Earth System Models are state of the art research tools to project future climate, and attribute the effects of humans on the land, oceans and atmosphere. Present day Earth System Models contain a detailed land surface model to simulate the two-way interactions between vegetation and climate. ORCHIDEE (Krinner et al. 2005) is the land surface model of the IPSL-CM Earth System Model and simulates the carbon cycle including photosynthesis, autotrophic and heterotrophic respiration of plants and soils, in addition to the biophysical processes such as the surface energy budget. Recent developments of the model focused on the nitrogen cycle (Zaehle et al. 2010), forest management (Bellassen et al. 2010) and stand structure (Naudts et al. 2015).
The increasing complexity of the model , requires new approaches to confirm that the model reproduces our current understanding of plant physiology, ecology and ecosystem dynamics. A novel and promising avenue is to include, in the model, the mixing and fractionation of C and N isotopes. This approach is hampered by two issues: (1) lack of a conceptual framework of 15N mixing and fractionation that could serve as the starting point of new model code and (2) scarcity of long-term data that can be used to confirm a correct process representation by ORCHIDEE.
At the end of the eighties an European research project called NITREX investigated the effect of atmospheric nitrogen on forests in several European countries. In some sites N deposition was experimentally increased, as well as decreased (Wright & Van Breemen, 1995). One of the forests was a Scots pine forest in the Peel, located near Ysselsteyn, Netherlands, where N deposition was decreased to pre-industrial values. In the early nineties, 15N was added to precipitation for a year in most NITREX sites, also in Ysselsteyn (Koopmans et al. 1996). This stable isotope of nitrogen served as a tracer and was used to investigate the pathway of deposited N through the ecosystem. In Ysselsteyn, the fate of 15N was measured 1, 8 and 19 years after application (Wessel et al. 2013). The data collected during this experiment are valuable to enhance our understanding of the long-term dynamics of 15N within a forest and is a unique opportunity to confirm and improve our capability to simulate the interactions between the C and N cycle (Nadelhoffer et al. 1999).
The ultimate goal of this study is to build the capacity to use long-term tracer experiments to validate the process representation of the C and N cycle in Earth System Models and as such contribute to better projections of future climate. This long trajectory will be divided into several more or less sequential phases. Each step phase can be carried out by a master student.
1. Literature review of 15N mixing and fractionation in forest ecosystems
2. Implementing 15N mixing and fractionation into the ORCHIDEE code.
3. Calibration and validation of the model using the 15N dataset
4. Spatially explicit global simulations with the improved version of ORCHIDEE
5. Quantifying the effect of N-deposition on the global climate
All topics are well within reach for physiology, ecology and earth science students with a fair interest in computers and coding. Likewise, students with a background in geography, physics, chemistry or computer science with a strong interest in ecology could contribute to this study.