Achievable agricultural soil carbon sequestration across Europe from country‐specific estimates

Abstract The role of soils in the global carbon cycle and in reducing GHG emissions from agriculture has been increasingly acknowledged. The ‘4 per 1000’ (4p1000) initiative has become a prominent action plan for climate change mitigation and achieve food security through an annual increase in soil organic carbon (SOC) stocks by 0.4%, (i.e. 4‰ per year). However, the feasibility of the 4p1000 scenario and, more generally, the capacity of individual countries to implement soil carbon sequestration (SCS) measures remain highly uncertain. Here, we evaluated country‐specific SCS potentials of agricultural land for 24 countries in Europe. Based on a detailed survey of available literature, we estimate that between 0.1% and 27% of the agricultural greenhouse gas (GHG) emissions can potentially be compensated by SCS annually within the next decades. Measures varied widely across countries, indicating differences in country‐specific environmental conditions and agricultural practices. None of the countries' SCS potential reached the aspirational goal of the 4p1000 initiative, suggesting that in order to achieve this goal, a wider range of measures and implementation pathways need to be explored. Yet, SCS potentials exceeded those from previous pan‐European modelling scenarios, underpinning the general need to include national/regional knowledge and expertise to improve estimates of SCS potentials. The complexity of the chosen SCS measurement approaches between countries ranked from tier 1 to tier 3 and included the effect of different controlling factors, suggesting that methodological improvements and standardization of SCS accounting are urgently required. Standardization should include the assessment of key controlling factors such as realistic areas, technical and practical feasibility, trade‐offs with other GHG and climate change. Our analysis suggests that country‐specific knowledge and SCS estimates together with improved data sharing and harmonization are crucial to better quantify the role of soils in offsetting anthropogenic GHG emissions at global level.

capacity of individual countries to implement soil carbon sequestration (SCS) measures remain highly uncertain. Here, we evaluated country-specific SCS potentials of agricultural land for 24 countries in Europe. Based on a detailed survey of available literature, we estimate that between 0.1% and 27% of the agricultural greenhouse gas (GHG) emissions can potentially be compensated by SCS annually within the next decades. Measures varied widely across countries, indicating differences in countryspecific environmental conditions and agricultural practices. None of the countries' SCS potential reached the aspirational goal of the 4p1000 initiative, suggesting that in order to achieve this goal, a wider range of measures and implementation pathways need to be explored. Yet, SCS potentials exceeded those from previous pan-European modelling scenarios, underpinning the general need to include national/ regional knowledge and expertise to improve estimates of SCS potentials. The complexity of the chosen SCS measurement approaches between countries ranked from tier 1 to tier 3 and included the effect of different controlling factors, suggesting that methodological improvements and standardization of SCS accounting are urgently required. Standardization should include the assessment of key controlling factors such as realistic areas, technical and practical feasibility, trade-offs with other GHG and climate change. Our analysis suggests that country-specific knowledge and SCS estimates together with improved data sharing and harmonization are crucial to better quantify the role of soils in offsetting anthropogenic GHG emissions at global level.
[Correction added on 12 October 2021, after first online publication: The ORCiD has been added and spelling corrected for author 'Thomas Kätterer '.]

