DIFFERENTIAL RATES OF INHIBITION OF N-2 FIXATION BY SUSTAINED LOW CONCENTRATIONS OF NH4- IN NORTHERN ECOTYPES OF WHITE CLOVER (TRIFOLIUM-REPENS L)( AND NO3)

An experimental study in flowing solution culture compared three Norwe gian ecotypes (from Saerheim, Pasvik and Bodo) and a commercial cultiv ar (Ac51) of white clover (Trifolium repens L.) and demonstrated diffe rences with respect to the rate and intensity with which N-2 fixation was affected when NO3- or NH4+ were supplied at 20 mmol m(-3) over 14 d, Plants were nodulated and N-2 fixation was established over 17 d pr ior to supplying mineral N, with shoot temperature 25/15 degrees C day /night and root temperature adjusted progressively to 12 degrees C, Co ntrol plants received no mineral N throughout, and did not grow as rap idly as those supplied with NH4+ or NO3-. Mineral N generally depresse d the total N-2 fixation per plant relative to control plants, with tw o exceptions. The effect of NO3- on N-2 fixation was more severe than that of NH4+ and, over the 14 d, NH4+-fed plants fixed more N-2 than t he comparable NO3--fed plants, Interpolated daily rates of N-2 fixatio n per plant and nodule dry wts were used to calculate specific rates o f N-2 fixation, These showed that (1) addition of either NH4+ or NO3- initially stimulated fixation relative to control plants and to minera l 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 N-2 fixation after 8 d compared with NO3- nutrition, under wh ich fixation all but ceased after 10 d in three of the genotypes, Ecot ype Bodo showed by far the most severe NO3--induced depression of N-2 fixation.