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

C Delon,C Galy-Lacaux,B Diop,M Adon,A Akpo,D Serça,C Liousse

doi:10.5194/bg-9-385-2012

Abstract

Abstract. Surface emission and deposition fluxes of reactive nitrogen compounds have been studied in five sites of West Africa during the period 2002 to 2007. Measurements of N deposition fluxes have been performed in IDAF sites representative of main west and central African ecosystems, i.e., 3 stations in dry savanna ecosystems (from 15° N to 12° N), and 2 stations in wet savanna ecosystems (from 9° N to 6° N). Dry deposition fluxes are calculated from surface measurements of NO2, HNO3 and NH3 concentrations and simulated deposition velocities, and wet deposition fluxes are calculated from NH4+ and NO3− concentration in samples of rain. Emission fluxes are evaluated including simulated NO biogenic emission from soils, emissions of NOx and NH3 from biomass burning and domestic fires, and volatilization of NH3 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.

Highlights

The vast majority of N in the atmosphere is N2, which is biologically unavailable to most organisms
Means of deposition velocities for NO2, HNO3 and NH3 have been calculated in order to reproduce the seasonal cycle of the deposition process at each site
The types of vegetation and soil are completely different between the tropical sites considered in this study and the european sites of Flechard et al (2011) study, and maximum values are found during the wet season, when the vegetation cover and the soil moisture are at their maximum, whereas soil moisture in european sites is less subject to such drastic seasonal changes

Summary

Introduction

The vast majority of N in the atmosphere is N2, which is biologically unavailable to most organisms. Most biological N fixation (BNF) in terrestrial systems occurs in tropical regions (Galloway et al, 2008), and the response of these systems to additional N inputs could result in rapid N losses to air and water (Matson et al, 1999). Chen et al (2010) have estimated that the mean BNF rate in savannas was 18.6 kgNha−1 yr−1. The flux of a trace gas between the soil and the atmosphere is the result of three basic processes: production, consumption and transport. In the case of Nr, production leads to emission of N compounds such as NO and NH3, and consumption is preceded by transport and deposition of compounds such as gases (NH3, NO2, HNO3, Peroxy Acetyl Nitrates,...), and particles (pNH+4 , pNO−3 ). The atmospheric nitrogen budget depends on emission and deposition fluxes both as reduced and oxidized compounds

Methods

Results

Discussion

Conclusion