Nitrogen nutrition effects on δ13C of plant respired CO2 are mostly caused by concurrent changes in organic acid utilization and remobilization

     In the short- or mid-term, the variation of leaf-respired δ¹³CO₂ has important consequences for δ¹³C of CO₂ in air in terrestrial ecosystems. Therefore, the isotope composition of plant respired CO₂ is of crucial importance for understanding plant and ecosystem carbon balance. It has previously been shown in tobacco (Nicotiana tabacum) that the balance between ammonium and nitrate has an influence on δ¹³C of leaf-respired CO₂. However, uncertainty remains as to whether (i) the effect of N nutrition is observed in all species, (ii) N source also impacts on respired CO₂ in roots, and (iii)  there is a relationship or equation predicting δ¹³C of respired CO₂ that can be applied regardless of N conditions and species, this uncertainty represents a hurdle in plant ¹³C budget modelling.

     Here, we carried out isotopic measurements of respired CO₂ and various metabolites using two species (spinach, French bean) grown under different NH₄⁺:NO₃^- ratios. Both species showed a similar pattern, with a progressive ¹³C-depletion in leaf respired CO₂ as the ammonium proportion increased, while δ¹³C in root-respired CO₂ showed little change. Supervised multivariate analysis showed that δ¹³C in respired CO₂ was mostly determined by organic acid (malate, citrate) metabolism, in both leaves and roots. We then took advantage of non-stationary, two-pool modelling that explained 73% of variance in δ¹³C in respired CO₂. It demonstrates the critical role of the balance between the utilization of respiratory intermediates and the remobilization of stored organic acids, regardless of anaplerotic bicarbonate fixation by phosphoenolpyruvate carboxylase and the organ considered.