For France, each group had to choose a type of farming system among the nine case studies defined in the report “Agriculture, Forest, Climate. Towards strategies of adaptation" carried out by the French Ministry of Agriculture (Centre d'Etudes et de Prospective, 2013). In this report, each case study was described based on technical, economic, and environmental variables, including SOC. The following four case studies were chosen by the participants: (i) Polyculture-breeding in Meuse, (ii) Cattle breeding in Creuse, (iii) Field crops in Cher, and (iv) Vineyards in Beaujolais. A brief description is given below, and a comprehensive description of these farming systems can be found in Centre d'Etudes et de Prospective (2013).

The cattle breeding farming system in Creuse (Central France) is based on cattle meat production with weanlings fattening in grasslands. Cereal silage (from only 5% of about 100 ha average farm size) provides additional fodder. Soils are sometimes hydromorphic with a 3% soil organic carbon content. Under high rainfall (at least 1,100 mm/year) and at an elevation of 700 to 1,000 m asl., the plant-growing season is never longer than 6 months. About 35% of the income comes from subsidies. Le Noë et al. (2019) indicates a relatively high increase in SOC stocks in the topsoil of cropland (0–30 cm) of + 2 to 3 % year⁻¹ for the period from 2000 to 2014 for this area. In this area, Pellerin et al. (2019) assesses an additional potential of SOC sequestration of 0.9% year⁻¹ for permanent grasslands.

The field crops farming system in Cher (Central France) is characterized by small farms (~25 ha) producing corn and rape (with wheat-barley rotations), without irrigation. Good soils with about 2.4% organic matter content have a high agronomic potential. The climate has oceanic influence due to western winds and the annual rainfall is 600 to 700 mm, at an elevation of <500 m asl. Farms show good economic conditions. Le Noë et al. (2019) indicates a moderate increase in SOC stocks in the topsoil of cropland (0–30 cm) of + 1 to 2% year⁻¹ for the period from 2000 to 2014 for this area. In this area, Pellerin et al. (2019) assesses an additional potential of SOC sequestration of 5.2% year⁻¹ for croplands.

The vineyard farms of Beaujolais are situated in Eastern France, north to the city of Lyon. The Beaujolais wine farming system studied here represents about a quarter of the farms in the area, with a farm size around 14 ha producing about 500 hl of wine per year, mostly (95%) for bulk sale to wine merchants, the rest being bottled at the farm. Shallow, stony soils hold a low organic carbon content (<2%). Rainfall is about 500 mm year⁻¹, with dry, cold winters and warm summers with frequent storms. The fluctuating wine market makes income irregular. Le Noë et al. (2019) indicates a low increase in SOC stocks in the topsoil of cropland (0–30 cm) of 0 to 1% year⁻¹ for the period from 2000 to 2014 for this area. In this area, Pellerin et al. (2019) assesses an additional potential of SOC sequestration of 3.7%year⁻¹ for vineyards related to inter-rank plant grass cover.

For Senegal, a selection of farming systems was done prior to the workshop based on expert knowledge from colleagues in Senegal, including past or on-going projects related to soil carbon sequestration in the country. The following four case studies were chosen: (i) Peri-urban agriculture in the Niayes, (ii) Irrigated agriculture in the Senegal River valley, (iii) Agroforestry in Faidherbia albida parklands, and (iv) Rain-fed agriculture and extensive livestock integration. A brief description is given here. Soil organic content values are given, when possible, as approximate values obtained from available publications on similar soil types in the area.

The coastal peri-urban agriculture farming systems of the Niayes supply the Senegalese capital Dakar and its suburbs with vegetables and fruits. It is a succession of sandy dunes and interdune depressions where the water table regularly outcrops, and the soil has a high salt content. Urban development and rising standards of living have also created a demand for animal products. Livestock farms contribute to the supply of manure and slurry to horticultural farms. Organic residual products and biowastes represent significant volumes. Soumare et al. (2002) reports a soil organic content of 1.2% for the Niayes area. The urban sprawl of the city of Dakar exerts a strong pressure on this peri-urban farming area. Climate is tropical semi-arid with a single rainy season from July to October. Based on Zomer et al. (2017), an indicative additional potential of SOC sequestration for the Niayes area should be around 2 to 3% year⁻¹.

