Abstract
A 15N dilution technique is proposed to determine the role of nitrogen reserves in the plant on the estimation of dinitrogen fixation, during regrowth of perennial legumes. It is based on labelling of nitrogen compartments of the plant. Since the kinetics of utilization of nitrogen reserves of ryegrass (Lolium italicum L.) and red clover (Trifolium pratense L.) appear similar, ryegrass would be a good control plant. Despite this observation, the comparison of two methods (based on two or three sources of nitrogen) to estimate dinitrogen fixation shows the difficulty in applying the technique using 15N-labelled substrate over earlier periods of regrowth. In this case, the 15N method described for determining symbiotic dinitrogen fixation could be applied in controlled conditions.