Summary• 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.

Summary• Understanding of the influences of root-zone CO 2 concentration on nitrogen (N) metabolism is limited.• The influences of root-zone CO 2 concentration on growth, N uptake, N metabolism and the partitioning of root assimilated 14 C were determined in tomato ( Lycopersicon esculentum ).• Root, but not leaf, nitrate reductase activity was increased in plants supplied with increased root-zone CO 2 . Root phosphoenolpyruvate carboxylase activity was lower with NO 3 --than with NH 4 + -nutrition, and in the latter, was also suppressed by increased root-zone CO 2 . Increased growth rate in NO 3 --fed plants with elevated root-zone CO 2 concentrations was associated with transfer of root-derived organic acids to the shoot and conversion to carbohydrates. With NH 4 + -fed plants, growth and total N were not altered by elevated root-zone CO 2 concentrations, although 14 C partitioning to amino acid synthesis was increased.• Effects of root-zone CO 2 concentration on N uptake and metabolism over longer periods (> 1 d) were probably limited by feedback inhibition. Root-derived organic acids contributed to the carbon budget of the leaves through decarboxylation of the organic acids and photosynthetic refixation of released CO 2 .