Rising atmospheric CO2 concentrations are affecting the cycling of carbon (C) and nitrogen (N)in ecosystems, which has the potential to alter the emissions of the stable greenhouse gases CO2 and N2O to the atmosphere. Despite the relevance of these processes to affect global warmingcurrent knowledge is fragmentary and relies mostly on short-term studies.At the Giessen Free Air CO2 Enrichment Experiment (Gi-FACE) the effect of +20% above ambient CO2 concentration (corresponds to conditions

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... d 2035-2045) in a temperate grassland has been investigated since 1998. Consequently, observations from this site allow to investigate long-term effects of elevated CO2 (eCO2).The main objective of the present work was to contribute to a better understanding of soil C and N dynamics under long-term eCO2, which are governing the formation and emission of CO2 and N2O from a temperate grassland ecosystem. Towards this objective we assessed the seasonal effects of long-term eCO2 on soil respiration (study I). We further elucidated the distribution of soil aggregate-size classes at different soil depths under eCO2 (within 13.5 years) by physical fractionation, estimated the associated mean residence time (MRT) under eCO2 by applying an isotope mixing model and measured the resulting soil organic carbon (SOC) content (study II). Moreover, we quantified N transformations via 15N labelling and by applying a 15N tracing model and measured the resulting N2O emissions (study III).The results of weekly soil respiration measurements for a period of three years (2008-2010) revealed a pronounced and repeated increase of soil respiration under eCO2 during late autumn and winter dormancy. Increased CO2 losses during the autumn season (September October) were 15.7% higher and during the winter season (November March) were 17.4% higher compared to respiration from ambient CO2 plots. However, during spring time and summer,which are characterized by strong above- and below-ground plant growth, no significant change in soil respiration was obse [...]