expense of SOC loss at another site (Gaudaré et al. 2023). To improve our understanding on soil organic carbon (SOC) stock outside Europe, standardized estimation methods, comprehensive data sets, and accurate mapping techniques is needed (Aksoy et al. 2016, Lorenz et al. 2019, Lugato et al. 2014, Wiesmeier et al. 2012).

To effectively address content and quality of soil organic carbon stock, several methods exist ranging from laboratory measurements to remote sensing modelling. In short, the determination of soil organic carbon stocks in soil requires measurements of bulk density, gravel content and soil organic C concentration. Soil organic C concentration can be determined making use of several methods where Visible–Near-Infrared (vis–NIR) Spectroscopy is one, the bulk density by Active Gamma-Ray Attenuation, whereas gravel content may require wet sieving (England and Viscarra Rossel 2018). Careful, repeated field sampling followed by laboratory analysis using dry combustion analysis and bulk density measurement, following standardized and procedural guidelines are, however necessary, for accurate reporting and verification. There is a challeng in developing cost-effective methods for detecting changes in soil organic C resulting from changes in management etc., but a number of direct field applicable methodologies exist, such as laser-induced breakdown spectroscopy (LIBS) (Cremers et al. 2001), inelastic neutron scattering (Wielopolski et al. 2000), Mid-Infrared and Near-Infrared Diffuse Reflectance Spectroscopy (McCarty et al. 2002). For remote sensing eddy covariance is a costy application usefull for measuring respiration and carbon fluxes, provideing insights into regional soil carbon sequstration when used in combination with simulation modelling (Zeng et al. 2020).

Research on forestry and practices require more long-term soil monitoring, experimental studies, and synthesis of existing data to provide evidence-based guidance for climate-smart forest management practices. Such as:

How different forest management practices affect soil carbon stocks and greenhouse gas emissions in different forest types, climates, and soil conditions.

There are several knowledge gaps on various aspects of agronomic practices for managing soil organic carbon stocks in agricultural soils, and long-term field experiments trying to elucidate the effect of different soil management practices on soil carbon stocks need long-term perspectives (and appropriate financing possibilities):

Soil management

Many studies focus on varying one or two management practices (e.g., different tillage intensity, fertilization strategies or straw management), and how they influence soil carbon stocks. While the picture gets clearer for some aspects, others require more systematic long-term studies, e.g., fertilizer strategies for more novel organic fertilizers (biogas digestates, fish sludge etc.).

System approaches

There is need for more long-term studies on a system level, e.g., comparing different types of agronomic systems in different climate zones and on different soil types with respect to processes affecting carbon stocks.

The knowledge gaps on climate adaptation to sustain food and fibre production, while maintaining or even increasing soil organic stocks is a broad and interdisciplinary field of research, involving various disciplines, methods, and perspectives concerning soil health, quantification of SOC stocks, regional variability, mitigation strategies and integration with agricultural polices:

Despite efforts, many European soils remain unhealth, failing critical thresholds for soil health indicators. Thus, is developing specific indicators that correlate with SOC storage and climate resilience essential.

More indirect knowledge-gaps are:

What is the effect of climate adaptation measures, e.g., breeding of more resilient varieties with larger root systems, longer-lasting plant covers.

The "Convention on Biological Diversity (CBD)" defines soil biodiversity as “the variation in soil life, from genes to communities, and the ecological complexes of which they are part, that is from soil micro-habitats to landscapes.” It encompasses the variety of life belowground, including microorganisms, microfauna, mesofauna, and macro/megafauna. Soil biodiversity plays critical roles in delivering ecosystem goods and services, such as nutrient cycling, water regulation, and soil structure maintenance. Biodiverse ecosystems may enhance SOC storage capacity and research can identify which plant species or microbial communities promote SOC accumulation. Plant communities enhance SOC through root exudates, litter quality and mycorrhizal associations, and investigating these feedback loops may help designing effective climate adaptation strategies. Moreover, does biodiversity affect SOC response to land use changes, and the relationships vary across ecosystems, climates, and soil types. There is an intricate relationship between SOC, soil biodiversity and ecosystem resilience in Europe, and some of the unknowns are:

There is a need for more knowledge on the relationship between biodiversity and soil organic stocks as biodiversity influence soil processes, nutrient cycling, and ecosystem functioning.

