Mineral nitrogen effect on nodulation and nitrogen fixation of the stem‐nodulating legume Aeschynomene afraspera

Abstract

The effect of increasing amounts of nitrogen (nitrate or urea) on the nitrogen fixing capacity (acetylene reduction assay = ARA), growth (fresh and dry weight) and the number of stem‐ and root‐nodules of the tropical legume Aeschynomene afraspera was studied in hydroponic cultures (in growth cabinet) as well as in pot experiments (field conditions). The experiments were carried out at Dakar in the rainy season of 1985. Plants were grown in the presence of 6 nitrate concentrations (0,3,6,9,12 and 15 mM N/l) in hydroponic solution and with 4 urea concentrations (0,50,100 and 200 kg N/ha) in pots. In both types of experiments, root nodulation and ARA were strongly inhibited by increasing amounts of mineral nitrogen. Stem nodulation and potential nitrogen fixation of stems, however, remained unaffected. Lower amounts of mineral nitrogen even enhanced growth as well as nitrogen fixation. The possible future of this remarkable plant as green manure or fodder in low input countries of the tropics is discussed.