The relative rates of ammonium and nitrate-N uptake and assimilation by creeping bent (Agrostis stolonifera), were investigated for plants grown in soil and supplied with three different ratios of ammonium and nitrate-N. Following two preliminary defoliations, plants were supplied with the equivalent of 150 kg N ha−1, given as 15N-(differentially) labelled NH4+ and NO3−-N in three different ratios (20:80, 50:50 and 80:20), followed by sequential destructive harvests of shoots and roots at four points during a 35-d regrowth period. Maximum use of labelled nitrogen and ‘exhaustion’ of soil mineral nitrogen reserves occurred much earlier when plants were supplied with half or more of their nitrogen as ammonium, than occurred when they were supplied predominately with nitrate-N. The lack of consistency in the patterns of ammonium and nitrate-N absorption, however, implied that the plants had no specific preference for either nitrogen form. Supplying plants with different combinations of ammonium and nitrate produced distinctive differences in plant morphology. In the high nitrate treatment, plants preferentially partitioned resources into shoot and stolon formation, whereas in the high ammonium treatment, resources were preferentially partitioned into root production. These changes in plant morphology might be adaptations to aid species survival in environments associated with a predominance of either nitrogen form.

A detailed study was carried out to obtain information on the relative rates of NH4+ and NO3−-N uptake and assimilation by perennial ryegrass (Lolium perenne L.), when grown in soil and supplied with different ratios of NH4+ and NO3−-N. Following two preliminary defoliations, plants were supplied with 15N-(differentially) labelled NH4+ and NO3−-N in three different ratios (20[ratio ]80, 50[ratio ]50 and 80[ratio ]20), followed by sequential destructive harvests of shoots and roots taken at four points during a 35-d regrowth period. When supplied with equimolar concentrations of NH4+ and NO3−-N, perennial ryegrass absorbed both forms of nitrogen at almost identical rates; and even when the two N forms were supplied in widely disproportionate concentrations, both forms appear to have been absorbed at equal rates, at least until the supply of the minor nitrogen component in each treatment was almost exhausted, i.e. by day 3 of regrowth. It is suggested that this matching of NH4+ and NO3−-N absorption rates by plants maximizes the advantages and minimizes the disadvantages associated with the exclusive use of either form of nitrogen, and thereby optimizes the potential for vegetative and reproductive regeneration. Fifteen days after the application of 15N-labelled fertilizer, shoot and root yields differed little between treatments. However, plants grown with NH4+ and NO3−-N at a ratio of 50[ratio ]50 had accumulated significantly more root (soluble) carbohydrate than those in the other two treatments. Consequently, when subjected to defoliation at day 15, these former plants were clearly in the best position to re-mobilize root carbohydrate and initiate shoot regeneration. However, further work is needed in order to determine whether or not this finding holds true for plants subjected to a multiple (bi- to tri-weekly) defoliation regime, e.g. in a grazing situation.