Description: Within the Critical Zone (the surface part of the Earth's crust directly impacted by human activities), the water, carbon and nitrogen cycles are strongly coupled. They interact in all parts of the earth system  (soils, vegetation, snowpack, surface water and atmosphere).  As a result, they  impact  on climate trajectories and environmental quality. Understanding these interactions and being able to model  them  is  therefore  essential  to  propose sustainable measures to adapt to ongoing global changes.

Mountain regions represent a particular challenge in this respect. Ecosystems are adapted to a  snow regime  under  change  due  to  the  rise  in  the  0°C  isotherm. In  addition,  atmospheric  nitrogen deposition, a product  of  industrial  activity carried  by  valley  winds  and  mesoscale  atmospheric circulation, already impacts some high-altitude ecosystems by modifying nutrient flows (nitrogen and carbon in particular). These combined forcings could lead to major ecosystem changes (distribution of water, carbon and nitrogen flows, growth rates, species, etc.). Anticipating this evolution, and the associated flows (CO2, nitrogen, water) under this double constraint, remains problematic due to the lack of  adapted models. The processes to be taken into account are influenced by topography, which induces strong spatial heterogeneity  and  significant  lateral flows through  surface  runoff  and underground transfers. However, these are rarely if ever taken into account in surface models.

Within the framework of this thesis, we propose to study the behaviour of a small instrumented hydrological unit (17ha), the site of the Charmasses meadow (Lautaret pass), for which we monitor the nutrients (water, carbon, nitrogen) and energy flows as well as the isotopic signature of nitrate (d15N, d18O and D17O) and ammonium (d15N) in aerosols, precipitations,soils and water. This data set,  unique  for  a  hydro-nival  watershed,  can  be  used  for  budget  and  mechanisms  assessments carried out at site level, and to force and evaluate a distributed Critical Zone model. The first step will be to integrate the measurements made on site, to establish the annual nitrogen budget for the watershed  and  to  inform  our  understanding  of  the  pathways  and  mechanisms  for  nitrogen transformation  in  the  ecosystem  through  the  joint  use  of  isotopes  measured  in  the  different compartmentsand metabolic activity tracers measured in the soils. This will build on the monitoring initiated in 2018 and to be continued as part of the thesis. In a second step, a surface model (CLM5) will be evaluated, which takes into account the interactions between the nitrogen and carbon cycle in soils and plants. This surface scheme with dynamic vegetation will be evaluated on the primary production  and  associated  water,  carbon  and  energy  flows  measured  by  eddy  covariance  at  the Charmasses  meadow.  This  scheme  will  then  be  introduced  into  the  distributed  ZC  Parflow/CLM model implemented in the catchment area to take into account the influence of topography on flow distribution. The model will be evaluated on water and nitrogen flows at the outlet, and carbon flows at the FluxAlp  tower. This will be the first physically-based model that explicitly  represents the couplings between the water, carbon and nitrogen cycles.

Application deadline: 10 June 2019

Preferred  profiles:  Master in atmospheric or climate  sciences, hydrology, or ecology or any equivalent; a distinct taste for interdisciplinary problems ; some numeric literacy (Python, Fortran, or Matlab preferred)

Contacts: didier.voisin@univ-grenoble-alpes.fr; jean-martial.cohard@univ-grenoble-alpes.fr