Soil - vegetation - atmosphere exchanges of carbon-water-nitrogen in an alpine grassland (col du Lautaret) : model - observations comparison

Laboratory : IGE

5 or 6 months paid internship

Supervisor : Didier Voisin (didier.voisin

Phone : 0 456 520 999

Co-supervisor : Arnaud Foulquier (LECA)

keywords : biogeochemical cycles ; land surface models ; atmosphere -
ecosystem exchanges

Context and objectives :

Carbon and nitrogen biogeochemical cycles are heavily disturbed by anthropogenic activities and they interact with each other and with the water cycle and climate in complex ways. Mountain areas are particularly sensitive to those perturbations. Their ecosystems are adapted to snow, whose presence will diminish, thereby changing the hydrologic cycle. They are also adapted to low nitrogen inputs. Yet, human activities (combustions, industrial fertilizers, agro-pastoral practices) have increased those inputs: in the Alps, they typically represent 10 kg ha-1 an-1, which is twice the amounts measured in Colorado, where they have a visible impact on the ecosystem and its functions.

Predicting the evolution of biosphere/ atmosphere interactions requires:
- a fine understanding of the bio-physico-chemical processes controlling the transfers of carbon, water, nitrogen and energy at the interface
- models of the critical zone representing those processes with enough details
- observations to evaluate those models.

IGE and LECA have initiated some years ago observations of those exchanges at the col du Lautaret. A flux tower (ICOS-Ecosystem standard) is operational in a "Festuca paniculata" grassland, and atmospheric nitrate have been isotopically traced from deposition to the outlet.

The objective of this internship is to evaluate the land surface model CLM4.5 with the existing data. This model couples water, carbon and nitrogen cycles through hydrology and plants physiology description. Data available to ensure the models forcing include local meteorology measured since 2012, and atmospheric deposition from MOCAGE model runs.
The model will be evaluated first by comparing its outputs on plant growth to NDVI measurements available on site, and then with CO2 and water vapor fluxes available since 2016.
Then, Plant Functional Types as they are described in CLM will be analysed against recent results on atmospheric nitrogen assimilation by local plants : these have been showed to assimilated nitrate directly through their leaves. As PFTs code in the model for the role of plants in the carbon and nitrogen cycle, and are therefore key in a correct representation of C, N, water cycles coupling, they will be modified and the model retested.

Pre-requisite :
- formation in atmospheric or climate sciences, hydrology, or ecology or any equivalent ;
- a distinct taste for interdisciplinary problems ;
- some numeric litteracy (FORTRAN, Python, or Matlab preferred).