Modelling land atmosphere daily exchanges of NO, NH<sub>3</sub>, and CO<sub>2</sub> in a semi-arid grazed ecosystem in Senegal

Abstract. Three different models (STEP-GENDEC-NOflux, Zhang2010 and Surfatm) are used to simulate NO, CO2, and NH3 fluxes at the daily scale during two years (2012–2013) in a semi-arid grazed ecosystem at Dahra (15°24′10′′ N, 15°25′56′′ W, Senegal, Sahel). Model results are evaluated against experimental results acquired during three field campaigns. At the end of the dry season, when the first rains rewet the dry soils, the model STEP-GENDEC-NOflux simulate the sudden mineralization of buried litter, leading to pulses in soil respiration and NO fluxes. The contribution of wet season fluxes of NO and CO2 to the annual mean is respectively 51 % and 57 %. NH3 fluxes are simulated by two models: Surfatm and Zhang2010. During the wet season, air humidity and soil moisture increase, leading to a transition between low soil NH3 emissions (which dominate during the dry months) to large NH3 deposition on vegetation during wet months, Results show a great impact of the soil emission potential and a close agreement between the two models. The order of magnitude of NO, NH3 and CO2 fluxes are correctly represented by the models, as well as the sharp transitions between seasons, specific to the Sahel region. The role of soil moisture on flux magnitude is highlighted, whereas the role of soil temperature is less obvious. The simultaneous increase of NO and CO2 emissions and NH3 deposition at the beginning of the wet season is attributed to the availability of mineral nitrogen in the soil and also to microbial processes which distribute the roles between respiration (CO2 emissions), nitrification (NO emissions), volatilization and deposition (NH3 emission/deposition). This objective of this study is to understand the origin of carbon and nitrogen compounds exchanges between the soil and the atmosphere, and to quantify these exchanges on a longer time scale when only few measurements have been performed.