| INTRODUC TI ON
To meet the Paris Agreement goal of limiting global average temperatures to below 2℃, preferably to 1.5℃, compared to pre-industrial levels (UNFCCC, 2015), the EU aims to reduce 40% of its domestic greenhouse gas (GHG) emissions by 2030 and become the world's first climate-neutral economy by 2050 ('Green Deal') (EC, 2019).
Today, 11% of total European GHG emissions derive from agriculture and measures to reduce and offset these emissions are urgently required to meet climate mitigation targets (EU NIR, 2021). The role of soils in the global carbon cycle and the importance of reducing GHG emissions from agriculture has been increasingly acknowledged (IPCC, 2018). Depending on land use and management, soils can act either as source or sink of CO 2 (Lal, 2004). Soils are the largest terrestrial pool of organic carbon, with global soil organic carbon (SOC) stocks estimated as 863, 1824 and 3012 Pg C in the upper 0.3, 1 and 2 m of soil respectively (Sanderman et al., 2017). Evidence from various long-term soil-monitoring and field experiments (FEs) across several European countries shows that organic carbon contents are decreasing in many agricultural soils (Bellamy et al., 2005;Heikkinen et al., 2013;Keel et al., 2019;Sleutel et al., 2007;Taghizadeh-Toosi et al., 2014).
However, there is general agreement that soil C losses can be reverted and C accumulation increased under selected agricultural practices, which could further benefit soil fertility and ecosystem service delivery (e.g. water infiltration and holding capacity, provision of food and ecological resilience, preventing soil erosion etc.) (Baveye et al., 2020;Lal, 2018). Hence, in the last two decades, more research has been focused on what certain agricultural practices (e.g. cover crops [CC], residue management, land use changes), might contribute to increase SOC stocks and help removing CO 2 from the atmosphere as effective climate mitigation measures (Bolinder et al., 2020;Lal, 2004;Paustian et al., 2016;Smith et al., 2005Smith et al., , 2007. The '4 per 1000' (4p1000) initiative has become a prominent soilbased climate mitigation action also to ensure food security through an annual increase in SOC stocks by 0.4%, or 4‰ per year, in the top 0-40 cm of soil. It was launched in 2015 at the United Nations Climate Change Conference (COP 21, CMP 11) together with the Paris agreement. Since then, the feasibility of the 4p1000 soil C sequestration annual target has been intensively debated. For example Rumpel et al. (2020) summarized the opportunities and limitations of soil carbon sequestration (SCS) as a sustainable development strategy and the implementation challenges of the 4p1000 initiative and Amelung et al.
(2020) stressed the aspirational nature of the sequestration target.
Specific criticism of the 4p1000 initiative relates to the role of biophysical barriers (Baveye et al., 2018;Poulton et al., 2018;Schiefer et al., 2018;de Vries, 2018;White et al., 2018), trade-offs (Lugato et al., 2018), climate change effects (Mondini et al., 2012) and socio-economic and political implications (Baveye et al., 2018;Poulton et al., 2018;White et al., 2018), which all together can greatly affect the 4p1000 SCS target. Besides these well-justified critiques and existing knowledge gaps, soil scientists generally agree that enhancing SCS comes with multiple benefits as associated with increased soil quality and greater resilience of soil ecosystems to human management. In this context, according to Olson et al. (2014) and Chenu et al. (2019), SCS is defined as 'the process of transferring CO 2 from the atmosphere into the soil of a land unit, through plants, plant residues and other organic solids which are stored or retained in the unit as part of the soil organic matter. The sequestered SOC should increase the net SOC storage during and at the end of a study to above the previous pre-treatment baseline'. However, there is still much uncertainty about the extent to which SCS can be enhanced under different management practices, especially in relation to region-specific potentials (Amelung et al., 2020).
To realistically estimate achievable and implementable SCS, the first step is to determine where and to which extent particular SCS measures can be mobilized. Furthermore, costs and practical applicability of these measures should be considered. The potential of SCS of agricultural land is generally estimated from modelling outputs calibrated with long-term experiment (LTE) data. The first estimates of technical/biophysical agricultural SCS potentials for Europe were given by Smith et al. (1998), based on results from several LTEs.
Under the assumption that 100% of the total arable land of Europe (EU15) is converted to no-tillage (NT), Smith et al. (1998)

Funding information
Horizon 2020 European Joint Programme SOIL (EJP-SOIL), Grant/Award Number: H2020-SFS-2018-2020/H2020-SFS-2019-1 European arable lands (EU28) would sequester from 27 to 91 Mt C by 2020 and from 150 to 583 Mt C by 2100 in the upper 30 cm of soil. This means that annual SCS potentials might range initially between 2.7 and 9.1 Mt C , thus offsetting 2.3%-7.8% of total European emissions from the agricultural sector (excluding land use and land use change and forestry [LULUCF] and fossil energy sources used for agriculture).
These results underpin the significant role that agricultural soils can play in mitigating GHG emissions at the European scale. To be realistic, SCS scenarios must be designed at large scale (national or even regional level) and take into account local pedo-climatic, socioeconomic and political environment, which are key for a sustainable and successful implementation. This is best achieved via estimates acquired from within countries, based on the premise that a more detailed knowledge on the individual applicability of SCS measures is available at national rather than international levels. Yet, countryspecific knowledge and estimates on SCS potentials are hitherto insufficiently exploited.
Here we present a reality check on where 24 European countries stand in relation to European GHG reductions targets through application of SCS management options and the 4p1000 initiative (knowledge, feasibility) using a bottom-up approach by exploiting country-specific knowledge and data sets. We give an overview of estimates of national SCS potentials for mineral agricultural soils related to a change in farming practices, the share of land for which such information is available and calculation methodology. The objectives of this study are to assess (1) the potential abatement of GHG emissions relative to GHG emissions of the agricultural sector through the implementation of country-specific SCS measures; (2) the feasibility of the 4p1000 initiative for each country; and (3) major knowledge gaps associated with the estimation of SCS potential across multiple countries.

