Questioned on the 4/1000 initiative on soil carbon (C) sequestration to face climate change (CC), several Labex Agro units and their partners propose to pull together their research capacities in order to provide new insights in soil C sequestration, the DSCATT project proposes to explore the potential for sequestering C in cultivated soils, taking into account the sustainability of agricultural practices in the context of global changes.
Soil Carbon Sequestration (SCS) is the net balance of all direct positive carbon fluxes coming from the atmosphere into the soil via the primary plant production and of all negative fluxes coming from the soil including indirect GHG emissions needed to produce biomass (Bernoux et al. 2006).
So, the enhancement of SCS concerns both increasing primary plant productivity and avoiding losses of C already present in the soil.Multiple functions are attributed to primary production or plant biomass (food, feed, fiber) and SOC (nutrients reserves, energy for soil life, etc.). Tradeoffs between these functions occurring at different scales of space and time govern peoples’ decisions in terms of agricultural activities and natural resources management, and consequently these drive SCS dynamics over the time (Giller et al. 2011, Tittonell et al. 2015).
Soil management to improve tradeoffs between ecosystem services and SCS need a systemic and transdisciplinary approach integrating biophysical, social, economic and political issues at different scales of time and space, confronting also multi-actors viewpoints to develop and to share a conceptual model that identifies the levers to be tackled (Scoones 2015, Tittonell 2016). The social and ecological systems are dependent through feedback mechanisms, and express complexity at different scales. Addressing this complexity through a transdisciplinary and iterative approach will allow the actors of a given farming system to define and assess agricultural practices relevant to conciliate several objectives in a changing context (Astier et al. 2011, Etienne 2014). Finally, multi-scale modeling including social behaviors as well as biophysical processes appears to be essential to provide a prospective outlook and help actors' decision towards a desirable future.
Several field management options are recognized to improve SOC contents in croplands, such as organic amendments and manure, cover crops, legumes, biochar, agroforestry, conservation agriculture (Vagen et al. 2005, Lorenz and Lal 2014, Poeplau and Don 2015, Paustian et al. 2016, Smith 2016, Cardinael et al. 2017, De Stefano and Jacobson 2017, Corbeels et al. 2018). Nutrient and water use efficiencies are key processes to maximize the primary productivity and therefore soil carbon storage potential. Soil-crop models help to simulate SOC dynamic at different time steps. Experiments on integrated soil fertility management, legumes (rotation, mixed cropping), crop livestock integration and agroforestry systems are useful to test some potentially best practices but also to calibrate models. GHG emissions under agricultural practices are generally poorly addressed (Chapuis-Lardy et al. 2007, Ahlström et al. 2015). Besides, the role of roots has been under-evaluated as a potential carbon supplier to soil while plants exhibiting deeper root systems are expected to better cope with climate changes or better use of soil resources (Iversen 2010, Duursma et al. 2011, Kell 2011, Kautz et al. 2013). Roots and all forms of deep carbon will open new perspectives in terms of soil C storage potential in agricultural systems.
Local and national institutions (norms, customs, traditions) defining social and sectoral arrangements (e.g. land tenure) act on farms or household functioning or territory management that finally drive natural resource management (Mazzucato and Niemeijer 2001, Mazzucato et al. 2001). Farm and landscape models based on multidisciplinary data will help to assess and improve the performance of farms and farming systems including co benefit from SCS (Vayssières et al. 2011). Despite expected co-benefits, adoption of SOC restoring practices by smallholders is low in sub-Saharan Africa (Giller et al. 2009, Jerneck and Olsson 2013). There are several technical, infrastructural, socio-economic, or policy barriers to adoption, which raise questions about the significance of potential impact of interventions on farm income (Harris and Orr 2014 Ehui and Pender 2005, Davis et al. 2008, Jayne and Rashid 2013)) and other aspects valued by farmers. In certain contexts, adequate financial incentives could help farmers to change their current practice. However, to be effective and efficient, incentives must take specific characteristics of SCS as non-linearity in time, site and farm specific of soil C accumulation. High uncertainty other a long time complicates also the monitoring of such economic incentives. Advocating on the importance of soil carbon management and moreover on sustainable soil management would be the first step to promote innovative practices.
Operating on three study casesin Senegal, in Zimbabwe and Kenya,and in France, the project addresses 3 interrelated scales of fields, farms or territories to answer to the question What are long-term efficient strategies to foster soil C sequestration in an agricultural system ?
At field level (Action 1), research focuses on how biomass production and soil C sequestration relate, in different soil and climate conditions. Two approaches complement each other. One studies at the soil-plant interface the processes regulating the forms and residence time of C in soils. It includes the analysis of interactions between nutrients and C storage, the role of deep roots and in soils with contrasting storage potentials. The other approach determines the C balances under different practices. Farms will be characterized in order to propose practices likely to improve complementarities amongst the activities of rural households. At this scale (Action 2), DSCATT research will focus on farmers' practices (for crops, livestock, forestry…) and assess the impacts of farmers' practices on their objectives (income, food security…), taking into account their main constraints (cash, labour…). The project will assess the social and economic determinants of farmers’ decisions and of trade-offs between farm activities.
At the territory (or farmers' network) level (Action 3), the different compartments of agroecosystems and the organic matter flows will be studied. The project will analyze the role of the socio-economic and biophysical contexts and will test several possible changes and their impacts on soil C sequestration dynamics, economic performance of farms and food security. This scientific knowledge and the viewpoints of the farmers involved will be shared and used for a transdisciplinary assessment of several C sequestration strategies in agricultural soils (Action 4). Considering changes and uncertainties, a multi-criteria and prospective evaluation approach is proposed. It will allow iterations between evaluation and redefinition of strategies to cope with global changes in agriculture. The sharing and dissemination of the knowledge (Action 5) enriched by the project will target several audiences (farmers, students and teachers, policy makers) through a variety of communication media and assessment tools.
DSCATT will will link the knowledge on processes governing the preservation of C sequestration and farmers' multiple objectives and constraints.
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