Abstract
A pot experiment with sand as a rooting medium was conducted to investigate the effect of four different nitrogen sources (urea, nitrate, ammonium, nitrate + ammonium) at two concentrations (5 and 10 mM nitrogen) on nodulation and nitrogen fixation in faba bean (Vicia faba L. cv. Fiord), white lupin (Lupinus albus L.) and medic (Medicago rugosa Desr. cv. Paraponto). Results showed that all nitrogen forms were able to suppress nodule formation, nodule growth and nitrogenase activity, measured by acetylene reduction (AR) in all species studied, and the higher concentration had a stronger depressive effect than the lower level. Medic was inhibited most in terms of number of active nodules and AR activity per plant. Among the nitrogen forms, the suppression by NH4Cl was the most severe on nodulation causing the lowest nodule number and nodule weight, and on AR activity in all crops, while urea at 5 mM level seemed to be the least inhibitory. The depressive effect on AR was mainly due to the depressive effect on nodule formation and nodule growth, rather than to a direct influence on nodule specific activity.
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