Nitrogen nutrition effects on δ 13 C of plant respired CO 2 are mostly caused by concurrent changes in organic acid utilisation and remobilisation.

Nitrogen (N) nutrition impacts on primary carbon metabolism and can lead to changes in δ 13 C of respired CO 2 . However, uncertainty remains as to whether (1) the effect of N nutrition is observed in all species, (2) N source also impacts on respired CO 2 in roots and (3) a metabolic model can be constructed to predict δ 13 C of respired CO 2 under different N sources. Here, we carried out isotopic measurements of respired CO 2 and various metabolites using two species (spinach, French bean) grown under different NH 4 + :NO 3 - ratios. Both species showed a similar pattern, with a progressive 13 C-depletion in leaf-respired CO 2 as the ammonium proportion increased, while δ 13 C in root-respired CO 2 showed little change. Supervised multivariate analysis showed that δ 13 C of respired CO 2 was mostly determined by organic acid (malate, citrate) metabolism, in both leaves and roots. We then took advantage of nonstationary, two-pool modelling that explained 73% of variance in δ 13 C in respired CO 2 . It demonstrates the critical role of the balance between the utilisation of respiratory intermediates and the remobilisation of stored organic acids, regardless of anaplerotic bicarbonate fixation by phosphoenolpyruvate carboxylase and the organ considered.