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)