Nitrogen compounds emission and deposition in West African ecosystems: comparison between wet and dry savanna

Abstract

Emission fluxes are evaluated including simulated NO biogenic emission from soils, emissions of NO x and NH 3 from biomass burning and domestic fires, and volatilization of NH 3 from animal excreta. This paper is a tentative to understand the eventual impact of the monsoon variability from year to year, with the natural variability of local sources, on the emission and deposition N fluxes, and to compare these evolutions between dry and wet savanna ecosystems. In dry savanna ecosystems where the rain season lasts mainly from June to September, the occurence of rain correlates with the beginning of emission and deposition fluxes. This link is less obvious in wet savanna ecosystems (wet season mainly from May to October), where the surface is less submitted to drastic changes in terms of water content. Whatever the location, the natural variability of rain from year to year does not exceed 15 %, and the variability of emission and deposition magnitude ranges between 15 % and 28 %. While quasi providing the same total N budget, and due to the presence of different types of soils and vegetation, wet and dry savanna do not present the same distribution in emission and deposition fluxes contributions: in dry savanna, the emission is dominated by ammonia volatilization, and the deposition is dominated by the dry contribution. In wet savanna, emission is equally distributed between ammonia volatilization, emissions from biomass burning and natural NO emissions from soils, and wet and dry deposition are equivalent. Due to the scarcity of available data on the African continent, and despite the numerous uncertainties resulting from the different calculations and assumptions, this work is a combination of data from different origins (surface measurements, satellite and modelling) to document the atmospheric Nitrogen cycle in tropical regions.

Dry deposition of nitrogen compounds (NO<sub>2</sub>, HNO<sub>3</sub>, NH<sub>3</sub>), sulfur dioxide and ozone in west and central African ecosystems using the inferential method

“…No specific trend in the variability is observed, as already concluded in the study of Delon et al (2012). The interannual variability is largely attributable to the variability of gaseous concentrations due to the potential variation of the intensity of atmospheric sources and the variability of meteorological data.…”

“…Differences between the two approaches are mainly due to the different resolutions of models and to the degree of details involved in the estimation of input parameters such as the ones detailed in the above paragraph. The previous studies of Delon et al (2010Delon et al ( , 2012 give a point of comparison for N compounds' V d calculated in the present work, as very few studies are available for African ecosystems. Furthermore, as explicated below, the NH 3 bidirectional exchange is applied in the present study, which was not the case in the previous studies of Delon et al (2010Delon et al ( , 2012.…”

“…The inferential technique has been widely used in other studies for different types of ecosystems (Shen et al, 2009;Pineda and Venegas, 2009;Jin et al, 2006;Zhang et al, 2005Zhang et al, , 2009Delon et al, 2010Delon et al, , 2012and Pan et al, 2012). This approach is more suited when routine monitoring data are available but the values of the derived fluxes are clearly dependent on the validity of the dry deposition velocity calculation.…”

“…However, the forcing in ALMIP is available at 10 m, whereas the concentrations of gases are measured at around 2 m for the savanna sites and 3 m for the forested ecosystems within IDAF network. As a first approximation, a logarithmic decrease of the wind forcing from 10 to 2 m, depending on the rugosity of the site, has been applied to calculate deposition velocities at 2 m for the savanna sites, as described in Delon et al (2010Delon et al ( , 2012. This approach to reduce the wind from 10 m to the height of 2 m is based on the constant flux assumption (Baldocchi, 1988;Zhang et al, 2009).…”

Modelling the effect of soil moisture and organic matter degradation on biogenic NO emissions from soils in Sahel rangeland (Mali)

“…In Table 3, they are compared to other measurements made in other types of semi-arid ecosystems, and to wet season measurements in Banizoumbou (Niger), situated in a dry savanna site and presenting the same type of vegetation . Data from this study are also compared to other simulations done with the ISBA model (Interactions Soil Biosphere Atmosphere) used to simulate emission and deposition N fluxes in dry savannas in a previous study Delon et al, 2012), where the N input (fertilization rate) was constant in time, and equivalent to 5.5 kg(N) ha −1 yr −1 (17.4 ng(N) m −2 s −1 ). These results show that the coupled STEP-GENDEC-NO model gives fluxes in the order of known experimental or simulated data, while not exactly equivalent to measurements at the Agoufou site in 2004 and 2005.…”