Conservation agriculture (CA) is often presented as a promising pathway for climate change mitigation. By reducing tillage and maintaining crop residues on the soil surface, CA is expected to enhance soil carbon storage and limit greenhouse gas emissions. However, its overall climate impact remains debated, because these practices also modify the land surface itself, notably by changing surface albedo the fraction of incoming solar radiation that is reflected back to the atmosphere. In this study, we explored how these biogeochemical and biogeophysical processes interact in real agricultural systems. Using two long-term experimental sites located in a subhumid tropical region and characterized by contrasting soil types, we assessed the combined climate effects of no-tillage and mulching. Our results show that no-tillage alone did not significantly affect soil organic carbon stocks, whereas mulching consistently increased carbon storage at both sites.

In contrast, neither practice substantially altered nitrous oxide emissions. Beyond these biogeochemical responses, clear biogeophysical effects emerged. No-tillage systematically increased surface albedo, leading to a cooling effect regardless of soil type. Mulching, however, produced contrasting outcomes: on dark Ferralsols, surface residues increased albedo and generated a cooling effect, while on brighter Lixisols, mulching provided little cooling and even led to a warming effect when residues were maintained after harvest. When these effects were integrated and upscaled to Sub-Saharan Africa, the picture became even more nuanced. Depending on soil brightness, mulching could either contribute to net cooling up to –0.75 Tg CO₂ equivalent per year on dark Ferralsol or, conversely, result in net warming of up to +0.51 Tg CO₂ equivalent per year on bright Lixisols.

Taken together, these findings highlight that the climate benefits of conservation agriculture cannot be evaluated solely through changes in soil carbon or greenhouse gas emissions. Changes in surface albedo can either reinforce or counterbalance these benefits, depending on local soil properties. This underscores the need to systematically integrate biogeophysical effects into assessments of agricultural climate mitigation strategies.

Souleymane Diop, LGENS.

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