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

摘要

Surface emission and deposition fluxes of reactivenitrogen compounds have been studied in five sites ofWest Africa during the period 2002 to 2007. Measurementsof N deposition fluxes have been performed in IDAF sitesrepresentative of main west and central African ecosystems,i.e., _3 stations in dry savanna ecosystems (from 15° N to1_2° N), and _2 stations in wet savanna ecosystems (from 9° Nto 6° N). Dry deposition fluxes are calculated from surfacemeasurements of NO_2, HNO_3 and NH_3 concentrations andsimulated deposition velocities, and wet deposition fluxes arecalculated from NH+4 and NO~-_3 concentration in samples ofrain. Emission fluxes are evaluated including simulated NObiogenic emission from soils, emissions of NOx and NH_3from biomass burning and domestic fires, and volatilizationof NH_3 from animal excreta. This paper is a tentative to understandthe eventual impact of the monsoon variability fromyear to year, with the natural variability of local sources, onthe emission and deposition N fluxes, and to compare theseevolutions between dry and wet savanna ecosystems. In drysavanna ecosystems where the rain season lasts mainly fromJune to September, the occurence of rain correlates with thebeginning of emission and deposition fluxes. This link is lessobvious in wet savanna ecosystems (wet season mainly fromMay to October), where the surface is less submitted to drasticchanges in terms of water content. Whatever the location,the natural variability of rain from year to year does notexceed 15 %, and the variability of emission and depositionmagnitude ranges between 15% and _28 %. While quasi providingthe same total N budget, and due to the presence ofdifferent types of soils and vegetation, wet and dry savannado not present the same distribution in emission and depositionfluxes contributions: in dry savanna, the emission isdominated by ammonia volatilization, and the deposition isdominated by the dry contribution. In wet savanna, emissionis equally distributed between ammonia volatilization, emissionsfrom biomass burning and natural NO emissions fromsoils, and wet and dry deposition are equivalent. Due to thescarcity of available data on the African continent, and despitethe numerous uncertainties resulting from the differentcalculations and assumptions, this work is a combination ofdata from different origins (surface measurements, satelliteand modelling) to document the atmospheric Nitrogen cyclein tropical regions.