Estimating the true cost of dinitrogen fixation by nodulated plants in undisturbed conditions

Either symbiotic or nonsymbiotic N 2 fixation in association with higher plants requires an appreciable amount of energy. This is mostly supplied by current photosynthesis, but an exact estimation of the true cost of the process is rather difficult. Simultaneous exposure of the association to 14 CO 2 and 15 N 2 for 1 day, analysis of labelled CO 2 produced by soil respiration, and determination of N 2 fixation make it possible to quantify the true respiratory cost associated with fixation. Preliminary trials with soybean suggest an amount of 2.5–7 mg C/mg of N 2 fixed.