PLANT-GROWTH, UPTAKE AND PARTITIONING OF NITROGEN IN ALFALFA SEEDLINGS AS AFFECTED BY THE FORM OF N-15-LABELED COMBINED NITROGEN

We investigated the growth, uptake and partitioning of nitrogen (N) in alfalfa seedlings grown in the pot soil where N-15 labelled combined N was applied as nitrate N or ammonium N by means of basal dressing or topdressing. Dry weight, N content, and N-15 recovery in the plant were greater when N was applied as ammonium N than nitrate N in both basal dressing and topdressing. Elongation of the roots was more strongly promoted by ammonium N than nitrate N, and this was associated with increased uptake of N-15 from labelled fertilizer topdressed in ammonium form. Absorbed N-15 was more preferentially allocated to the roots than the stem in the plant when ammonium N was topdressed, while a reverse trend was found in the plant to which nitrate N was topdressed. These results indicate that ammonium N is more effective source of N for the growth of alfalfa seedlings as compared with nitrate N.