New Phytol. (1998), 139, 71–86

Consequences of high loads of nitrogen for

spruce (Picea abies) and beech (Fagus

sylvatica) forests

B HEINZ RENNENBERG"*, KARL KREUTZER# , HANS PAPEN$

PAUL WEBER "

" Institut fuX r Forstbotanik und Baumphysiologie, Professur fuX r Baumphysiologie, Albert-

Ludwigs-UniversitaX t Freiburg, Am Flughafen 17, D-79085 Freiburg i. Br., Germany

#Lehrstuhl fuX r Bodenkunde, Ludwig-Maximilians-UniversitaX t MuX nchen, Hohenbachernstr.

22, D-85354 Freising, Germany

$Fraunhofer Institut fuX r AtmosphaX rische Umweltforschung, Kreuzeckbahnstr. 19, D-82467

Garmisch-Partenkirchen, Germany

(Received 5 September 1997)

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 ni-

trogen 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-freiburg.de

environments and the demand by plants for N is met

almost exclusively by the uptake of NH%

+ and}or

NO$

− (Glass & Siddiqi, 1995) originating from

mineralization processes in the soil under these

conditions. As a consequence, a competition for

NH%

+ and NO$

− in the soil between plant roots and

microbial immobilization, nitrification, and denitri-

72 H. Rennenberg and others

Atmosphere

Removalby cropping

or fire

Plants

Plantlitter

Plantuptake

NH4+/NH

3NO

3–/NO

x

NO/N2O/N

2

Pedosphere

Organicnitrogen

Microbialbiomass

Ammonification

Immobilization FixationExchange

sites

Symbiotic andnon-symbiotic

fixation

DenitrificationNitrification

NH4+ NO

2– NO

3–

Ground water Leaching Leaching

Figure 1. Interaction of atmospheric nitrogen with ecosystem processes.

fication determine N nutrition of plants (Fig. 1).

Atmospheric pollution from agriculture (Wellburn,

1990; Fangmeier et al., 1994) and fossil fuel

combustion (Crutzen, 1979; Logan, 1983) are

thought to interact with this competition in several

ways. Uptake of dissolved NH%

+ and NO$

− by above-

ground parts of plants (Brumme, Leimke & Matzner

1992), but also the influx of atmospheric NH$

(Pearson & Stewart, 1993; Pe! rez-Soba, Stulen & van

der Eerden, 1994) and NOx

into plant shoots

(Wellburn, 1990; Nussbaum et al., 1993; Muller,

Touraine & Rennenberg, 1996) can contribute

significantly to N nutrition. Wet deposition of NH%

+

and NO$

− to the soil alters the physical, chemical and

biological properties of the pedosphere and might

affect N uptake by the roots of plants (Marschner,

Ha$ ussling & George, 1991).

In many areas of Europe the patchiness of

landscape has resulted in densely populated areas,

agricultural land and forests being in close vicinity.

As a consequence, many European forest ecosystems

are exposed to high loads of N, often dominated by

NHy

(NH$

plus NH%

+) from agricultural sources

(Erisman & Heij, 1991; Kreutzer, 1992; Fangmeier

et al., 1994). Therefore, the question has to be

addressed as to whether forest trees are able to

defend themselves at least to some extent from

overnutrition when exposed to high loads of N. This

question is of particular significance for forests, since

the availability of inorganic N is one of the major

environmental factors limiting growth of woody

plants not exposed to atmospheric pollution (Cole &

Rapp, 1981; Dickson, 1989; Kreutzer, 1992) and

tree metabolism as well as forest ecosystem func-

tioning are thought to be adapted to N-limitation

rather than to cope with high loads of N.

