Differential rates of inhibition of N2fixation by sustained low concentrations of NH4+and NO3-in northern ecotypes of white clover (TrifoliumrepensL.)

An experimental study in flowing solution culture compared three Norwegian ecotypes (from Saerheim, Pasvik and Bodø) and a commercial cultivar (Ac51) of white clover (Trifolium repens L.) and demonstrated differences with respect to the rate and intensity with which N2 fixation was affected when NO3- or NH4+ were supplied at 20 mmol m−3 over 14 d. Plants were nodulated and N2 fixation was established over 17 d prior to supplying mineral N, with shoot temperature 25/15 °C day/night and root temperature adjusted progressively to 12°C. Control plants received no mineral N throughout, and did not grow as rapidly as those supplied with NH4+ or NO3-⁠. Mineral N generally depressed the total N2 fixation per plant relative to control plants, with two exceptions. The effect of NO3- on N2 fixation was more severe than that of NH4+ and, over the 14 d, NH4+-fed plants fixed more N2 than the comparable NO3--fed plants. Interpolated daily rates of N2 fixation per plant and nodule dry wts were used to calculate specific rates of N2 fixation. These showed that (1) addition of either NH4+ or NO3- 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 NH4+-fed plants; and (3) NH4+ nutrition sustained higher residual rates of N2 fixation after 8 d compared with NO3- nutrition, under which fixation all but ceased after 10 d in three of the genotypes. Ecotype Bode showed by far the most severe NO3- induced depression of N2 fixation.