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 (' Ho$ glwald ', Germany). In addition, uptake experiments were 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 true for beech except during September 1995. Apparently, beech trees was capable of taking up nitrate, but the environmental condition prevailing at the field site usually prevented net uptake. Net uptake of ammonium in both tree species showed a seasonal course, with maximum rates in mid summer. Rates of ammonium uptake by both species correlated with soil temperature at the field site.Laboratory experiments on the influence of root temperature on uptake of nitrate indicated that uptake rates at temperatures found in the field were low compared with the uptake capacity at optimum temperature. At temperatures of 10 and 15 mC, frequently found in the soil at the field site, net uptake of nitrate by spruce and beech amounted to c. 16 % and 11 %, respectively, of maximum uptake at 25 mC. By contrast, net uptake of ammonium at 10 mC reached 73 % and 31 % of the maximum uptake for spruce 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 solution at the forest site. Preincubation of roots of both species, with amino acids present in the phloem of adult trees at the field site, led to an increase in the amino acid pool in the roots. For spruce trees a correlation between inhibition of uptake of nitrate and enrichment of the roots with the amino compounds Glu, γ-amino butyric acid (Gaba), Gln, and Asn was observed. In beech trees, enrichment of Asp and 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 acid enrichment in the roots are discussed with special emphasis on the patterns of net uptake of ammonium and nitrate observed in the field.Key words : Nitrate uptake, ammonium uptake, soil temperature, nitrate to ammonium ratio, amino acids. In forest ecosystems, nitrate and ammonium are the most abundant N compounds available to the roots * To whom correspondence should be addressed. E-mail : here!sun2.ruf.uni-freiburg.de of trees. Total amounts of nitrate and ammonium, and the ratio of nitrate to ammonia depend on quality and quantity of N input and on the balance of ammonification, nitrification, immobilization and denitrification processes in the soil (Haynes & Goh,

summary During the vegetation period 1994, the nitrogen (N) composition and contents of leaves, xylem sap and phloem exudates of twigs from a coniferous (Picea abies (L.) and a deciduous (Fagus sylvatica L.) tree species were analysed at a field site (Höglwald, Germany) exposed to high loads of N. In April, total soluble non‐protein N (TSNN) in the xylem sap of twigs from spruce and beech reached a maximum of about 7.0 and 15.0 μmol ml−1, respectively, probably owing to remobilization of stored N. After bud break, TSNN in the xylem sap of both tree species decreased rapidly, to a minimum of c. 2.0 μmol ml−1 in July. Subsequently, TSNN increased two‐fold in the xylem sap of beech until September, but remained constant in the xylem sap of spruce at the low level observed in July. In both tree species, amides were the predominant TSNN compounds transported in the xylem. In xylem sap of beech. Gin, Asn and Arg were most abundant, together comparing more than 60% of TSNN. Gin and Asp, but not Arg, prevailed in the xylem sap of spruce, together comparing more than 50 % of TSNN. In addition, a number of other proteinogenic and non‐proteinogenic amino acids, and also nitrate and ammonium, were detected in small amounts in the xylem saps of both tree species. Remobilization of stored N during growth of the new flush resulted in a 4.5‐fold decrease of TSNN in previous year's needles from 9.1 μmol g−l f. wt in April to 2.0 μmol g−1 f. wt in May. This decrease was mainly attributed to Arg, the prevailing amino compound in needles of spruce. Subsequently, a 5.5‐fold increase was observed in July. High TSNN contents remained constant until September, Within current year's needles TSNN remained relatively constant during the entire vegetation period. In leaves of beech, TSNN amounted to c. 5.6 μmol g−1 f, wt from April to July and increased to 9.5 μmol g−1 f. wt in September. In April and May the predominant amino compounds in leaves of beech were Asn and Glu, in September they were Arg and Gin. Concomitant with the increase in Arg content of the leaves, its content in the phloem also increased, suggesting a transport of Arg from the senescencing leaves to storage tissues. Nitrate was found neither in needles nor in leaves, whereas ammonium amounted to up to 25 % of TSNN in both needles and leaves. In phloem exudates of twigs from beech, TSNN amounted to c. 34.0 μmol g−1 f. wt in April and decreased 10‐fold until May, during growth of the new flush. Subsequently, until September a threefold increase in TSNN was observed. In phloem exudates of twigs from spruce, TSNN decreased from 13.0 μmol g−1 f. wt in April to 4.8 μmol g−1 f. wt in May, By September, TSNN contents were doubled. In April, Arg was the prevailing amino acid in phloem exudates of beech. Concomitant with the decline in total TSNN the Arg content decreased. In September a significant increase in the Arg content was observed. In phloem exudates of spruce, Gin and Arg were the predominant N compounds in April and May. In July and September a decrease of the Gin conte...

From laboratory experiments with seedlings and young trees of Norway spruce (Picea abies L. Karst.), a cycling pool of soluble non‐protein N compounds is thought to be indicative of the N‐nutritional status of trees. In order to test whether this assumption can be transferred to mature trees grown in the field, xylem sap and phloem exudate were collected from spruce trees in two remote forest stands: (1) a N‐limited stand (Villingen site), and (2) a stand where trees are sufficiently supplied with N from the soil (Schluchsee site). Trees at these sites were c. 80–100 (Villingen site) and c. 40–60 (Schluchsee site) yr old. In addition to untreated control areas, one entire watershed area at both sites was subjected to (NH2)2SO4 fertilization to the soil. In the xylem sap of the spruce roots at both sites Gln, Asp and Arg were the dominant total soluble non‐protein nitrogen (TSNN) compounds. In the xylem sap of the trunk and the twigs Arg was virtually absent and Gln plus Asp dominated TSNN. On average, TSNN in the xylem sap of trees at the Schluchsee site was 1·5–2‐fold higher than those at Villengen. Highest TSNN contents in the xylem sap were found during growth and development of current year tissues, while the lowest TSNN contents were found in summer. At the Villingen site (NH4)2SO4 fertilization caused an increase in the Gln content in the xylem sap of all tree sections analysed as well as an increase in the Arg content in the xylem sap of the roots. At the Schluchsee site only a small increase in TSNN contents of the xylem was observed, mostly in the xylem sap of the roots. In phloem exudates TSNN contents were much higher in trees on the Schluchsee than on the Villingen site. The seasonal pattern of TSNN in phloem exudates was similar to the seasonal pattern found in the xylem sap. During spring and early summer Gln was the predominant TSNN compound in phloem exudates, but during late summer and autumn Arg became predominant. At the Villingen site (NH4)2SO4 fertilization caused a significant increase in TSNN contents in phloem exudates of twigs and roots, but at Schluchsee an increase in TSNN was found only in phloem exudates of the roots. At both field sites Arg that was not transported to the shoot by xylem transport, was allocated from the leaves to the roots by phloem transport and was cycled within the root system by both xylem and phloem transport. From these results it is calculated that shoot‐to‐root signalling by long‐distance transport of amino compounds can also contribute to the regulation of N‐nutrition of mature spruce trees. Apparently, the internal cycling of individual N compounds within spruce trees differs considerably.

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