B value and isotopic fractionation in N2 fixation by chickpea (Cicer arietinum L.) and faba bean (Vicia faba L.)

Abstract

A prerequisite for the calculation of the proportion of a legume’s nitrogen derived from the atmosphere (%Ndfa) using the 15N natural abundance technique (NA) is the determination of the B value (δ15N value for a legume when completely dependent on N2 fixation for growth). With this objective, Kabuli-type chickpea seeds were inoculated with Mesorhizobium ciceri, strain ISC-6, and an indigenous strain mixture (ISM) from the Guadalquivir Valley (southern Spain); and mayor-equina-type faba bean seeds were inoculated with Rhizobium leguminosarum strain ISL-19. Both legumes were grown in perlite with an N-free nutrient solution and in a growth chamber; uninoculated plants were also grown under the same conditions. Samples of the inoculated plants were collected at full flowering and maturity, analyzing total N and 15N content of the nodules, root, shoot and seeds. The seeds sown and uninoculated plants at wilting were also analyzed. The uninoculated chickpea and faba bean plants showed strong 15N discrimination in the remobilization of reserve proteins from the cotyledons to the shoot. Only 52% of the 15N from the chickpea seed and 46% from the faba bean seed were translocated to the shoot. The Bwhole plant value for both inoculated species (−1.44‰ in chickpea and −1.33‰ in faba bean) demonstrated the existence of a significant isotopic discrimination of N in the fixation of atmospheric N2. The Bshoot value did not show significant differences between full flowering and seed maturity for either species, which allowed field sampling to be performed for the calculation of %Ndfa between both growth stages. Each species presented different 15N discrimination for the different parts analyzed. In chickpea, strong discrimination was observed between the nodules (+3.11‰) and the rest of the plant (−1.67‰); this discrimination was less pronounced in faba bean and always with negative values between the underground parts (nodules and root, with a mean value of −0.75‰) and the shoot (−1.71‰). The Bshoot value obtained was −1.80‰ in chickpea and −1.71‰ in faba bean. However, given the importance of the amount of seed N in chickpea and faba bean, seed 15N contribution should be considered in the calculation of the Bshoot value using a mass balance model which considers 50% of this amount. In this case, the adjusted Bshoot values would increase to −2.05‰ and −1.89‰ in chickpea and faba bean, respectively.