Pertinence de l'agriculture de conservation pour tamponner les aléas climatiques : cas des systèmes de culture en riz pluvial au Lac Alaotra, Madagascar

Abstract : Conservation agriculture (CA) is widely disseminated at large scale in sub-Saharan Africa (SSA) in order to restore soil fertility and sustainably increase crop production of family farming. As defined by the FAO, CA is based on the three principles of minimal soil disturbance, permanent soil cover, and complex crop rotation. Among all the beneficial functions of CA, its ability to improve water balance through mulching can buffer water stress during crop cycle, and hence secure yields when rainfall are limited or poorly distributed. In the Lake Alaotra region of Madagascar, the area under rainfed upland rice has expanded lately. The region being characterized by an erratic rainfall distribution, intra- and inter-annually, CA practice seems appropriate to secure rainfed production. The main objective of this study is to evaluate to which extent this climate hazard can be buffered through the practice of CA. Firstly, we ran a review of the scientific literature to better understand the impact of CA in a wider context. Regarding the projections of population growth and climate change for SSA, CA is considered as a climate-smart option, i.e. an agriculture able to simultaneously mitigate climate change, adapt to this change, and sustainably increase productivity. The different studies illustrated the capacity of CA to maintain, or even increase production in the long-term, and in the shorter-term under limited or poorly distributed rainfall African contexts. These results suggested an ability of CA to adapt to climate change, predicting an increase in rainfall variability in SSA. However, the climate change mitigation through carbon sequestration under CA remained unclear because of a superficial storage which may be unsteady. Then, we focused on the study area to evaluate the potential of CA to buffer rainfall hazard. Using a 4-year dataset monitoring farmers’ fields transitioning to CA, we observed a gradual increase in upland rice average yield with a decrease in variability over the consecutive years of CA practice. The data also suggested a capacity of CA to secure early or late sowing. But agro-environmental factors were mainly impacting yields in the region. This exploratory analysis allowed us to observe positive impacts of CA under the climate conditions of the Lake Alaotra region, suggesting an impact on water balance but no information was available to validate this hypothesis. Finally, we focused more precisely on the impact of mulch on water balance and upland rice yields under the climate conditions of the region, using a modeling approach. We ran an virtual experiment with the model PYE-CA to simulate a range of soil and climate conditions met in the region. We confirmed the ability of mulch to reduce, or even suppress, efficiently surface water runoff. We identified the sowing period within which rice growing would be the least impacted by water stress in the region. The results indicated that water availability for rice cropping was slightly impacted by a decrease in runoff for the majority of soil conditions and farmers’ usual sowing dates. Beneficial effects of runoff reduction appeared under higher water stress conditions such as early sowing date or crop intensification and yield variability was decreased. This study allowed us to better apprehend the impacts of CA on water balance in the specific climate context of the study area. It would be interesting to integrate these results at the farm-level to identify the pros and cons of adopting CA under the socio-economic context of the Lake Alaotra region of Madagascar.
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Guillaume Bruelle. Pertinence de l'agriculture de conservation pour tamponner les aléas climatiques : cas des systèmes de culture en riz pluvial au Lac Alaotra, Madagascar. Ecologie, Environnement. Université d'Antananarivo; Montpellier SupAgro, 2014. Français. ⟨tel-01123862v2⟩

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