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Sugar beet ( Beta vulgaris ssp . altissima Döll) was grown in the field under free-air CO2 enrichment (FACE, 550 ppm) and different nitrogen (N) supply (2001: 126 (N100) and 63 kg·ha−1 (N50); 2004: 156 (N100) and 75 kg·ha−1) during two crop rotations. Canopy CO2 exchange rates (CCER) were measured during the main growth phase (leaf area index ≥2) using a dynamic chamber system. Canopy CO2 exchange data were analysed with respect to treatment effects on seasonal means and light use efficiency and light response characteristics. CO2 enrichment enhanced CCER throughout the season. However, in both years, CCER declined after the second half of August independent of radiation and [CO2]. Elevated [CO2] strongly stimulated CCER on a seasonal basis, whereas the reduction of CCER caused by low N was below 10% and not significant. There were no effects of N on daily radiation use efficiency of carbon gain calculated from CCER data, but a strong enhancement by CO2 enrichment. CCER closely tracked diurnal variations in incident photosynthetic photon flux density (PPFD, μmol·m−2·s−1). The relationship between CCER and incident PPFD was curvilinear. In both seasons, initial slopes and maximum rates (CCERmax) were determined from two 6-day periods using these relationships. The first period was measured after canopy closure (first half of July) and the second in the second half of August. In the first period, elevated [CO2] increased the initial slopes. Low N supply affected neither the initial slopes nor their response to elevated [CO2] in either period. In contrast to initial slopes, N stress limited the [CO2] response of CCERmax in the first period. In the second period, however, this interaction of [CO2] and N on CCERmax was completely dominated by a general decline of CCERmax whereas no general decline of the initial slopes occurred in the second period. This response of light response parameters to [CO2] and N suggests that, in sugar beet, the decline of CCER in the late season may rely on limiting mechanisms such as photosynthetic acclimation responses to elevated [CO2] caused by sink limitations. [ABSTRACT FROM AUTHOR]