Variation in root-zone CO 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 and NH uptake and assimilation has not been adequately quantified. •  The influence of varying root-zone CO concentrations on tissue δ C and δ 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 in NO and NH -fed plants than with 0% root-zone CO . The plant δ C values were not altered by 1% CO with δ C = -29.00‰, but they were increased when supplied with 1% CO with δ C = -10.91‰. The δ N values of NO -fed plants were unchanged by variation in root-zone CO concentration. In NH -fed plants the δ N values were c.  1.5‰ higher at 1% CO . •  Changes in δ C values with increased CO concentration (δ C = -10.91‰) were ascribed to root incorporation of CO . 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 nutrition at 1% CO could be related to increased exudation of NH and organic nitrogen and also reduced uptake.