Abstract: | An experimental study in flowing solution culture compared threeNorwegian ecotypes (from Saerheim, Pasvik and Bod) and a commercialcultivar (Ac51) of white clover (Trifolium repens L.) and demonstrateddifferences with respect to the rate and intensity with whichN2 fixation was affected when or were supplied at 20 mmol m3 over 14 d. Plants were nodulated and N2 fixation wasestablished over 17 d prior to supplying mineral N, with shoottemperature 25/15 C day/night and root temperature adjustedprogressively to 12C. Control plants received no mineral Nthroughout, and did not grow as rapidly as those supplied with or . Mineral N generally depressed the total N2 fixation per plantrelative to control plants, with two exceptions. The effectof on N2 fixation was moresevere than that of and, over the 14 d, -fed plants fixed more N2 than the comparable -fed plants. Interpolated daily rates of N2 fixation per plant andnodule dry wts were used to calculate specific rates of N2 fixation.These showed that (1) addition of either or initially stimulated fixation relative to control plants and to mineral N uptake;(2) this stimulation was greatest and was delayed by 1-2 d in -fed plants; and (3) nutrition sustained higher residualrates of N2 fixation after 8 d compared with nutrition, under which fixation all but ceased after10 d in three of the genotypes. Ecotype Bode showed by far themost severe induced depression of N2 fixation. Key words: N2 fixation, nitrate, ammonium, white clover, northern ecotypes |