The δ15N value of N2 fixing actinorhizal plants and legumes grown with N2 as the only nitrogen source

The aim of this study was to investigate the effects of different plant parts and the age of plants at harvest as well as N-2 fixing bacterial strains on the N concentration in symbiotic plant parts, especially on the delta N-15 signature of the actinorhizal plants and legumes. The N-15 natural abundance method was used. Two actinorhizal plants were studied: Alnus incana (L.) infected with the Frankia strains ArI3 or "lsF" (local source of Frankia) and Hippophae rhamnoides (L.) infected with the Frankia strains T1 or E15b. Two legume species were studied: Hedysarum coronarium (L.), infected with a soil suspension, and Robinia pseudoacacia (L.), infected with a crushed nodule suspension. It was particularly interesting that in A. incana, the two Frankia strains affected not only N concentration and delta N-15 signature of leaves and roots, but also had an impact on plant growth at first harvest. In Hippophae rhamnoides plants inoculated with the Frankia strains T1 and E15b, N concentrations and delta N-15 values did not differ at any harvest time. However, plants nodulated by the Frankia strain T1 showed a higher nitrogen fixation rate and higher plant dry matter at all harvesting times. Based on our results for the quantification of N-2 fixation with the "B" value, that is the delta N-15 value of the N-2 fixing plants relying only on N-2 fixation, plant parts, ages and strains should be carefully considered.