Apart from changes in temperature and precipitation patterns 'climate change' is driven by and entails marked changes in atmospheric chemistry. The future increase of the atmospheric CO ₂-concentration is the most prominent and undisputable change of the atmosphere. Elevated CO ₂ (e[CO ₂]) is known to stimulate leaf level photosynthesis and to reduce leaf transpiration. This may result in altered biomass production of agricultural plants and subsequent secondary feedback effects on ecosystem properties, as for example, water relations, carbon (C) turnover and soil biology. In the framework of the Braunschweig Carbon Project effects of e[CO ₂] under different levels of nitrogen (N) supply are studied in an arable crop rotation system (winter barley - sugar beet - winter wheat, 1999-2005) under field conditions by means of a Free Air Carbon dioxide Enrichment (FACE) approach. Preliminary results obtained during the first 3 years of experimentation are shown here. Canopy CO ₂ uptake was stimulated by e[CO ₂] by ca. 18 % (barley) to ca. 45 % (sugar beet), while canopy H ₂O loss (evapotranspiration) was reduced by 2.6 % (wheat) to 19.8 % (sugar beet). The effects of e[CO ₂] resulted in a stimulation of above ground biomass production between ca. + 6.1 % (sugar beet, reduced N fertilization) to + 14.4 % (winter wheat, full fertilization). Fine root biomass production was stimulated by e[CO ₂] but this effect was not consistent during the growing seasons. While no significant effects of e[CO ₂] on soil microbial biomass were observed, a CO ₂-induced shift in the ratio of bacterial to fungal soil respiration was observed indicating an increase of the percentage bacterial respiration. Species number and abundances of collembolans were enhanced by e[CO ₂] during the wheat growing season. Total soil CO ₂ efflux (soil respiration) was stimulated under e[CO ₂], however the magnitude of this effect differed between sugar beet and wheat. As evidenced from stable carbon 13 isotope analysis changes in soil carbon content were not yet detected.