A dynamic whole-plant model of integrated metabolism of nitrogen and carbon. 1. Comparative ecological implications of ammonium-nitrate interactions

The dynamics of ammonium (NH4+) and nitrate (NO3-) concentrations in the so il solution is an important determinant of the species composition of natur al vegetation. A mathematical model of uptake, assimilation and translocati on of NH4+ and NO3- is presented to assess the performance of species with respect to NO3-/NH4+ feeding characterised by physiologically defined param eters. Nitrate efflux is explicitly considered. The capacities of NO3-, [ U ], and NH4+ influx, [U], and NO3- reduction, [A], appear sufficient to char acterise whole-plant N metabolism including NO3- translocation. The paramet er space made up by these parameters is represented by 276 parameter combin ations ('species'). Simulated total net N uptake rate and C costs for uptak e 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 for ms with similar total N concentration for external concentrations up to 1.6 mM. Five response categories were identified and contrasted with categorie s defined by Bogner (1968) on the basis of experimental results on forest p lants. The largest category comprises species that respond positively to NO 3- and positively or indifferently to NH4+. These species have intermediate to high [U] and [A] and variable [U] and correspond to woodland edge speci es 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]; s everal species from oak-hornbeam woodland (Carpinion) belong to this group. Many parameter combinations were found that responded positively to NH4+ a nd indifferently to NO3-: low to medium [U], medium to high [ U] and variab le [A]. This category includes all heathland species. No species were found which responded negatively to NO3-. The physiological background of differ ences between the categories is explained with respect to the equilibrium N O3- concentration in roots, influx, efflux, translocation and assimilation of NO3- and uptake and assimilation of NH4+. The relationship between NO3- accumulation capacity and morphology is discussed. Some slow-growing specie s with high [U] and low [A] use NO3- mainly as an osmotic solute. Respirato ry costs in roots of inherently slow-growing species are discussed with res pect to patterns in NH4+ and NO3- availabilities of their habitat.