Nitrogen emission and deposition budget in West and Central Africa

Abstract

Atmospheric nitrogen depends on land surface exchanges of nitrogen compounds. In Sub Saharan Africa, deposition and emission fluxes of nitrogen compounds are poorly quantified, and are likely to increase in the near future due to land use change and anthropogenic pressure. This work proposes an estimate of atmospheric N compounds budget in West and Central Africa, along an ecosystem transect, from dry savanna to wet savanna and forest, for years 2000−2007.The budget may be considered as a one point in time budget, to be included in long term studies as one of the first reference point for Sub Saharan Africa. Gaseous dry deposition fluxes are estimated by considering N compounds concentrations measured in the frame of the IDAF network (IGAC/DEBITS/AFrica) at the monthly scale and modeling of deposition velocities at the IDAF sites, taking into account the bi directional exchange of ammonia. Particulate dry deposition fluxes are calculated using the same inferential method. Wet deposition fluxes are calculated from measurements of ammonium and nitrate chemical content in precipitations at the IDAF sites combined with the annual rainfall amount. In terms of emission, biogenic NO emissions are simulated at each IDAF site with a surface model coupled to an emission module elaborated from an artificial neural network equation. Ammonia emissions from volatilization are calculated from literature data on livestock quantity in each country and N content in manure. NO x and NH 3 emission from biomass burning and domestic fires are estimated from satellite data and emission factors. The total budget shows that emission sources of nitrogen compounds are in equilibrium with deposition fluxes in dry and wet savannas, with respectively 7.40 (±1.90) deposited and 9.01 (±3.44) kgN ha −1 yr −1 emitted in dry savanna, 8.38 (±2.04) kgN ha −1 yr −1 deposited and 9.60 (±0.69) kgN ha −1 yr −1 emitted in wet savanna. In forested ecosystems, the total budget is dominated by wet plus dry deposition processes (14.75 ± 2.36 kgN ha −1 yr −1 ), compared to emissions processes (8.54 ± 0.50 kgN ha −1 yr −1 ).

First Assessment of Inorganic Nitrogen Deposition Budget Following the Impoundment of a Subtropical Hydroelectric Reservoir (Nam Theun 2, Lao PDR)

“…That variability results from the natural variability of rainfall and integrated the variability of the different atmospheric sources of gases and particles influencing nitrogen rain content. This percentage of variability is close to the ±25% maximum variation for total N wet deposition fluxes found by Galy‐Lacaux and Delon () based on a synthesis on the different African IDAF sites (dry savanna, wet savanna, and forest). The amount of N wet deposition on the NT2R was estimated to be 5.01 ± 0.92 kgN·ha −1 ·year −1 after the impoundment.…”

“…Wet deposition fluxes are directly linked to the annual rainfall that varied from 2,182 to 3,420 mm/year with an interannual variability of 22% over the period 2004–2007 before impoundment and from 2,165 to 3,162 mm/year with an interannual variability of 15% over the period 2008–2012 after impoundment. Knowing that estimated uncertainty on wet deposition fluxes measurement is about 10% (Galy‐Lacaux & Delon, ), it seems reasonable to consider a total variability of ±30% for N wet deposition fluxes. That variability results from the natural variability of rainfall and integrated the variability of the different atmospheric sources of gases and particles influencing nitrogen rain content.…”

“…Wet deposition fluxes uncertainties rely on uncertainties on rainfall amount measurements and on ionic chromatography chemical analysis. Combining these uncertainties, we estimate that the uncertainty on wet deposition fluxes measurements is about 10% (Galy‐Lacaux & Delon, ).…”

Contrasting nitrogen fluxes in African tropical forests of the Congo Basin

“…), and some other sites in South America and Southeast Asia (Bruijnzeel ), with very few data on African tropical forest (Galy‐Lacaux et al. ).…”

“…This biases our perception of forest N cycling, since it depends on geographic location (deposition; Dentener et al 2006), forest type , climate (Weintraub et al 2016), and topography (Weintraub et al 2014). The availability of studies that extensively characterize the N cycle and its fluxes in tropical forests is also geographically biased, being studied in Costa Rica (Taylor et al 2015), Hawaii (Vitousek 1984, Hedin et al 2003, and some other sites in South America and Southeast Asia (Bruijnzeel 1991), with very few data on African tropical forest (Galy-Lacaux et al 2014).…”