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.