1. Liebermann R 1, Kraft P 1, Houska T 1, Müller C 2,3, Haas E 4, Kraus D 4, Klatt S 4, Breuer L 1 1 Institute for Landscape Ecology and Resources Management, Research Centre for BioSystems, Land Use and Nutrition (IFZ), Justus Liebig University Gießen, Germany 2 Institute for Plant Ecology, Research Centre for BioSystems, Land Use and Nutrition (IFZ), Justus Liebig University Gießen, Germany 3 School of Biology and Environmental Science, University College Dublin, Ireland 4 Institute of Meteorology and Climate Research – Atmospheric Environmental Research, Karlsruhe Institute of Technology, Garmisch-Partenkirchen, Germany Uncertainty analysis of a coupled ecosystem response model simulating greenhouse gas fluxes from a temperate grassland Motivation Among anthropogenic greenhouse gas emissions, CO 2 is the dominant driver of global climate change. Next to its direct impact on the radiation budget, it also affects the climate system by triggering feedback mechanisms in terrestrial ecosystems. Such mechanisms – like stimulated photosynthesis, increased root exudation and reduced stomatal transpiration – influence both the input and the turnover of carbon and nitrogen compounds in the soil. The stabilization and decomposition of these compounds determines how increasing CO 2 concentrations change the terrestrial trace gas emissions, especially CO 2, N 2 O and CH 4. To assess the potential reaction of terrestrial greenhouse gas emissions to rising tropospheric CO 2 concentration, we make use of LandscapeDNDC 1,2 as a comprehensive ecosystem model, integrating known processes and fluxes of the carbon- nitrogen cycle in soil, vegetation and water. Field Site & Data The data have been attained from a FACE (Free Air Carbon dioxide Enrichment) experiment running since 1998 on a temperate grassland in Giessen, Germany. Management and soil data from the FACE rings as well as weather records are used to drive the model. Observations on cut plant biomass and GHG emissions have been used for calibration. Acknowledgements This work is part of the LOEWE-Schwerpunkt “FACE2FACE”. We thank the Institute for Plant Ecology (JLU Gießen) for providing data from the Gießen FACE installation and the State of Hesse (Germany) for its financial support. l Calculating Soil chemistry Microclimate Water cycle Plant physiology Microbiology LandscapeDNDC grassland realization Input:  Weather data  Soil properties  Management  Air chemistry  Species composition Output:  Biomass  Greenhouse gas emissions  Soil humidity  … The Ecosystem Model Results: preliminary GLUE simulation Outlook: N deficit? Measured sum of N sinks > measured sum of N sources (difference: 52 kgN/(ha*a)) Biomass production and ecosystem respiration are underestimated Possible explanation: N supply via groundwater and capillary rise have not been considered so far (not yet measured or modelled) Fig. 1: From top: carbon and nitrogen content of harvested grass biomass (two cuts per year), daily mean of CO 2 emissions, monthly mean of N 2 O emissions. Red and green dots and lines denote measurements from two FACE rings; black lines denote best simulations aiming at adequate match between simulations and the 8 measured time series (4 variables at 2 rings). Current simulations underestimate measured biomass and CO 2 emissions, especially in the 2nd half of the time series. Corresponding author: Ralf.Liebermann@umwelt.uni-giessen.de GLUE = Generalized Likelihood Uncertainty Estimation 3 2434 simulations were performed by random variation of 112 parameters Likelihood was calculated with objective functions MSE (for CO 2 ), Agreement Index (N 2 O) and RRMSE (C and N content) Simulations in Fig. 1 showed best (top 20%) likelihood values for all objective functions N deposition & fertilization N emissions N harvest (grass cutting) N leaching groundwater N supply by capillary rise? ? References 1) Li et al. 1992, J Geophys Res 97:9759-9776, DOI: 10.1029/92JD00509 2) Haas et al. 2013, Landscape Ecol 28: 615-636, DOI:10.1007/s10980-012-9772-x 3)Beven & Freer 2001, J Hydrol 249: 11-29, DOI: 10.1016/S0022-1694(01)00421-8 Photo made by FACE2FACE Fig. 2: Basic nitrogen fluxes of the plant-soil system.