| ME THODS
This study was conducted within the framework of the Horizon 2020 European Joint Programme SOIL (EJP-SOIL; https://ejpso il.eu/). We that 'the sequestered SOC should increase the net SOC storage during and at the end of a study to above the previous pre-treatment baseline'. (c) Relevant results: SCS (e.g. Tg C year −1 of Tg CO 2-eq year −1 ) must be expressed regarding a specific spatial scale (ha). To meet this goal, literature was first sorted by title and abstract and second by full text screening. All relevant studies were included in a meta-database describing the reference, study settings, measures involved, temporal and spatial scale, methods and quantitative data (available in supporting information).

| Analysis of articles
The studies were analysed according to: a. the methodology used to determine SCS potentials, sorted by level of complexity (Tiers 1-3 according to IPCC) b. parameters involved in the estimate of SCS (reasoning behind the determination of area on which measures are implemented [realistic area], technical and practical feasibility, trade-offs with other GHG, climate change). In this study, we refer to technical feasibility when measures used by the studies are relevant to the country farming specificities. This includes measures which are already applied or the readiness of a technology for its implementation is given (including farmers knowledge, equipment, extension services) (Smith et al., 2008). To practical feasibility, we refer to the consideration of factors such as implementation costs, local policies, regulations and assessment of willingness of farmers.
c. the value of potentials in relation to domestic emissions from the agricultural sector (excl. LULUCF), henceforth referred to as EA (i.e. Emissions from the agricultural sector). The EA was taken from the national GHG inventory reports of each country submitted in 2020 (National Inventory Submissions 2020|UNFCCC). TA B L E 1 Achievable domestic soil carbon sequestration (SCS) reported for specific measures and temporal (years) and spatial scale  (Leifeld et al., 2005) and Bavaria in Germany (Wiesmeier et al., 2020) and Baden Württemberg (Poeplau et al., 2020). Last column also indicates the language of the source.  Estimates are reported for a certain soil depth, area and time period and therefore can hardly be compared directly to each other (Table 1). For this reason, the shares (%) of the potentials to the yearly domestic EA were calculated (Figure 1; Table 1).  (Figure 1) for at least the next 9 years, which is the shortest time scale reported. In the context of the 4p1000 initiative, estimates were also converted into shares of current SOC stocks for agricultural land as reported by   (Figure 2). Annual SCS potentials ranged from 0.03‰ to 2.8‰ of the respective national soil carbon stocks, falling short of the 4p1000 target ( Figure 2). The detailed description of countryby-country potentials including measures used is described in the supporting information.

| Measures
We evaluated the role of different measures in affecting national SCS potentials (Figures 1 and 3; Table 1). In total, 23 different measures were identified and studied to estimate SCS of mineral soils (see Table S1)  Table S1)  which incorporate local information (country-or region-specific data), to more advanced modelling and field-based data included in the Tier 3 level. Figure 4a shows the tier level for each country where data are available and Table 1  The degree of complexity of the studies was evaluated here based on the number of parameters (Score 1 to 5) used within a particular study ( Table 2 and Table S1 for each individual country), visualized in Figure 4b. More than half of the studies described in Table 1 used one single realistic parameter and the other 40% of the studies included between two and three parameters (Figure 4b). The highest score and thus most complete studies are given by Belgium (Dendoncker et al., 2004) and France (Pellerin et al., 2019) with Score 3 including a realistic area, technical and practical feasibility (Table S1; Figure 4b).

Realistic area
The determination of the available area for implementation of a measure is mainly based on expert knowledge and available estimates from literature. Specific approaches for determination of the available area were applied by only four countries (BE, DE, DK and FR) ( Table 2). These approaches include accounting for resource limitations (i.e. manure and straw availability) (Dendoncker et al., 2004;Pellerin et al., 2019;Taghizadeh-Toosi & Olesen, 2016), national regulations that mandate specific measures or the use of fertilizers (Pellerin et al., 2019;Taghizadeh-Toosi & Olesen, 2016) and specific soil properties and soil depths (i.e. non-hydromorphic soils for NT) (Dendoncker et al., 2004;Pellerin et al., 2019). Concerning intra-plot AGF, available plot size and soil depth seem to be important parameters for a realistic estimate of the implementation area (Pellerin et al., 2019). For the expansion of CC, the area occupied by winter crops and spring crops harvested too late to allow the sowing of a winter CC (e.g. potatoes, sugar beets and chicory), are taken into account for the state of Bavaria in Germany (Wiesmeier et al., 2020).

