Articles - Biogeosciences

The Sahel is one of the largest semi-arid regions in the world and it is a transition zone between the Sahara desert in the north and the more humid Sudanese savanna in the south. In semi-arid zones, the exchanges of trace gases are strongly influenced by hydrologic pulses defined as temporary increases in water inputs (Harms et al., 2012). In the West African Sahel (between 12∘ N/18∘ N, 15∘ W/10∘ E), soil water availability strongly affects microbial and biogeochemical processes in all ecosystem compartments (Wang et al., 2015), which in turn determines the exchange fluxes of C and N (Austin et al., 2004; Tagesson et al., 2015a; Shen et al., 2016). After a long dry period (8 to 10 months in the Sahel), the first rainfall events of the wet season cause strong pulses of CO2, N2O, NO, and NH3 to the atmosphere (Jaeglé et al., 2004; McCalley and Sparks, 2008; Delon et al., 2015; Shen et al., 2016; Tagesson et al., 2016b). Anthropogenic activities have a strong impact on N and C cycling, and in large parts of the world, deposition of N compounds has several damaging impacts on ecosystem functions, such as changes in species biodiversity (Bobbink et al., 2010). The Sahel is still a protected region from this N pollution (Bobbink et al., 2010), but climate change could create an imbalance in biogeochemical cycles of nutrients (Delgado-Baquerizo et al., 2013).

The emission of NO from soils leads to the formation of N2O and O3 in the troposphere. Soil NO biogenic emissions from the African continent expressed in teragrammes of nitrogen per year are considered as the largest in the world (Fowler et al., 2015) because of extended natural areas. The pulses of NO from the Sahel region at the beginning of the wet season have been shown to strongly influence the overlying N2O tropospheric column (Jaegle et al., 2004; Hudman et al., 2012; Zörner et al., 2016), indicating the urgent need for improved understanding of the dynamics of NO pulses from this region. NH3 emissions lead to the formation of particles in the atmosphere, such as ammonium nitrates (NH4NO3), whose vapour phase dissociation further produces NH3 and HNO3 (Fowler et al., 2015). The land–atmosphere exchange of ammonia varies in time and space depending on environmental factors such as climatic variables, soil energy balance, soil characteristics, and plant phenology (Flechard et al., 2013). Emissions of these compounds involve changes in atmospheric composition (ozone and aerosol production) and effects on climate through greenhouse gas impacts.

The N exchange fluxes are also influenced by the soil N content, and the main inputs of N compounds into the soil in semi-arid uncultivated regions are biological nitrogen fixation (BNF), decomposition of organic matter (OM), and atmospheric wet and dry deposition (Perroni-Ventura et al., 2010). Soil N losses to the atmosphere involve N2O, NH3, and NO gaseous emissions, whereas within the soil, N can be lost via erosion, leaching, and denitrification. NO emissions to the atmosphere are mainly the result of nitrification processes, which is the oxidation of NH4+ to nitrates (NO3-) via nitrites (NO2-) through microbial processes (Pilegaard et al., 2013; Conrad, 1996). In remote areas, where anthropogenic emissions such as industrial or traffic pollution do not happen, NH3 bidirectional exchanges are regulated through diverse processes: NH3 is emitted by livestock excreta, soil, and litter and is regulated by the availability of NH4+ and NH3 in the aqueous phase (NHx), by the rate of mineralization of NH4+, and by the availability of water, which allows NHx to be dissolved, to be taken up by organisms, and to be released through decomposition (Schlesinger et al., 1991; Sutton et al., 2013). Additionally NH3 can be dry and wet deposited on soil and litter (Laouali et al., 2012; Vet et al., 2014), leaf cuticles, and stomata and regulated by chemical interactions within the canopy air space (Loubet et al., 2012). The N cycle is closely linked to the C cycle, and it has been suggested that C–N interactions may regulate N availability in the soil (Perroni-Ventura et al., 2010). The link between N and C cycles in the soil, and their effects on OM decomposition, affect the emissions of C and N compounds to the atmosphere. These cycles are interlinked by respiration and decomposition processes in the soil, and the balance between C and N is controlled by biological activity, mainly driven by water availability in drylands (Delgado-Baquerizo et al., 2013). Indeed, the decomposition of soil OM, and its efficiency, regulates the amount of CO2 that is released to the atmosphere (Elberling et al., 2003).

Biogeochemical regional models have been applied for N compound emissions mostly in temperate regions (Butterbach-Bahl et al., 2001, 2009), where the spatial and temporal resolution of data is well characterized. Global approaches have also been developed, with a simplified description of processes and with coarse spatial resolution (Yienger and Levy, 1995; Potter et al., 1996; Yan et al., 2005; Hudman et al., 2012). Considering the weak number of experimental data in semi-arid regions about trace gas exchanges and their driving parameters, one-dimensional modelling is a complementary, essential, and alternative way of studying the annual cycle dynamics and the underlying processes of emission and deposition. The specificity of the semi-arid climate needs to be precisely addressed in the models used to be able to correctly represent the pulses of emissions and the strong changes in C and N dynamics at the transition between seasons. Improving the description of processes in 1-D models in tropical regions is therefore a necessary step before implementing regional modelling.

In this study, three main modelling objectives are focused on (1) investigating the links between N and C cycles in the soil and consecutive daily exchanges of NO, NH3, and CO2 between the soil and the atmosphere, at the annual scale and specifically at the transition between seasons, (2) comparing two different formalisms for NH3 bidirectional exchange, and (3) highlighting the influences of environmental parameters on these exchanges. Different one-dimensional models, specifically developed or adapted for semi-arid regions, were used in the study. As a study site, representative of the semi-arid region of the western Sahel, we selected the Dahra field site located in the Ferlo region of Senegal (Tagesson et al., 2015b). The one-dimensional models were applied for the years 2012 and 2013 to simulate the land–atmosphere exchange fluxes of CO2, NO, and NH3. Model results were compared to flux measurements collected during three field campaigns in Dahra in July 2012 (7 d), July 2013 (8 d), and November 2013 (10 d), and presented in Delon et al. (2017).