NITROGEN NUTRITION OF DOUGLAS-FIR (PSEUDOTSUGA-MENZIESII) ON STRONGLY ACID SANDY SOIL .1. GROWTH, NUTRIENT-UPTAKE AND IONIC BALANCE

Through ionic balance calculations, the effect of different sources and levels of nitrogen on nutrient uptake by Douglas-fir was studied. With ammonium as the sole source of N, growth of the plant was very poor. Increasing the levels of ammonium supply strongly decreased the surplus of total inorganic cations (C) over total inorganic anions (A). This decrease in C-A value, corresponding to the level of carboxylates in the plant, implies that in the long term the plant will run out of carboxylates and will then no longer be able to eliminate protons in the cytoplasm, produced during assimilation of ammonium. This can lead to internal acidification of the plant, toxic concentrations of free ammonium and an unbalanced amino acid composition. Values for the ratio of net carboxylate production and organic nitrogen production were in the same range as commonly found for other species. This did not support the theory of a conifer-specific ionic balance regulation as posed by others. © 1990 Kluwer Academic Publishers.