ED 173 - SDU2E
Proposition de Sujet de thèse 2014
Nom du laboratoire (et n° de l’unité) dans lequel se déroulera la thèse :
LABORATOIRE D'AEROLOGIE (UMR UPS-CNRS 5560)
Titre du sujet proposé :
Surface atmosphere Nitrogen cycle in semi arid ecosystems
Financement :
x mis au concours (contrat doctoral ministériel)
Spécialités de l’école doctorale : (cocher une seule spécialité sans la modifier)
x Climat,
Océan, Atmosphère, Surfaces Continentales
Nom et statut (PR, DR, MCf, CR, …) du (des) responsable(s) de thèse (préciser si HDR) :
Dominique Serça (MC HDR UPS), Claire Delon (CR CNRS) and Corinne Galy-Lacaux (IR HDR
CNRS)
Coordonnées (téléphone et e-mail) du (des) responsable(s) de thèse :
D Serça (05 61 33 27 04, dominique.serca@aero.obs-mip.fr)
C Delon (05 61 33 27 44, claire.delon@aero.obs-mip.fr)
C Galy-Lacaux (05 61 33 27 06, corinne.galy-lacaux@aero.obs-mip.fr)
Résumé du sujet de la thèse
The Nitrogen (N) cycle is closely linked to water and Carbon cycle, and participates to
surface-atmosphere interactions. Model studies try to reproduce the mechanisms responsible
for these exchanges, to link hydrology, vegetation, chemistry, dynamics and biogeochemistry,
in order to integrate all these surface processes and their impacts.
A single molecule of reactive N can “cascade” through the environment, and can affect
climate, chemistry of the atmosphere, and the composition and function of terrestrial and
aquatic ecosystems (Galloway et al., 2008).
In semi arid regions, far from large cities, reactive nitrogen comes from sources such
as the soil surface, where organic or synthetic fertilisation can be applied (origin of ammonia
volatilization), and from domestic or vegetation fires (responsible for Nitrogen oxides and
ammonia emissions).
Natural biogenic nitrogen emissions from soils to the atmosphere result from
biogeochemical processes in the soil and at its surface (Delon et al., 2012). These processes
are governed by temperature and moisture conditions, directly linked to precipitations, and by
the quantity of organic matter, linked to the quantity of vegetation, the presence of livestock
or not, the fertilization (organic or synthetic), and to agriculture practices.
This Nitrogen reacts with other compounds in the atmosphere, and therefore deposits
on bare soil or vegetation, through wet and dry (gaseous and particulate) deposition processes.
ED 173 - SDU2E
It participates also to ozone formation in the lower troposphere, which will be transported far
away from the continent. As an example, Delon et al. (2010) have estimated that biogenic
nitrogen compound emissions are about 2 TgN/yr (in West African Sahel), approximately 2%
of total anthropogenic and biogenic emissions in the world. This result emphasizes the
importance of better quantifying nitrogen compounds emissions and deposition in semi arid
regions.
Deposition is a key element of the cleaning potential of the atmosphere from
pollutants emitted at the surface, and plays a great role in chemistry and ecosystems
dynamics. Indeed, reactive nitrogen deposition brings essential nutrients to plants
development, but can also act as a toxic substance if this deposition is too important. This
constitutes a major threat on ecosystems in terms of eutrophication and acidification, and may
affect the variety and composition of vegetal communities (biodiversity). Each ecosystem has
a critical load, above which the N deposition will be damageable. In semi arid regions, this
critical load has been rarely studied and necessitates dedicated studies, along with studies on
N emissions and deposition processes.
The change of carbon, nitrogen, and other nutrients in the soil is strongly dependant on
organic matter and vegetation dynamics. Organic matter quantity and quality need to be
assessed in terrestrial ecosystems and soils. This natural resource needs to be considered and
managed ecologically and economically for a sustainable agriculture.
This PhD subject is about Nitrogen cycle study and N cascade in semi arid
ecosystems. The main objective is to better understand and analyse biogeochemical processes
in soils, and to link these processes to emission and deposition fluxes of reactive nitrogen
compounds between surface and atmosphere. In a first step, this study deals with the
understanding of N cascade at rural sites, to further extrapolate the processes to disturbed
ecosystems (under anthropogenic influence of synthetic fertilizers for example).
This PhD subject relies on ongoing existing programs:
1- In the framework of long term observation networks: IDAF (IGAC-DEBITS-Africa),
initiated in 1995 is composed by a network of 9 sites representative of the great
African ecosystems to study the biogeochemical cycles of Carbon and Nitrogen (focus
on deposition processes), and AMMA-CATCH (African Multidisciplinary Monsoon
Analysis-Couplage de l’Atmosphère Tropicale et du Cycle Hydrologique), which aim
is to study vegetation dynamics and water cycle .
2- In the framework of the DACCIWA program (Dynamics-aerosol-chemistry-cloud
interactions in West Africa, FP7), concerning the impact of emissions and deposition
of Nitrogen compounds on ecosystems.
Moreover, this subject is in the following of several studies on N compounds emissions
and deposition processes in West Africa lead by our team in Toulouse.
The topics of this PhD subject lies on ecosystem sustainability, nitrogen cycle,
exchanges between surface and atmosphere, soil fertility, and nitrogen deposition on
ecosystems in semi arid regions. This work will be achieved with a combined approach
including measurements and modeling. Field campaigns should be conducted in semi arid
regions in West Africa (Senegal) at specific seasonal periods, for emission and deposition
fluxes measurements of reactive N compounds, samples and data analysis in the laboratory.
Modelling of processes of emission and deposition specific to semi arid regions, with
ecosystem and regional chemistry models should be developed and validated using collected
data sets.
References:
C. Delon, C. Galy-Lacaux, A. Boone, C. Liousse, D. Serça, M. Adon, B. Diop, A. Akpo, F. Lavenu, E. Mougin,
F. Timouk, Atmospheric Nitrogen budget in Sahelian dry savannas, Atmos. Chem. Phys., 10, 2691-2708, 2010.
Delon, C., Galy-Lacaux, C., Adon, M., Liousse, C., Serça, D., Diop, B., and Akpo, A.: Nitrogen compounds
emission and deposition in West African ecosystems: comparison between wet and dry savanna, Biogeosciences,
9, 385-402, DOI: 10.5194/bg-9-385-2012, 2012.
James N. Galloway, et al., Transformation of the Nitrogen Cycle: Recent Trends, Questions, and Potential
Solutions Science 320, 889 (2008); DOI: 10.1126/science.1136674.