The effects of N deposition on forests have been

analysed in numerous field studies at the ecosystem

level (Johansson, 1987; Go$ ttlein & Kreutzer, 1991;

Schulze, Gebauer & Katz, 1991; Brumme et al.,

1992). In these studies, deposition of N compounds

and the effects of this deposition on pedospheric

processes have been intensively studied, but trees

were usually treated as a ‘black box’ (Fig. 1). As a

consequence, information from field studies on the

interaction between high loads of atmospheric N and

physiological processes in forest trees that control N

nutrition is scarce. In order to overcome this lack of

knowledge, N nutrition of mature Norway spruce

(Picea abies) and beech (Fagus sylvatica) trees has

been studied at a forest site (‘Ho$ glwald’) exposed to

high loads of atmospheric N by dry and wet

deposition (Go$ ttlein & Kreutzer, 1991). At this site,

long-term studies on physical, chemical and mi-

crobial properties of the soil, including trace-gas

exchange with the atmosphere, have been performed

and could be used for the interpretation of the results

achieved in tree physiological field studies (Kreutzer,

Effects of high nitrogen loads on forests 73

1995). In the present report the data obtained at the

‘Ho$ glwald’ site are compared with studies

performed at a site in the Black Forest (‘Villingen’)

not exposed to high loads of atmospheric N and

considered to be limited by N availability (Feger,

1992). Experimental NH%

+ fertilization on part of

this site provided the opportunity to test the

assumptions drawn from the comparison between

the ‘Ho$ glwald’ site and the unfertilized area of the

‘Villingen’ site. In addition, laboratory studies with

young spruce and beech trees were performed to

permit a more detailed analysis of the physiological

observations made in the field.

Site and stand description

The field site ‘Ho$ glwald’ is located 50 km west-

north-west of Munich (Bavaria, Germany) at

11° 10« E longitude and 48° 30« N latitude within the

hilly prealpine region c. 70 km north of the Alps.

The elevation is 540 m above sea level. Climate is

suboceanic with relatively high precipitation during

summer, mean annual precipitation of 800 mm, and

mean annual air temperatures of 7±3 °C. Long-term

mean air temperatures amount to ®2 °C in January

and 16±5 °C in July. During the period of the present

study winter air temperatures were unusually high.

In 1770 most of the natural vegetation, a sub-

montane Asperulo–Fagetum luzuletosum, was

replaced by a Norway spruce plantation. The second

generation of spruce had reached an age of 90 yr in

1997. Despite this relatively advanced age, trees have

been growing vigorously for the last 30 yr, exceeding

the best yield class of the regional yield tables

(Ro$ hle, 1991). The trees are apparently healthy

without symptoms of defoliation. Elemental analyses

of the needles provided no indications of nutrient

deficiencies or imbalances (Table 1). The stand is

full-stocked and has a closed canopy. In a small area,

the spruce trees (Picea abies L.) were replaced by

beech trees (Fagus sylvatica L.) after the first

generation of spruce. These beech trees approached

an age of 97 yr in 1997. A small area of the spruce

plantation was limed in 1994 by application of

4000 kg of calcium-magnesium-carbonate ha−" con-

taining 22 kmol Ca and 20 kmol Mg.

Table 1. Elemental concentrations (mg g−" d. wt) in

half-year-old spruce needles from the ‘HoX glwald ’ site

N P K Ca Mg Mn

1986 15±6 1±7 4±3 3±5 1±2 2±11988 13±9 1±8 4±8 3±2 1±4 2±11989 14±1 1±6 5±1 3±1 1±2 2±11994 15±3 1±6 4±1 4±1 1±2 2±7

The soil of the spruce plantation is a typic

‘Hapludalf ’, covered by a moder layer of 6-cm

thickness (weakly spodic, weakly pseudogleyic para-

brown earth). It is derived from Lo$ ss over Tertiary

silty sand deposits. The pH(H#O) is 3±5–3±9 in the

O-layer; 3±6–4±2 in the A-layer and 4±2–4±8 in the

B-layer. Base saturation is 50–80% in the O-layer;

2–5% in the A-layer; 20–60% in the B-layer. Mean

depth of the root zone is 35 cm in the interstem space

(c. 70% of the area) and c. 70–120 cm below the

stems. Further details are described by Kreutzer &

Go$ ttlein (1991).

