The Influence of Ammonium on Nitrate Uptake and Assimilation in 2-Year-Old Ash and Oak Trees - A Tracer-Study with 15N

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Abstract
Interactions between ammonium and nitrate as competitive N sources depend on various biotic and abiotic factors. The preference for one of these N sources and the influence of ammonium on nitrate uptake and nitrate reductase activity was investigated in a 15N labelling experiment using 2-year-old potted plants of ash (Fraxinus excelsior L.) and oak (Quercus robur L.) under greenhouse conditions. Seedlings of both tree species use ammonium and nitrate in equal amounts when both N forms are supplied in a 1:1 ratio (1.5 mM NH4 + + 1.5 mM NO3 −), although there is a slight tendency that ammonium is preferred. In both species total N uptake is higher if ammonium and nitrate are supplied simultaneously when compared with uptake of nitrate alone (3 mM nitrate). If nitrate is the sole N source N uptake is only half as high as if ammonium and nitrate are supplied in a ratio of 1:1. The distribution of nitrate reductase between shoot and roots is not influenced by the N-form: nitrate reductase activity is always highest in the roots of both species under the conditions of this experiment. Xylem sap analyses showed that both species transport higher concentrations of amino acids than of nitrate from the roots to the shoot. The amino acid composition is independent of the type of N source. Furthermore, ash trees contain more nitrate in the xylem sap than oak trees, reflecting the higher N uptake and the higher nitrate reductase activity in the leaves of this species.