Assessing total atmospheric nitrogen input to a subalpine prairie at the col du Lautaret

Background :

Long Term Ecological Research stations are set up to routinely monitor an extensive set of environmental parameters such as meteorological and soil physical parameters together with exchange fluxes between the surface and the atmosphere in well characterized ecosystems.

These long time series contribute the observational basis for understanding ecosystems physiology and biogeochemical cycles (carbon and nitrogen). These stations have been organizing in networks in order to harmonize their practices and add value to the data through intercomparability.

The Jardin du Lautaret is involved in several such networks (ICOS-ETC) for greenhouse gases, and eLTER for socio-ecosystems). In the recent years, a small watershed has been equiped to enable budgets at the catchment scale : continuous measurements of atmosphere-surface exchanges of carbon, energy and water ; measurements of nutriments at the outflow and in atmospheric deposition...

An important step towards integrated understanding of biogeochemical flows was recently taken during A. Gupta’s thesis with the setup of a process based model of the catchment. PARFLOW-CLM explicitly couples subsurface hydrology to surface exchanges of water and energy, and shows very good ability in predicting outflow, snow cover evolution and evapotranspiration from the atmospheric forcings (temperature, humidity, precipitations) and the time evolution of the observed vegetation. This makes this model a very sound basis for further integrated modelling of the catchment’s biohydrochemical fluxes, which is our current objective, with a focus on Nitrogen.

Measurement based preliminary budgets on nitrogen estimate total atmospheric deposition in the range 12 - 18 kg ha^-1 an^-1 and export at the outflow 10 kg ha^-1 an^-1. In addition to those fluxes, biological fixation of nitrogen can be estimated from litterature to 2 - 4 kg ha^-1 an^-1. The 4 - 12 kg ha^-1 an^-1 imbalance must be either exported by agricultural operations, stored in the soils, or reemited to the atmosphere by denitrification, as NO, N₂O or N₂.

Internship’s objectives :

Deposition fluxes have been estimated on a one point measurement, although deposition depends on land cover and micrometeorological parameters that vary across the catchment. Those small scale meteorological variations play a role on the catchment hydrology ( see https://hess.copernicus.org/preprints/hess-2021-639/ ), and most probably also influence deposition. A first objective of the internship will be to produce space-time resolved atmospheric deposition maps from the measured one point chemical and meteorological measurements, using both A. Gupta’s spatialisation method for meteorological parameters and known parametrisations for deposition velocities, including those implemented in the CLM model interfaced to PARFLOW.

Denitrification yields are know to depend strongly on soil humidity and temperature, and are therefore very variable in space and time. The second objective of the internship will be to produce space time resolved nitrogen reemission maps based on our hydrological model ouput maps of soil properties and nitrogen biogeochemical parametrizations taken from the CLM model. The aggregated reemissions from the resulting maps will be compared to our preliminary nitrogen budgets as a preparation for future complete coupled CLM modelling of the catchment.

Student profile :

Expected student should be numerically apt, preferably with python.

Their scientific background can be varied, from ecology to earth and environmental sciences to chemistry or engineering as long as it includes some basic physics, a reasonable ability to work with process based models, and a capacity to evolve in a pluridisciplinary environment.

Aside from the expected data analysis tasks described above, the student will help in the team general monitoring of the site, which includes some assisting in the field during sampling sessions and possibly some sample handling back in the lab, for a minor fraction of his time.