Plant physiological processes alter with elevated carbon dioxide concentrations in the atmosphere (eCO2), which affects the growth and yield potential of crops. Among plants with C3 carbon fixation, root crops show highest yield response under eCO2 which is suggested to be linked to their large carbon (C) sink strength. The high C gain under eCO2 can be limited by processes that constrain sink capacity, such as nitrogen (N) supply. Different N sources may interact with eCO2 and thus have variable impacts on carboxylation activity, N uptake efficiency and plant development. This study aims at contributing to a better understanding of sink-driven assimilation, re-allocation and finally growth processes under eCO2 as a function of N-form. Radish (Raphanus sativus L. var. sativus), a C3 tuber plant with strong sink strength, was used. The plants were grown in pots in climate chambers at 400 ppm (aCO2) and 1000 ppm CO2 (eCO2) with either pure nitrate or ammonium-dominated nutrition. A split plot design was applied. Plants were harvested after four weeks and physiological, morphological and chemical parameters in leaves and tubers were assessed. The N-form had no effect on N acquisition, but affected C and N partitioning into plant organs differently under eCO2. N assimilation processes of nitrate-fed plants were focussed on leaves being both source and sink, and those of ammonium-fed plants on tubers, the strongly pronounced sink. Acclimation of CO2 fixation due to eCO2 did not occur for both N forms, probably due to altered sink strength and low N content in leaves. The N-form influences the sink-driven C and N balance and thus enables unrestricted carbon gain as well as the variation of organ development under future eCO2 conditions. These processes can be utilized in cultivation and breeding. •High sink strength can delay photosynthetic acclimation at high CO2.•Nitrate nutrition is not adverse due to its non-restricted assimilation at high CO2.•C and N partitioning in organs indicates reallocation of N assimilation at high CO2.•Ammonium-dominated nutrition fosters tuber formation of root crops at high CO2.•Nitrate nutrition fosters formation of leaf biomass over tuber biomass at high CO2.