Varying the ratio of ¹⁵N-labelled ammonium and nitrate-N supplied to creeping bent: effects on nitrogen absorption and assimilation, and plant growth

Abstract

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.