Atmospheric nitrogen budget in Sahelian dry savannas 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
Received: 30 April 2009 – Published in Atmos. Chem. Phys. Discuss.: 30 June 2009 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 NOx and NH3 from
domestic fires and biomass burning, and volatilization of NH3 from
animal excreta. Uncertainties are calculated for each contribution of the budget.
Revised: 05 March 2010 – Accepted: 09 March 2010 – Published: 23 March 2010
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
NHx. 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.
Citation: Delon, C., Galy-Lacaux, C., Boone, A., Liousse, C., Serça, D., Adon, M., Diop, B., Akpo, A., Lavenu, F., Mougin, E., and Timouk, F.: Atmospheric nitrogen budget in Sahelian dry savannas, Atmos. Chem. Phys., 10, 2691-2708, doi:10.5194/acp-10-2691-2010, 2010.