N-15-LABELED AMMONIUM AND NITRATE UPTAKE BY THE GRASS CALAMAGROSTIS-VILLOSA

Calamagrostis villosa dominates the understory vegetation in declining spruce forests at higher elevations of the Central European mountain areas which show symptoms of needle yellowing and associated magnesium (Mg) deficiency. It was hypothesized that grasses would preferentiall y take up nitrate-nitrogen (NO3-N) over ammonium-nitrogen (NH4-N) whic h would support the cation balance in Mg deficient soils. In order to test this hypothesis, growth experiments were carried out in the green house using plants which were cultivated in sand for nine weeks with f ull nutrient solution containing 0.2 or 2 mmol of N with different NH4 + to NO3- ratios (1:0, 0.5:0.5, 0:1). In a short term experiment with labelled (NH4+)-N-15 and (NO3-)N-15, uptake of NH4+ and NO3- was measu red. When NO3- was the only N source it was taken up at similar rate p er g dry mass as in the experiment in which NH4+ was the only N source . However, at high supply pure NO3- nutrition resulted in higher bioma ss. In contrast, supply of only NH4+ caused accumulation of N in the r oots but growth remained restricted. If NH4+ and NO3- were supplied at equal amounts, NH4+ was the preferred form for N uptake. Biomass of t he plants with mixed supply did not differ from the plants with pure N O3- nutrition. The results point to an interesting interaction of carb on and nitrogen relation, but they do not support the initial hypothes is that grasses may prefer NO3- over NH4+.