Elevated atmospheric CO ₂ concentrations [CO ₂] are known to change plant growth by stimulation of C ₃ photosynthesis and by reduction of transpiration of both C ₃ and C ₄ crops. In comparison to the information on above ground plant responses only limited knowledge exists on the response of root growth of arable crops to elevated [CO ₂] which is particularly true for temperate crop species under real field conditions. A free air CO ₂ enrichment (FACE) study (550ppm at daylight hours) was carried out in a crop rotation of winter barley, sugar beet and winter wheat repeated twice in the course of six years on a sandy loam soil at Braunschweig, Northern Germany. Winter barley and sugar beet were included for the first time in a FACE study. A possible interaction with restricted nitrogen (N) supply was studied by fertilizing the CO ₂ treatment plots with adequate and 50% of adequate N supply. Fine root samples were taken in the plough layer and below at 3-4 sampling dates during the vegetation period and root dry matter (excluding sugar beet storage root), shoot root ratio, root length density, specific root length and root tissue composition (CN ratio) were determined. Main effects of elevated [CO ₂] on the investigated variables were slightly significant. Significant CO ₂ effects were observed in interaction with the sampling date. In most cases elevated [CO ₂] increased root dry matter early in the vegetation period with a maximum growth stimulation of up to 54% as compared to ambient [CO ₂]. Concomitantly, root length densities were increased in both winter wheat and sugar beet. For winter barley also a significant decrease in root dry weight and significant increase of shoot root ratio was detected at final harvest while such an effect was not significant for sugar beet. Specific root length as an indicator of root morphology was mainly influenced by crop species. As a result, there was no consistent overall effect of elevated [CO ₂] on biomass partitioning in this study as changes in shoot root ratio only occurred at specific sampling dates indicating a similar stimulation of roots and above-ground biomass due to elevated [CO ₂]. Nitrogen supply did not alter the effect of elevated [CO ₂] on any of the root variables apart from CN ratios. A significant increase of root CN ratios in wheat and sugar beet was observed under elevated [CO ₂], but this effect was much smaller than the effect of N supply.