Irrigated rice cultivation in the Senegal River valley is strongly influenced by the structural organization of cultivated areas and the social organization of producers (Le Gal, 1995). Farms are part of local irrigation schemes and farmers are grouped in peasant organization for water management, agricultural inputs, and mechanization of work. Tillage and harvesting operations are mechanized in large areas, where the most common method of planting is broadcast sowing in water, and weed control is either manual or chemical. For small perimeters, transplanting is the most general rule as well as manual weeding. Soils are mainly Vertisols with swelling clays and frequent high salinity (Boivin et al., 1998). Two cropping seasons are generally carried out: the hot dry season, from February to July, and the wintering season, from July to November-December. Based on Zomer et al. (2017), an indicative additional potential of SOC sequestration for the Senegal River valley area should be around 1 to 2% year⁻¹.

Faidherbia albida is a leguminous tree widely distributed in semi-arid West Africa, as a scattered tree in cropland (agroforestry “parklands”). The best-known peculiarity of F. albida is its reverse phenology (Wickens, 1969). Trees are in leaf, growing, and fruiting during the dry season, whereas leaves are shed after the first rains and growth resumes only at the end of the wet season. This phenology is advantageous for agroforestry, because competition with associated crops growing during the wet season (mainly pearl millet and groundnut in Senegal) is minimized. F. albida is thus a multipurpose tree widely grown for soil fertility improvement but also for fodder production (nitrogen-rich pods) in rural communities. Most parkland soils are Arenosols with low carbon content. Participants to the workshop mainly rely on their experience in the central part of Senegal (Niakhar; rainfall ~ 500 mm) where parklands of F. albida (12.5 m high; 7 trees ha⁻¹; 5% canopy cover) are part of the local agro-silvo-pastoral systems. In agroforestry “parklands,” Corbeels et al. (2018) assess a mean additional potential of SOC sequestration slightly above 4% year⁻¹, ranging from 3 to 22% year⁻¹.

Extensive livestock breeding in Senegal has been a traditional activity based on common use of resources (water, rangelands) in which managed herd movements are necessary for sustainability. The highly seasonal rainfalls brought by the convective storms of the West African monsoon, drive resource production in these rangelands with poor soils showing low soil carbon content. Livestock mostly adapt to the local resource availability and graze under the guidance of a herder or freely within communally exploited grazing areas (Ickowicz et al., 2012). Extensive breeding varies largely by the diversity of animal species, herd size, herding practices, and by the amplitude of their mobility. Agriculture is primarily rain-fed. It has undergone significant changes in the past 50 years. From food and family farming, it has been strongly oriented toward cash crops (peanuts and cotton). The groundnut industry, which has long been the engine of the Senegalese economy, is going through a major crisis. Production is increasing, however, in the cereal (millet, rice, and corn), pulses (peanut, cowpea), fruit and vegetable, and cassava sectors, meeting growing local demand. In rain-fed agriculture, if the three principles of conservation agriculture (i.e., minimum or no tillage/crop residue mulching/intercrop or crop rotation) are implemented, Corbeels et al. (2018) assess a mean additional potential of SOC sequestration above 4% year⁻¹, this being however highly variable.

Each group first had to identify the main sources of GHG emissions corresponding to the studied farming system, and to identify potential practices for soil carbon sequestration. For France, participants were invited to refer to the report “How can French agriculture contribute to reducing greenhouse gas emissions? Abatement potential and cost of ten technical measures” published in 2013 by INRA (Pellerin et al., 2013). For Senegal, each group comprised a specialist of the studied farming system to help in the identification of sources of GHG emissions and therefore potential 4 per 1,000 practices. Next, each working group was asked to identify: (i) barriers to adoption of innovations contributing to the objectives of the 4 per 1,000 Initiative, and (ii) actions to overcome these barriers. All barriers and actions were classified by participants into major “categories” representing scientific disciplines or fields of action.

