Ammonium and nitrate uptake by Eucalyptus nitens: effects of pH and temperature

Ammonium and nitrate uptake by roots of Eucalyptus nitens was characterised with respect to pH and temperature. Uptake of ammonium and nitrate was mea sured as depletion from solutions by roots of intact 11 week old solution-c ultured seedlings. Uptake rates of ammonium were consistently higher than t hose of nitrate in all experiments. Uptake rates for ammonium were 200% hig her at pH 4 than at pH 6, but for nitrate were unchanged. Uptake rates of a mmonium and nitrate were both reduced to a similar extent (70%) with a decr ease in temperature from 20 degrees C to 10 degrees C. For ammonium uptake, there was rapid (< 24 hr) adaptation to a reduction in root temperature. T he apparent preference shown here for ammonium over nitrate could be indica tive of E. nitens growing in cold, acidic forest soils where ammonium is co mmonly more available than nitrate. These results suggest that N uptake rat es of E. nitens may be maximised under a wide variety of conditions if N is supplied predominantly in the ammonium form.