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,

No abstract

On the basis of a field experiment in Norway spruce with acid irrigation and compensatory liming of the soil surface (H6glwald, S-Bavaria), liming effects are described as lime dissolution rate, transformation of carbonate buffer to exchange buffer, time required for deacidification of soil and drainage water, mobilization of Cu and Pb, changes in soil organisms, humus decomposition, and nitrogen turnover. It was shown that lime dissolution followed an exponentially decreasing curve. 4 t ha-l dolomitic lime were dissolved within 6 years. Additional acid irrigation of 4 kmol H + ha -1 yr -1 as sulphuric acid speeded up the lime dissolution to about 4 years. After dissolution of lime about 70% of Ca and about 30% of Mg, both originating from lime dissolution, are retained in the surface humus layer, loading the exchange buffer capacity there. Liming acted as a protection against acid irrigation but the extension of soil deacidification downwards proceeded slowly due to the high base neutralizing capacity of protonated functional groups of the organic matter. The main depth effect is caused by Mg translocation. A significant increase of organic Cu complexes occurred due to mobilization of water soluble humus decomposition products. The effect of liming on litter decomposing organisms is demonstrated with microorganisms, collembolae and earthworms regarding the abundance and the structure of dominance. It was shown that liming may induce unusually large changes in biocenoses of forest soils. The decay of surface humus accounted for 7.2 t ha-1 or 23% of the store within 7 years. Within the same time span, liming caused a loss of about 170 kg N ha -l or 14% of the store of the surface humus layer. The nitrate concentration in the drainage water thus increased by about 50 to 60 mg NO 3 L -1. Site-specific conditions are discussed, which produce such negative liming effects as increased nitrate concentration of seepage, humus decay and heavy metal mobilization. Redistribution of tree roots, induction of boron deficiency and root rot are also considered. It is indicated that liming may aggravate the increasing problem of nitrate contamination of forest ground water resources which is associated with deposition of atmogenous nitrogen compounds. Some recommendations are given regarding forest practice.

High loads of nitrogen to spruce and beech forests can result in a complete inhibition of NO $ − uptake by the roots of the trees. This conclusion is based on (a) a comparison of a field site continuously exposed to high loads of N and a N-limited site, (b) the results of N fertilization of a N-limited field site, and (c) laboratory experiments under controlled environmental conditions. From fertilization experiments in the field it appears that NH % + uptake might become inhibited subsequent to an excessive uptake of NH % + . Apparently, the inhibition of NO $ − uptake by high loads of N to forests is a consequence of an accumulation of organic amino compounds in the roots originating from phloem transport from the shoot to the roots. These amino compounds seem to signal the N demand of the shoot to the roots. At present this function cannot be attributed to an individual organic amino compound in beech or spruce, but Gln is a likely candidate in both species among other compounds, e.g. Glu in spruce or Asp in beech trees. Direct inhibition of NO $ − uptake by NH % + can be excluded from the present studies. The mechanism(s) by which elevated levels of particular organic amino compounds interact with NO $ − uptake remains to be elucidated. This (these) mechanism(s) seem to affect NO $ − influx rather than NO $ − efflux. As a consequence of this (these) mechanism(s), spruce and beech trees can prevent, within a certain physiological window, N over-nutrition when the roots are exposed to excessive amounts of inorganic N. However, inhibition of NO $ − and NH % + uptake by the roots makes more N available for leaching into the ground water and, in addition, for soil microbial processes that result in the production and re-emission of volatile N compounds into the atmosphere.At the ' Ho$ glwald ' site, continuously exposed to high loads of N, 20 % of the N input from throughfall into the spruce and beech plots is re-emitted as NO and N # O. However, the NO to N # O ratio is highly dependent on the tree species, with a preference for NO in the spruce and a preference for N # O in the beech plot. Since at least part of the NO emitted from the soil will be converted inside the canopy in the presence of ozone to NO # that might then be absorbed by the leaves, the portion of the N in the throughfall that will be released from the forest by gaseous N emission is higher in the beech than in the spruce plot. Leaching of NO $ − into the ground water is high in the spruce, but minute in the beech plot. However, this positive effect of beech on ground water quality is achieved at the expense of an enhanced release of radiatively active N gases into the troposphere.Key words : Atmospheric pollution, forest ecosystem, nitrogen allocation, nitrogen fluxes, regulation, nitrogen oxides, ammonia. Both atmospheric and pedospheric sources of nitrogen are available to plants. The contribution of atmospheric N is considered to be low in remote * To whom correspondence should be addressed E-mail : here!sun2.ruf.uni-fre...

No abstract