F I G U R E 5 (a) Reduction potential (%)
of greenhouse gas emissions from the agricultural sector for the three scenarios of    Rasse et al., 2019). Both studies recognize that biochar has a strong potential for SCS but also point out that the design of biochar application to soil has to primarily consider its effect on soil fertility, which is still poorly studied for temperate regions.
Practical feasibility is partly met by three countries (BE, IE and FR) which also take into account environmental and policy restrictions (BE; Dendoncker et al., 2004)

Climate change and trade-offs with other GHG (i.e. N 2 O)
Climate change is considered only in one study for Italy by Mondini et al. (2012). Changes in climate (temperature, precipitation and evapotranspiration) between 2001 and 2100 were predicted by 12 different scenarios, based on data from three different global circulation models (GCMs), namely HadCM3, PCM and GCM2 (Mitchell et al., 2004) and four different CO 2 emission scenarios as defined in the IPCC Special Report on Emissions Scenarios (Nakicenovic et al., 2000). The study shows that SOC increase per unit area was negatively correlated with temperature. Therefore, the response to compost application in Italy was only 0.13 t C ha −1 year −1 when considering climate change. This rate is three times lower than values (0.4 ha −1 year −1 ) estimated by Smith et al. (2008) for manure and biosolids application and Freibauer et al. (2004) for compost amendment, where climate change was not accounted for (Mondini et al., 2012).
The results show that GHG emissions from soils and soil C sequestration are rarely measured simultaneously, even though they are both strongly affected by different management practices . Two studies (FR and PL) include GHG emission other than CO 2 to calculate net carbon emissions or removals.
The Polish study of Faber and Jarosz (2018) presents simplified balances of carbon and gas absorption and emission of GHGs (CO 2 , CH 4 and N 2 O) on regional levels over 20 years. Simulations were performed using the DNDC model for the different administrative units of Poland using a 20-year series of meteorological data. The French study by Launay et al. (2021) takes into account several factors including GHG balance, biomass production and nitrogen-and water-related impacts in addition to soil carbon stock changes. By using a high-resolution modelling approach it was shown that current systems in France, even though they are accumulating some C in soils are on average strong sources of GHGs.

| Modelled technical and policy-oriented scenarios versus bottom-up national estimates
Panel (a) of Figure 5 shows

| DISCUSS ION
Estimates of achievable SCS across Europe remain highly uncertain mainly because a common SCS measuring and monitoring approach is not in place yet and because of the lack of data harmonization among countries in relation to the SCS potential of specific measures. Half of the partner countries included in this study lacked data on SCS at higher spatial levels (i.e. regional to national scale), while most of the remaining countries (BE, CH, DK, ES, FR, IR, IT, NL, NO, PO, SE and UK) provided nation-wide achievable SCS potential estimates produced using different approaches. It is remarkable how data and country-based knowledge on SCS potentials is highly heterogeneous regarding the choice of measures in each country, the methodological approach to assess SCS and the evaluation of other relevant parameters (e.g. economic and regulatory challenges) potentially affecting practical achievable SCS.

