Atmospheric nitrogen budget in Sahelian dry savannas

Abstract. The atmospheric nitrogen budget depends on emission and deposition fluxes both as reduced and oxidized nitrogen compounds. In this study, a first attempt at estimating the Sahel nitrogen budget for the year 2006 is made, through measurements and simulations at three stations from the IDAF network situated in dry savanna ecosystems. Dry deposition fluxes are estimated from measurements of NO₂, HNO₃ and NH₃ gaseous concentrations and from simulated dry deposition velocities, and wet deposition fluxes are calculated from NH₄⁺ and NO₃⁻ concentrations in samples of rain. Emission fluxes are estimated including biogenic emission of NO from soils (an Artificial Neural Network module has been inserted into the ISBA-SURFEX surface model), emission of NO_x and NH₃ from domestic fires and biomass burning, and volatilization of NH₃ from animal excreta. Uncertainties are calculated for each contribution of the budget.

This study uses original and unique data from remote and hardly-ever-explored regions.The monthly evolution of oxidized N compounds shows that emission and deposition increase at the beginning of the rainy season because of large emissions of biogenic NO (pulse events). Emission of oxidized compounds is dominated by biogenic emission from soils (domestic fires and biomass burning of oxidized compounds account for 0 to 13% at the most at the annual scale, depending on the station), whereas emission of NH₃ is dominated by the process of volatilization from soils. At the annual scale, the average gaseous dry deposition accounts for 47% of the total estimated deposition flux, for both oxidized and reduced compounds. The average estimated wet plus dry deposition flux in dry savanna ecosystems is 7.5±1.8 kgN ha⁻¹ yr⁻¹, with approximately 30% attributed to oxidized compounds, and the rest attributed to NH_x. The average estimated emission flux ranges from 8.4(±3.8) to 12.4(±5.9) kgN ha⁻¹ yr⁻¹, dominated by NH₃ volatilization (72–82%) and biogenic emission from soils (11–17%), depending on the applied volatilization rate of NH₃. While larger, emission fluxes are on the same order of magnitude as deposition fluxes. The main uncertainties are linked to the NH₃ emission from volatilization.

When scaled up from the 3 measurement sites to the Sahelian region (12° N:18° N, 15° W:10° E), the estimated total emission ranges from 2(±0.9) to 3(±1.4) TgN yr⁻¹, depending on the applied volatilization rate of NH₃ and estimated total deposition is 1.8(±0.4) TgN yr⁻¹. The dry savanna ecosystems of the Sahel contribute around 2% to the global (biogenic + anthropogenic) nitrogen budget.