Final Report Summary - JULIA (Joining ecophysiological Understanding and global ecosystem modelling for improved simulation of Land surface Interactions with the Atmosphere) This project aimed at contributing to closing the gap between observational science and large scale biosphere modelling in two key domains (canopy conductance and plant-soil interactions) by developing representations of relevant ecophysiological processes at a level of detail suitable for an Earth system model. A thorough understanding of feedbacks between the terrestrial biosphere and the climate system is pivotal for any climate change mitigation strategy. The key outcome of the project was an assessment of the role of the nitrogen cycle in the climate system both through its effects on the dynamics of the natural carbon cycle, as well as due to the consequences of anthropogenic nitrogen additions on terrestrial greenhouse gas fluxes. In a study published in Geophysical Research Letters, the researcher demonstrated that nitrogen dynamics constrain the terrestrial carbon cycle under future climate change such that it increases the projected rate of climate change due to its limiting effect on CO2 fertilisation induced terrestrial carbon sequestration. In a follow-up study published in Nature Geoscience, the researcher showed that the positive effect of anthropogenic nitrogen additions on terrestrial C storage, which tends to slow climate change due the reduction in atmospheric CO2, was about compensated by the radiative effect of N2O emissions associated with these nitrogen additions. These studies were also input to a review on terrestrial feedbacks within the climate systems, published in Nature Geosciences, to which the researcher contributed. The two wider-reaching consequences of these studies are that firstly even higher climate mitigation actions have to be made than previously thought estimated using carbon-cycle only assessments as these studies were too optimistic about the carbon sequestration capacity of the terrestrial biosphere, and secondly, that nitrogen management is important not only for control of environmental pollution and human health, but also for the anthropogenic perturbation of the climate system. A synthesis of the effects on the European scale was put together within the framework of the European Nitrogen Assessment's chapter 19 on radiative forcing from reactive nitrogen, to which the research was one of the three lead authors. These studies were only made possible by detailed work on ecophysiological process representations for terrestrial biosphere models, and in particular by evaluating and constraining model results using flux observations, plant trait characteristics and the outcomes of ecosystem monitoring and manipulation studies. This work, important though less high impact, resulted in a series of publications on canopy conductance modelling under drought stress (Keenan et al., 2009), analyses of plant trait characteristics for global modelling (Kattge et al., 2011), nitrogen cycle parameterisations and plant-soil interaction modelling (Zaehle and Friend, 2010), the use of ecosystem monitoring and manipulation studies to evaluate ecosystem models (Zaehle et al., 2010, GBC; Zaehle et al., 2010, GRL). This work has resulted in a model system that is now applied as part of global terrestrial research programmes under the Global Carbon Project, which will provide information for the AR5 of the Intergovernmental Panel on Climate Change.
