Variation in root-zone CO₂ concentration modifies isotopic fractionation of carbon and nitrogen in tomato seedlings

•  The contribution to the carbon budget and growth by root acquisition of inorganic carbon and the influence that this has on NO ₃ ^- and NH ₄ ⁺ uptake and assimilation has not been adequately quantified. •  The influence of varying root-zone CO ₂ concentrations on tissue δ ¹³ C and δ ¹⁵ N was used to estimate the contribution to the carbon budget of root-assimilated carbon in tomato ( Lycopersicon esculentum ) seedlings. •  Biomass accumulation was greater at 0.5% and 1% (v/v) root-zone CO ₂ in NO ₃ ^- and NH ₄ ⁺ -fed plants than with 0% root-zone CO ₂ . The plant δ ¹³ C values were not altered by 1% CO ₂ with δ ¹³ C = -29.00‰, but they were increased when supplied with 1% CO ₂ with δ ¹³ C = -10.91‰. The δ ¹⁵ N values of NO ₃ ^- -fed plants were unchanged by variation in root-zone CO ₂ concentration. In NH ₄ ⁺ -fed plants the δ ¹⁵ N values were c.  1.5‰ higher at 1% CO ₂ . •  Changes in δ ¹³ C values with increased CO ₂ concentration (δ ¹³ C = -10.91‰) were ascribed to root incorporation of CO ₂ . Less than 5% of carbon was derived from root dark fixation and thus cannot explain increases in growth on a mass basis. Reduced discrimination with NH ₄ ⁺ nutrition at 1% CO ₂ could be related to increased exudation of NH ₄ ⁺ and organic nitrogen and also reduced uptake.