Impact of wet nitrogen deposition on soil nitrogen emissions in West African ecosystems

Human activities such as fertilization of agricultural lands and human-induced biomass burning strongly impact nitrogen (N) dynamics and losses, with many consequences on the environment. The quantification of N budgets (N inputs and outputs) between the surface and the atmosphere is a prerequisite to understand the N biogeochemical cycle, i.e. how N is transferred from the atmosphere to the biosphere, through the soil and back to the atmosphere from surface emissions. Sub Saharan Africa (SSA) is characterized by an increase in demography, with strong impacts on biodiversity, and on the sustainability of human activities including agriculture. In Africa, the increase in demography and the associated increased fertilizer inputs (to supply growing food and energy demands) will lead to increased emissions from amended soils, which will in turn increase atmospheric N deposition and induce feedbacks to the ecosystems and the atmosphere.

In this context, the NitroAfrica project (2023-2026) is designed to study the impact of N wet deposition on the soil – plant – atmosphere continuum. We make the hypothesis that changes of wet N deposition in West African ecosystems over the 21^th centuries will induce important changes in biogenic emissions from the ecosystems to the atmosphere with impacts on regional atmospheric chemistry and further N deposition. Indeed, increasing trends of N wet deposition has already been observed, especially in the NH₄⁺ form. Three ecoclimatic zones in West Africa are studied, in Guinean (Lamto, Côte d’Ivoire), Sudanese (Korhogo, Côte d’Ivoire) and Sahelian (Dahra, Senegal) zones, where solutions with different NH₄⁺/NO₃^- partition are used to mimic the increase in N wet deposition.

Results on N (N₂O, NO) and CO₂ emissions from soils from plots amended with solutions as well as control plots will be presented. N wet deposition fluxes from recent years will also be presented within the context of existing long-term studies on N wet deposition. This comparison is particularly relevant for the Lamto station where the International Network to study Deposition and Atmospheric chemistry in Africa (INDAAF) is based and provides long-term data since 1995.

This study contributes to fill in the lack of studies in SSA, and to understand the processes involved in N emissions and deposition in tropical regions.