Cultivar-specific dinitrogen fixation in Vicia faba studied with the nitrogen-15 natural abundance method

Abstract

N fixation by different faba bean (Vicia faba) cultivars was studied using the natural abundance method. The delta 15N (δ15N) values of the faba beans and the reference plants differed by 4.6–7.0‰. The non-nodulating V. faba cv. F48 seems to be the best reference plant for nodulated and N2-fixing V. faba. Significant differences occurred in the quantity of N2 fixation of six V. faba cultivars. The average fraction of N derived from air (FNdfa) estimated from leaf material ranged between 69 and 80%. Shoot-based estimates of N fixation varied between 200 and 360 kg N ha–1. N fixation was affected more by differences in FNdfa than by differences in total N accumulation. Fixation data calculated with the non-nodulated reference plant V. faba cv. F48 were lower than those calculated with cabbage (Brassica oleracea) and ryegrass (Lolium perenne) as reference plants. Of all reference plants, non-N2-fixing V. faba cv. F48 has a root system and temporal pattern of N assimilation that is the one most similar to that of N2-fixing V. faba plants. Cv. F48 showed senescence as did the other V. faba cultivars after pod-fill was complete, whereas cabbage, ryegrass and camomile had a later senescence period. N fixation during pod-filling appears more important for a good yield than N2 fixation abilities in the earlier growth period. The best V. faba cultivars left about 100 kg N ha–1 in residual material on the field as fertilization for the following crops.