FIELD AND LABORATORY EXPERIMENTS ON NET UPTAKE OF NITRATE AND AMMONIUM BY THE ROOTS OF SPRUCE (PICEA-ABIES) AND BEECH (FAGUS-SYLVATICA) TREES

During the vegetation periods 1994 and 1995, net uptake of nitrate and ammonium by roots of adult spruce (Picea abies (L.) Karst) and beech (Fagus sylvatica L.) trees was studied at a field site exposed to high loads of N ('Hoglwald', Germany). In addition, uptake experiments wer e carried out under controlled conditions with young spruce and beech trees grown at normal N supply. In the field, nitrate was not taken up by the roots of spruce trees in appreciable amounts. This was also tr ue for beech except during September 1995. Apparently, beech trees was capable of taking up nitrate, but the environmental condition prevail ing at the field site usually prevented net uptake. Net uptake of ammo nium in both tree species showed a seasonal course, with maximum rates in mid summer. Rates of ammonium uptake by both species correlated wi th soil temperature at the field site. Laboratory experiments on the i nfluence of root temperature on uptake of nitrate indicated that uptak e rates at temperatures found in the field were low compared with the uptake capacity at optimum temperature. At temperatures of 10 and 15 d egrees C, frequently found in the soil at the field site, net uptake o f nitrate by spruce and beech amounted to c. 16 % and 11 %, respective ly, of maximum uptake at 25 degrees C. By contrast, net uptake of ammo nium at 10 degrees C reached 73 % and 31 % of the maximum uptake for s pruce and beech trees, respectively. Independent of temperature, rates of nitrate uptake were considerably lower than those of ammonium. In young spruce and beech trees, net uptake of nitrate was significantly inhibited by ammonium at nitrate:ammonium ratios found in the soil sol ution at the forest site. Preincubation of roots of both species, with amino acids present in the phloem of adult trees at the field site, l ed to an increase in the amino acid pool in the roots. For spruce tree s a correlation between inhibition of uptake of nitrate and enrichment of the roots with the amino compounds Glu, gamma-amino butyric acid ( Gaba), Gln, and Asn was observed. In beech trees, enrichment of Asp an d Gln in the roots correlated with a decrease in net uptake of nitrate . The results of laboratory experiments on the effects of temperature, the nitrate to ammonium ratio in the nutrient solution, and amino aci d enrichment in the roots are discussed with special emphasis on the p atterns of net uptake of ammonium and nitrate observed in the field.