| Management options for SCS across Europe
The most frequently suggested measures identified in previous studies for SCS at the European scale are CC, residue management (RES), conversion to permanent GR, reduced tillage (RET) and NT (Freibauer et al., 2004;Vleeshouwers & Verhagen, 2002) but also the regular addition of animal manures to soils (Fornara et al., 2016;Freibauer et al., 2004;Smith et al., 1997). Other less common measures such as AGF in France and bioenergy crops in Belgium appeared more important in terms of SCS potential than prominent measures like CC and permanent GR. This suggests that the potential to adopt realistic management practices also depends on current farms structure and resource availability.
The study of Dendoncker et al. (2004), for instance, shows that the conversion of arable land to GR in Belgium is not likely to happen and that current GR area is rather decreasing. Taghizadeh-Toosi and Olesen (2016) show that despite RES may increase SCS in Denmark, its potential is limited by the alternative fate of straw for fuel production, feed or bedding.
Several studies included RET and NT even though many European FE have shown that there is no or little effect on C stock changes when considering the whole soil profile (Anken et al., 2004;D'Hose et al., 2016;Dimassi et al., 2013;Feiziene et al., 2018;Hermle et al., 2008;Martínez et al., 2016;Meurer et al., 2018;Willekens et al., 2014). However, the effect of NT practices depends on pedoclimatic conditions (Chenu et al., 2019;Kochiieru et al., 2020). NT and RET may be best suited for warm and dry conditions where positive effects on soil carbon stocks have been shown (Farina et al., 2018;López-Bellido et al., 2020;Moreno-García et al., 2020).
NT, however, is associated with other agronomic benefits such as shorter periods with bare soil and better soil structure. This reduces soil erosion and increases water conservation, which may further justifies its application. This is particularly true for regions where water scarcity and erosion risks are limiting factors for agriculture (Álvaro-Fuentes et al., 2014). Moreover, NT also allows longer periods for intercrops, which also promotes soil protection from erosion.
Improving water availability and decreasing erosion risks might be a critical point in the context of climate change (Baveye et al., 2020), with the increasing occurrence of extreme climatic events such as prolonged drought and stormy rainfalls.
A highly debated measure currently adopted by some countries (BE, FR, NL and PL) for their SCS estimates is the application of animal manure. Manure is not always considered a SCS measure because it rather redistributes organic matter from one pool to another and is therefore not a net sink for CO 2 (Chenu et al., 2019;Leifeld & Fuhrer, 2010;Schlesinger, 2000). However, Smith et al. (1997Smith et al. ( , 2000 state that using agricultural by-products such as animal manure is crucial to recycling organic matter and to sequester carbon in soils. Also, the redistribution of manure previously applied on GR to arable land would lead to SCS as croplands have lower SOC contents (Dendoncker et al., 2004). In general, availability of manure is bound to animal husbandry, which has an intrinsically higher internal C return as compared to pure cash crop production where the share of exported biomass is much higher (Haberl et al., 2007).
Alternative measures like biochar and deep ploughing which are potentially interesting for many countries, were only mentioned by two countries (CH and NO). In recent years, biochar has been promoted as C negative emission technology and can also have positive effects on soil fertility and crop production (Lehmann, 2007;Smith, 2016). Biochar does not require extra additions of N fertilizer to sequester C in soils and thus can directly contribute to reducing N 2 O emissions .
However, biochar is a technology under development and questions of production costs and competition for feedstock with other bioenergy technologies need to be addressed. As biochar lasts in soil for centuries to millennia, a precautionary approach is indispensable and the in-depth study of the conditions of biochar application to soil for a successful use, as well as an evaluation of associated risks, is urgently needed. Furthermore, biochars ability to influence the stabilization of plant-derived C and other C inputs needs to be assessed across different pedo-climatic conditions (Rasse et al., 2019).
Deep ploughing is a method used to improve soil structure and/ or overcome hardpans of podzols, and could contribute to increase SOC stocks significantly (Alcántara et al., 2016;Schneider & Don, 2019). Nevertheless, it is essential to note that deep ploughing is an irreversible soil intervention influencing several soil ecosystem services (either positively or negatively), which needs to be assessed when optimizing soil fertility for crop production (Schiedung et al., 2019). Accordingly, the extent of the area where this specific measure is expected to be beneficial for soil fertility (podzols with hardpans) is relatively limited in European countries but might be significant at global scale.

