SCaT

Considering soil for its multiple benefits to Society
While soils are managed locally, their ecosystem services are globally relevant as they address food security and climate change challenges (Bouma 2014; McBratney et al 2014; Adhikari & Hartemink 2016).
In face of global warming there is a strong interest in stabilizing greenhouse gas (GHG) emissions to the atmosphere, which the Conference of Parties further emphasized (COP21, Paris, 2015). Because soils emitted large amounts of carbon (C) to the atmosphere through organic matter decomposition and because approximately 10% of the carbon dioxide (CO2) in the atmosphere passes through the soil each year (Raich & Potter 1995), storing back C into soils appears a promising strategy for lowering atmospheric CO2 content and mitigate climate change. Soils may constitute a major C sink owing to plant photosynthesis and the subsequent transfer of C to soils by living and dead organic matter (e.g. Kirschbaum 2000; Lal 2004; Luo et al 2010; Stockmann et al 2013). Globally, soils are estimated to contain 1500 Pg of soil organic carbon (SOC) to a depth of 1 m, approximately double the amount of C in the atmosphere. Arable soils contain 2 to 30 g C kg-1 (Feller 1993). The “4 per 1000“ initiative launched by France during the COP21 refers to an increase of 4 parts per thousand of soil organic carbon (in the upper 30-40 cm soil layer and each year for the next 25 years) to balance the annual increase of atmospheric CO2 concentration (http://4p1000.org ; IRD-Cirad-INRA CGIAR 2015). According to Lal et al (2013), managing the soil functions that affect the potential for C sequestration could not only mitigate climate change, but also have positive impacts on plant productivity and global food security and thus on adaptation to climate change. The C sequestration potential depends spatially and temporally on many factors such as climate, soil type, and land-use. Thus, the capacity for C sequestration can differ considerably (Schlesinger 1999; Sauerbeck 2001) calling for more research on biophysical limitations of ecosystems in C sequestration, e.g. SOC potential saturation vs. critical thresholds (Stockmann et al 2013) and on multiscale governing mechanisms (Lal et al 2015).
Beside the bio-physical dimension, the on-farm perspective
Many agricultural areas in the tropics have suffered severe depletion of their SOC stocks, offering a potential to recover (e.g. Lal 2004, 2011). Land management can affect soil C sequestration through restoration of degraded soils and adoption of recommended management practices (e.g. Guo & Gifford 2002; Binkley & Menyailo 2005; Hart et al 2005). Technical options can be proposed at a local scale to maintain or enhance soil C sequestration in agricultural land with co-benefits for other ecosystem services. These options target optimal plant productivity (through crop selection, appropriate nutrient management, plant microbial symbiotes, irrigation); reducing SOC loss (reduced tillage, erosion control, cover crops); and increasing C inputs to the soil (e.g. returning crop residues to the soil or importing organic matter such as animal manures, biochar or domestic wastes, after consideration of the potential risks associated with using these materials; Smith et al 2014). Other prominent options in tropical areas refer to agroforestry and improved livestock and manure management (Steinfeld et al 2006; Nair et al 2010; Herrero et al 2010; Lal 2011).
In agricultural landscapes, farmers’ involvement is instrumental for the success of C sequestration, based on the decisions they will make in terms of choice and management of farming activities. We assume that these decisions are mostly driven by economic rather than by environmental objectives (such as C sequestration). Organic amendments to agricultural soils are generally targeted at increasing soil fertility and plant performance, although this practice can also lead to C sequestration and add resilience to the agricultural production systems. More generally, implementation of the management options considered as the “best management practices” by the scientific community face different frames of reference, knowledge, ideas, values and interests of the various 2 stakeholders (Bouma et al 2011). Within multiple agricultural production sectors (crop, livestock, fruits & vegetables), the farmer’s decisions reflect compromises between short-term income maximization, land-tenure securing, resources (seeds, livestock, manpower…) limitations, social duties and risk aversion.
The spatial and organizational scales at which farmers negotiate and collectively organize the different aspects of land management, with direct consequences on management practices at the field level, are the agricultural landscape and the local community respectively (Harvey et al 2013; Torquebiau 2015). Moreover, the time span of C sequestration dynamics are quite specific and needs to be set against (confronted with) the temporal horizons of decisions determining the agricultural practices. Different methods target a fruitful collaboration between actors (research, farmers, advisory services, policy makers, etc.) through arenas or platforms (Innovation Platforms, Learning Alliances, Local Agricultural Research Committees, among others; Hounkonnou et al 2012). By facilitating exchanges between actors to address concrete problems, these arena/platforms provide space for experimentation, learning and negotiation (Kilelu et al 2013). Dedicated farm models are also useful to explore innovative scenarios (Le Gal et al, 2013; Semporé et al 2015), and can serve to evaluate ex-ante impacts of specific technical changes on farms’ economic performances and C sequestration.
The challenges
Promoting C sequestration in soils implies the application of agricultural practices adapted to local situations both environmentally, economically and socially. Linking soil functions and land management with soil C sequestration and connecting them successfully with needs and demands of farmers and other stakeholders can only be successful with multidisciplinary integrated research efforts and when continuing attention is given to interaction, co-constructing and co-learning processes. Thus the challenges are (i) to better understand farmers’ decision making processes and practices by taking into account their diversity of objectives and contexts at farm and agricultural landscape levels (Blanchard et al 2013; Dugué et al 2015); (ii) to support farmers in designing and implementing both technical and organizational innovations that would achieve both economic and C sequestration objectives, for instance through soil fertility or tree management (Le Gal et al 2011); and (iii) to develop in a multidisciplinary approach the next step change in practice that will further improve system management and deliver increased soil carbon (alongside with other co benefits), and to engage farmers and other stakeholders in this transition.