A dynamic whole-plant model of integrated metabolism of nitrogen andcarbon. 1. Comparative ecological implications of ammonium-nitrateinteractions

The dynamics of ammonium (NH₄⁺) and nitrate (NO₃^-) concentrations in the soil solution is an important determinant of the species composition of natural vegetation. A mathematical model of uptake, assimilation and translocation of NH₄⁺ and NO₃^- is presented to assess the performance of species with respect to NO₃^-/NH₄⁺ feeding characterised by physiologically defined parameters. Nitrate efflux is explicitly considered. The capacities of NO₃^-, [ U], and NH₄⁺ influx, [U], and NO₃^- reduction, [A], appear sufficient to characterise whole-plant N metabolism including NO₃^- translocation. The parameter space made up by these parameters is represented by 276 parameter combinations ('species'). Simulated total net N uptake rate and C costs foruptake and assimilation per mole total net N taken up are used to decide on how a species profits or suffers from NO₃^-+NH₄⁺ mixtures relative to pure N forms with similar total N concentration for external concentrations up to 1.6mM. Five response categories were identified and contrasted with categories defined by Bogner (1968) on the basis of experimental resultson forest plants. The largest category comprises species that respond positively to NO₃^- and positively or indifferently to NH₄⁺. These species have intermediate to high [U] and [A] and variable [U] and correspond to woodland edge species and forest plants on rich soil including typical 'nitrophilic' species. This category fades into a group of species that respond positively to NO₃^- and negatively to NH₄⁺. These species have high [U] and low [U] and [A]; several species from oak-hornbeam woodland (Carpinion) belong to this group [ABSTRACT FROM AUTHOR]