The relative effects of soil N, water supply and elevated atmospheric CO₂ on foliar δ¹⁵N values were examined. Phalaris arundinacea L. (Holdfast) and Physalis peruviana L. (Cape Gooseberry) were grown for 80 d with three water availability treatments, two atmospheric CO₂ concentrations and four N supply rates. Elevated CO₂ increased total plant biomass and N for each treatment and decreased allocation to roots, leaf N concentrations and stomatal conductance. Leaves had less negative leaf δ¹³C values under low water supply associated with decreased stomatal conductance and increased leaf N concentration, which decreased the ratio of intercellular to ambient CO₂ concentration. The δ¹⁵N value of the supplied nitrate (4.15‰) was similar to the value for Phalaris leaves (4.11‰), but Cape Gooseberry leaves were enriched (6.52‰). The effects of elevated CO₂ on leaf δ¹⁵N values were small, with Phalaris showing no significant change, while Cape Gooseberry showed a significant (P < 0.05) decline of 0.42 ‰. Variation in δ¹⁵N values was unrelated to stomatal conductance, transpiration, differential use of N forms or denitrification. Plants with low foliar N concentrations tended to be depleted in ¹⁵N. We suggest that changes in N allocation alter foliar δ¹⁵N values under different CO₂ and water treatments.