Pea is the first grain legume grown in Europe for its seeds rich in protein. As a legume, it has the ability to fix atmospheric N2 by the symbiosis with Rhyzobium bacteria. Still, N nutrition can be a limiting factor of pea yield, due to a high sensitivity of symbiotic N2 fixation to biotic stress like Aphanomyces that leads to root rots. The consequences of root damage on nodules and their N2 fixing activity and the possible adaptive response of plants still remain unknown. Root pruning could be an innovative experimental way to study these effects, resulting both in nodules and N2 deprivation. Still, researches described the adaptive responses of plants to a local and temporary N2 deprivation. Conversely to NO3-, Medicago truncatula response to local deprivation of N2 does not involve any short term increase of nodule specific activity (Jeudy et al, 2010). Thus, nodule specific activity could be limited by the C fluxes reaching the nodules (Ruffel et al. 2008). The Mtr plant response to N2 deprivation only involves long term increase of nodule biomass and nodule number (Jeudy et al, 2010. Regulation of nodulation was shown to be the main component to legume plant adjustment to its N requirements (Voisin et al, 2010). Hypernodulating mutants are defective in regulation of nodulation, thus producing an excess number of nodules (Sagan and Duc, 1996). Nodule specific activity in these mutants was usually shown to be lower than for wild type (Voisin et al. 2007). Still, in split root experiments, hypernodulating mutant response to local deprivation of N2 was associated with higher nodule specific activity compared with mutant without N2 deprivation (Jeudy et al. 2010). Our hypothesis is that hypernodulating mutants could have a higher reactivity of nodule specific activity to root pruning than a wild type. Our objective was to dissect the effect of root pruning on nitrogen fixation and to analyze plant adaptive response to this stress. To that purpose, half of the nodulated roots was removed for 4 pea genotypes: Frisson and 3 of its hypernodulating mutants: P64, P118 and P121, respectively mutated on sym28, sym29 and nod3 genes. We used different atmospheric CO2 concentration to modify C input by photosynthesis. Plant response was evaluated 5 days after root pruning; the relationships between C and N fluxes were analyzed using 13C/15N simultaneous labeling. Under ambient CO2, we showed that root pruning resulted in a sharp decrease of N uptake by symbiotic fixation for the four genotypes. But the adaptive response varied among genotypes. Mutant P121 displayed higher nodule growth in response to nodulated root removal than P118 while P64 mutant displayed no differential nodules growth but higher differential root growth compared to control plants. The wild type displayed no differential nodule or root growth. P118 was the only genotype to maintain nodule specific activity for N2 fixation in response to nodulated root removal whereas it was decreased compared to control intact plant for other genotypes. Finally, there was no impact of root pruning on carbon intake but C partitioning among shoots, roots and nodules differed in response to root pruning, with variations among genotypes. We will also present plant responses to root pruning under elevated CO2 up to 750ppm.