More specific examples of unknowns are:

Does the benefit of plant biodiversity on soil carbon storage depend on soil texture and parent material?

The EUSO soil health dashboard, reveals that 61% of EU soils are unhealthy. However, gaps remain due to limited data on various soil degradation issues, including contamination. Developing comprehensive and updated knowledge for identifying healthy and degraded soils is essential. Essential knowledge gaps are:

While we know that EU soils are both gaining and loosing carbon, the specific factors driving these changes remain a knowledge gap.

The soil plays a key role in a circular economy and sustainable society, but there is significant lack of knowledge concerning safe and energy-efficient recycling of waste materials in soil, and its impact on soil organic carbon stocks and soil health. We cannot make use of organic waste transferring contaminants, pathogenic organisms, and unwanted plant residues such as weed in healthy soils. While circular economy principles emphasize resource efficiency, their direct influence on SOC stocks remains an area of study.

Research is needed to understand how circular economy practices, such as recycling, remanufacturing, and waste reduction, affect soil carbon sequestration.

Identifying synergies between circular economy strategies and SOC preservation is crucial.

Urbanization significantly alters land use patterns, leading to changes in soil properties, and SOC stocks vary widely across different urban environments (e.g., parks, sealed surfaces, green spaces). Furthermore, does urban soils face unique challenges due to compaction, pollution, and limited space. Urban systems involve material flows (e.g., waste, organic matter) that impact SOC dynamics. Thus, will integrating soil health and carbon sequestration goals into urban planning and policies be challenging. Several unsolved questions remain though:

Understanding how urbanization affects different SOC pools (e.g., particulate organic matter, mineral-associated fractions) remains a gap. Preservation of high-quality soil should be an important principle. In some situations moving soil may be considered the last option when other measures to protect the productive land have failed. Hence:

Researchers need to explore the dynamics of SOC storage and turnover in urban soils.

Ongoing projects, such as the PREPSOIl, target the need to create awareness and knowledge on soil needs among stakeholders in regions across Europe. There is a great need for education that increases awareness and knowledge of the conservation of soil organic carbon stocks in sustaining life and natural resources from the individual to the societal level. Soil is generally strongly under-communicated in society, including institutes of education at all levels, and there is an obvious link to the 8^th mission objective on soil literacy in society, focusing on soil in general but also on its carbon stocks. .

There is a lack of awareness and appreciation of the multiple benefits and co-benefits of increasing and maintaining soil carbon stocks.

Not only for climate change mitigation, but also for soil health, food security including its adaptation to climate change, biodiversity.

Current knowledge gaps to improve our understanding on soil organic carbon (SOC) stock outside Europe highlight the need for standardized estimation on the effect of global land use change, European carbon fooprints, how policy making affect SOC coservation, global bodiversity, the importance in having access to comprehensive data sets and accurate mapping techniques.

European imports of agricultural products can indirectly influence land use changes and soil carbon dynamics in exporting countries, and there need to provide knowledge on:

Quantifying the extent of carbon leakage (i.e., when reduced emissions in Europe lead to increased emissions elsewhere due to trade) caused by European imports.

Several studies collectively point to the need for standardized, cost-efficient, and reliable methods that can be applied at various scales, and the potential of data-driven approaches and global frameworks to address these gaps:

Temporal Variability: Soil carbon levels can fluctuate seasonally and annually due to factors like weather, land use changes, and management practices. Research should focus on understanding these temporal variations and their impact on long-term carbon sequestration.