Barriers and solutions identified were categorized, using keywords (e.g., technical, financial, social…). Visualization of the occurrence of barriers and actions for each farming systems was performed running a Correspondence Analysis (CA) on data from Tables 1, 3 converted in contingency tables, using the R.3.5.3. environment for statistical computing (R Core Team, 2017).

For virtually all cases, the number of identified barriers is surprisingly high and diversified. Despite the different case studies addressed and the different socio-economic contexts between France and Senegal, social and economic contexts were identified as the major barriers to the adoption of 4 per 1,000 practices in both countries and most farming systems (Table 1).

The first and second principal components of the CA explained, respectively, 41.55 and 22.52% of the total inertia (Figure 1). Axis 1 was positively correlated with “Environmental” and “Organizational” barriers and was negatively correlated with “Political” and “Capacity building” barriers. Thus, vineyards in Beaujolais were mainly characterized by “Environmental” and “Organizational” barriers, while polyculture-breeding in Meuse and agroforestry in Senegal were correlated with “Political” and “Capacity building” barriers. According to the results of the CA (Figure 1), most of the farming systems shared barriers related to Axis 1 (Figure 1). Axis 2 was positively correlated with “Cultural” barriers. As livestock and agroforestry in Senegal were the only two farming systems in the upper part of the diagram, the “Cultural” barrier made these farming systems specific. Most farming systems shared “Social,” “Economic,” “Technical” and “Biophysical” constraints (Table 1). These constraints therefore appeared near the center of the CA diagram and did not help in differentiating the farming systems.

The compilation of detailed answers appearing in Table 2 shows that frequently mentioned topics refer to: “Lack of knowledge,” “Lack of training and capacity building,” “Difficulties of fieldwork and workload,” “Risk/change aversion,” “Social pressure” or “Legal, political, and professional compliance.” In both countries, economic, social, and capacity building appeared as major barriers related to an enabling environment. Biophysical barriers (e.g., low capacity of soil carbon storage in sandy soils, availability of organic matter, monsoon variability, fire, erosion) were also identified, mainly in Senegal. In France, the absence of additional income (i.e., economic barrier) resulting from the implementation of 4 per 1,000 practices was considered an impediment to their adoption. In Senegal, land availability and tenure can also be a constraint.

As for barriers, the number of proposed actions to overcome the identified barriers to 4 per 1,000 is high and diversified, ranging from training needs to fund raising or the need for new fodder sources. Identified actions to foster soil carbon sequestration mainly consisted in economic, political, and capacity-building actions (Table 3), in both countries. The first and second principal components of the CA explained, respectively, 31.65 and 29.74% of the total inertia (Figure 2). Axis 1 was positively correlated with “Regulations” and “Organizational” actions and was negatively correlated with “Communication” and “Demonstration” actions. Axis 2 was positively correlated with “Consultation” and “Research” actions and was negatively correlated with “Organizational” actions. Most farming systems shared “Capacity building,” “Political,” and “Economic” actions (Table 3). These actions therefore appeared near the center of the CA diagram and did not help in differentiating the farming systems.

One striking result was the difference of potential actions between farming systems in France and in Senegal. Indeed, all French farming systems were gathered on the diagram (Figure 2), while the Senegalese farming systems were more scattered in several parts of the diagram. According to the results of the CA (Figure 2), most of the farming systems in France shared actions related to “Communication,” “Demonstration,” and “Economic” actions (Figure 2). Furthermore, the diversity of actions identified was higher for farming systems in Senegal, ranging from “Regulations” and “Organizational” (i.e., peri-urban agriculture in the Niayes) to “Research” actions (i.e., agroforestry in Faidherbia albida parklands).

Specifically, actions such as “Adopting territorial approach to improve land planning,” “Coordination between stakeholders and actions,” or “Building networks of demonstrators from researchers to farmers, or from farmers to consumers” (Table 4) show a concern for interactive, perhaps collective, solutions where different persons are connected. Labeling and communication actions on value chains and marketing were mentioned in France, while diagnostic studies, or the need for research and organizational support, were highlighted in Senegal.