1Laboratoire d'Aérologie, Université de Toulouse and CNRS, Toulouse, France
2CNRM/GMME, Météo-France, Toulouse, France
3LAboratoire de Physique de l'Atmosphere, Abidjan, Côte d'Ivoire
4Université de Bamako, Mali
5Université Abomey Calavi, Cotonou,Bénin
6CESBIO, Toulouse, France
*formerly at: CESBIO, Toulouse, France
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 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 NOx and NH3 from domestic fires and biomass burning, and volatilization of NH3 from animal excreta.
This study uses original and unique data from remote and hardly-ever-explored regions. The monthly evolution of oxidized N compounds shows that deposition increases 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 account for 27% at the most, depending on the station), whereas emission of NH3 is dominated by the process of volatilization. Deposition fluxes are dominated by gaseous dry deposition processes (58% of the total), for both oxidized and reduced compounds. The average deposition flux in dry savanna ecosystems ranges from 8.6 to 10.9 kgN ha−1 yr−1, with 30% attributed to oxidized compounds, and the other 70% attributed to NHx. The average emission flux ranges from 7.8 to 9.7 kgN ha−1 yr−1, dominated by NH3 volatilization (67%) and biogenic emission from soils (24%). The annual budget is then balanced, with emission fluxes on the same order of magnitude as deposition fluxes.
When scaled up to the Sahelian region (10° N:20° N, 15° W:10° E), the estimates of total emission range from 3.6 to 4.5 TgN yr−1 and total deposition ranges from 3.9 to 5 TgN yr−1. The N budget gives a net deposition flux ranging from 0.2 to 0.6 TgN yr−1. If scaled up to the global scale (in the tropical band), it is possible to calculate a total budget of oxidized and reduced N compounds for dry savannas, with a global nitrogen deposition flux ranging from 11.1 to 14.1 TgN yr−1, and a global emission flux ranging from 10.1 to 12.5 TgN yr−1. These ecosystems contribute a significant amount (around 12%) to the global nitrogen budget.