The ‘Ho$ glwald’ site is surrounded by intensively

used agricultural land with high applications of

liquid manure from pig and cattle farming. As a

consequence, this forest ecosystem is exposed to an

excess of N by wet and dry deposition from the

atmosphere (Kreutzer, 1995). The surplus amount

of N in the ecosystem can be estimated from the

surplus Ninorg

(NO$

− ­ NH%

+) content in the soil

solution of the root zone per area and time (S) as

follows:

S¯T­M­Gin®U®G

out,

where T is the rate of throughfall of Ninorg

, M the

rate of mineralization, Gin

the absorption of NH$and

NOxby the canopy and the soil, U the rate of uptake

of NO$

− and NH%

+ by the roots for maximum

growth, and Gout

the release of gaseous N compounds

produced by nitrification, denitrification and other

processes. These parameters are quantified in an

interdisciplinary ecosystem study at the ‘Ho$ glwald’

site.

Exchange of nitrogen with the atmosphere

Since 1993 complete annual cycles of N#O-, NO-

and NO#-fluxes between the soil and the atmosphere

have been determined at the ‘Ho$ glwald’ site on a

spruce and a beech plot (controls) as well as on a

limed spruce plot. For this purpose, fully automated

measuring systems have been installed at the site

applying the closed-chamber technique for the

determination of N#O-flux rates and the dynamic-

chamber technique for determination of NO-}NO#-

flux rates. Time resolution is 2 h for N#O-, and 1 h

for NO-}NO#-fluxes (Butterbach-Bahl et al.,

1997).

In contrast to spruce forests which receive low N

inputs from the atmosphere and are N-limited, high

emission rates of N#O and NO from the soil to the

atmosphere were observed at the ‘Ho$ glwald’ site

(Butterbach-Bahl et al., 1997, 1998). A highly sig-

nificant correlation was demonstrated between the

actual N input by wet deposition from the at-

mosphere to the soil and the actual in situ flux rates

of NO and N#O from the soil to the atmosphere

(Butterbach-Bahl et al., 1998). This correlation was

74 H. Rennenberg and others

(a)

(b)

400

300

200

100

0

0

–50

–100

60

(c)40

20

10

0

30

20

0

–10

10

(d)

1 6 11 16 21 26 31

Day in September 1995

air

moss layer

NO

NO2

O3

–10

0

10

20

30

0

10

20

30

–100

–50

0

0

100

200

300

400

Tem

pera

ture

(°C

)

NO

/NO

2 c

on

cen

trati

on

(pp

bv)

NO

2 f

lux r

ate

(µg

NO

2-N

m–2 h

–1)

NO

flu

x r

ate

(µg

NO

-N m

–2 h

–1)

O3 c

on

cen

trati

on

(pp

bv)

Figure 2. Time courses of NO- and NO#-flux rates, concentrations of NO, NO

#and O

$in ambient air, and

air and soil temperature of an untreated and a limed spruce plot at the ‘Ho$ glwald’ site in September 1995 (data

from Gasche (1997)). (a) Hourly means of NO-flux rates of the untreated (control) spruce plot (*) and of

the limed spruce plot (+). Each data point shown was calculated from five individual flux rates obtained with

five dynamic chambers. (b) Hourly means of NO#-flux rates of the untreated (control) spruce plot (-D-) and

of the limed spruce plot (-E-). Each data point shown is calculated from five individual flux rates obtained with

Effects of high nitrogen loads on forests 75

highly significant for both NH%

+ and NO$

− input by

wet deposition, with a stronger correlation for NH%

+

than NO$

−. From this finding it may be concluded

that nitrification is the predominant source of the

N#O and NO emitted from the soil of the spruce

forest at the ‘Ho$ glwald’ site (Butterbach-Bahl et al.,

1998). This conclusion is supported by a highly

positive correlation (P!0±001; r#¯0±65) between

in situ net-nitrification rates and in situ NO-fluxes

(Gasche, 1997).

As an example of the extremely high exchange

rates of N oxides between the soil of the spruce plots

and the atmosphere, the time courses of the hourly

mean NO- and NO#-flux rates during September

1995 are shown in Figure 2 for the control and the

limed plot. In addition, the concentrations of NO,

NO#and ozone in ambient air as well as the air- and

soil-temperatures in the moss layer are given. This

example was chosen because the data measured in

September 1995 are representative for the mean

annual NO fluxes from the spruce plot in 1995.