| Methodological approaches and assessment of SCS
The comparability of results among countries remains challenging due to the different methodological approaches implemented.
There is, for example, a large variation in both soil depths and periods of time considered when estimating SCS. By definition, SCS following a change in farming practice will only last for a certain period until soil carbon stocks will reach a new steady state. The IPCC (2019) recommends a time horizon of at least 20 years to estimate SCS potentials. Findings from different LTEs, however, show how SCS can continue over many decades (Fornara et al., 2016;Poulton et al., 2018) and that soils may reach a new C equilibrium only after 100 years since a land-use change (Johnston et al., 2017). Because rates of soil carbon accumulation are not linear and tend to decrease over time, annual rates of C accumulation (Tg C year −1 ) will be larger if calculated for a shorter period of time (e.g. 10 or 20 years) and will decrease if calculated for longer periods (e.g. 100 years). To harmonize methodologies and reduce variability, Smith et al. (2020) (Amundson & Biardeau, 2018;MacLeod et al., 2010).
Improved information about the feasibility of implementing agricultural measures with high SCS potential, including the economic efficiency and social acceptability on a country scale, is needed.
The study of Pellerin et al. (2019) for instance, shows that the implementation of most practices that sequester C will result in a cost to the farmers, whereas other measures are associated with essential co-benefits (e.g. biodiversity, regulation of the water cycle, erosion reduction and other societal benefits), which are not yet monetized. Agroforestry, a relatively expensive measure for instance, comes with the simultaneous production of food and fibre, an increase in biodiversity, water and soil conservation, and improved resilience against climate change. Therefore, further efforts are needed to estimate the change in the value of co-benefits as a result of soil management changes (Amelung et al., 2020;Pellerin et al., 2019). The valorization of co-benefits could be key to meet social and economic acceptance. At the same time, it is also important that trade-offs are considered.
There is an urgent need to consider the impact of climate change on agriculture to correctly design achievable SCS scenarios. At present, almost no studies include climate change scenarios for their estimates. Achievable C sequestration and GHG emissions from the agricultural sector must be considered in the context of climate change, which is expected to significantly affect, land use, production systems and farming practices in the near future. The study of Mondini et al. (2012) clearly shows the importance of including climate change scenarios and indicates that sequestration rates could be three times lower than expected because of climate change.
Climate change will unevenly affect European regions and hence soil carbon dynamics (Kovats et al., 2014;Meersmans et al., 2016).  . In addition, estimation of realistic areas of implementation is mostly missing and is probably overestimated.

| Comparing country-specific estimates to the modelled potentials
By comparing three policy-oriented scenarios by  with the national estimates we find that most of national potentials are considerably higher than modelled potentials (BE, ES, FR, NL, PL, PT and UK). Results show that the sum of the reported potentials would amount to 15.2 Tg C year −1 , which would offset 13% of the total European EA. This value is considerably higher than the previously suggested maximal annual potential of 9.1 Mt C (10 years; , which corresponds to 7.8% of the total European emissions from the agricultural sector. We find two possible reasons for the higher SCS estimates: One reason is that some countries (ES, PL and UK) assumed a considerably larger area of implementation. There is still high uncertainty concerning estimates of realistic areas. In some countries, ES and PL for instance,  (Figure 5b). The relatively high average SCS rate of Belgium is achieved by a combination of measures, of which bioenergy crops seems to play a key role with rates of 0.61+4.2 (fossil fuel savings) t C ha −1 year −1 applied on an area of 20 kha (Dendoncker et al., 2004).
Our study clearly shows that bottom-up approaches and countryspecific expert knowledge are crucial to evaluate the achievable SCS and therefore, are complementary to homogeneous modelling approaches. The national potentials differ considerably from country to country and the considered measures go far beyond the most prominent measures (CC, GR, RES, RET and LEY) assessed by Lugato and others (Freibauer et al., 2004;Vleeshouwers & Verhagen, 2002). The measures with the highest modelled technical potentials (conversion to GR) are, in most cases, limited to relatively small areas because of country-specific farming situations.

| Lessons for future SCS studies from available data on national estimates
Available country-based knowledge on national achievable SCS potentials is still limited, and only half of the analysed partner countries have explored nationwide achievable SCS potential. Information provided often does not consider practical and socio-economic implications, which are vital for sustainable implementation. The feasibility of the 4p1000 target seems highly questionable. However, national SCS potentials do suggest potentially important contributions to climate mitigation offsetting national GHG emissions from the agricultural sector in the range of 0.1% and 27% annually. Furthermore, national SCS estimated potentials presented here exceeded those from previous pan-European modelling scenarios, underpinning the need for considering country-and region-specific knowledge and expertise as a means for improvement and as complementary approach.
Comparisons among countries are limited by methodological heterogeneity. Although guidelines for technical assessment of C stock changes at various complexities (tiers) exist, a standard protocol to measure and compare achievable SCS is still missing. Even though many studies already use Tier 3 approaches, the degree of complexity of the studies, which is dependent on the five defined controlling factors is in general low. The degree of complexity of a study is, however, crucial for realistic and practical estimates of potentials. Efforts should be taken, not only to move towards higher tiers, but also to achieve higher degree of complexity in order to better inform policy makers and implement feasible and effective SCS measures. Future studies should also account for co-benefits, when calculating costs. Many SCS measures are costly, and valorization of co-benefits could be key for social and economic acceptance. Finally, the high heterogeneity of data urgently requests a harmonized approach to evaluate the achievable SCS.
Relevant literature comprising national SCS potentials was provided by the following experts of the partner countries. We therefore gratefully acknowledge Alberto Sanz Cobeña, Alice Budai, Andis