During the entire month NO was mainly emitted

from, and NO#

mainly deposited to, the soil. Mean

monthly NO-emission rates were 95±8³28±1 µg NO-

N m−# h−", mean monthly NO#-deposition rates

28±5³13±6 µg NO#-N m−# h−". From these data, a

net NOx-emission from the soil to the atmosphere of

almost 70 µg NOx-N m−# h−" (i.e. equivalent to

6±1 kg NOx-N ha−" yr−") is calculated. This emission

rate is more than 50 times higher than NO-emission

rates reported for the N-limited spruce plot

‘Villingen’ in the Black Forest (Hermann, 1995).

The monthly NO-emission rates from soil of the

limed spruce plot at the ‘Ho$ glwald’ site of

41±2³11±6 µg NO-N m−# h−" were significantly

lower than the emission from the unlimed control

plot. Since significant differences in the NO#

de-

position rates between the limed spruce plot and the

control were not observed (Fig. 2), liming resulted in

a reduction of the NOx-emission rate of almost 57%.

But even after liming NO-emission rates from the

soil were more than 30 times higher than NO-

emission rates reported for the N-limited spruce plot

‘Villingen’ (Hermann, 1995).

Generally, NO-emission rates from the soil of the

control spruce plot at the ‘Ho$ glwald’ site exceeded

N#O-emission tenfold. By contrast, N

#O emission

rates exceeded NO emission rates on the beech plot

(NO}N#O emission ratio 0±6; Butterbach-Bahl et al.,

1997). Since beech and spruce grow on the same soil

type at the ‘Ho$ glwald’ site, these results indicate a

strong impact of the forest tree species on the N

oxide (N#O or NO) preferentially emitted from the

five dynamic chambers. (c) Hourly means of concentrations of NO, NO#and ozone in ambient air (n¯5). (d)

Hourly means of air temperature and soil temperature in the moss layer. A detailed description of the complete

measuring system is given by Butterbach-Bahl et al. (1997).

soil. Mean annual N#O emission rates of the

untreated spruce plot were "10 µg N#O-N m−# h−"

(equivalent to "0±9 kg N#O-N ha−" yr−"), whereas

the corresponding rates from the soil of the beech

plot were "30 µg N#O-N m−# h−" (equivalent to "

2±6 kg N#O-N ha−" yr−"). Apparently, the soil under

beech was a much stronger source of N#O than that

of the untreated spruce plot. The N#O-emission

rates from the spruce and the beech control plots of

the ‘Ho$ glwald’ site were considerably higher than

previously described for forest ecosystems of the

temperate zone exposed to lower levels of atmos-

pheric N (Butterbach-Bahl et al. 1997, 1998).

Uptake of nitrogen by the roots

Net uptake of NO$

− and NH%

+ by the roots of spruce

and beech trees growing in the humus and the upper

soil layer, i.e. the predominant rooting zone of both

species, was studied at the ‘Ho$ glwald’ site during

the growing seasons 1994 and 1995. For this purpose

the depletion technique was applied in the field

(Rennenberg, Schneider & Weber, 1996; Geßler et

al., 1998a). This technique is based on the removal

of NO$

− and NH%

+ from artificial soil water solutions

by the tips of roots still attached to the tree. The

artificial soil water solutions contained similar

concentrations of inorganic cations and anions as the

natural soil water at the site and the time of analysis.

During the growing season of 1994, neither the roots

of spruce nor those of beech trees took up NO$

−

in appreciable amounts (Geßler et al., 1998a).

From the limit of detection of the method applied,

net uptake of NO$

− was estimated to be

!0±1³15 nmol g−" root f. wt h−". Also during the

growing season of 1995 NO$

− net uptake was usually

below the limit of detection of the depletion

technique with the exception of beech roots in

September, when NO$

− net uptake amounted to c.

170 nmol g−" root f. wt h−". This exception seems to

be connected to fructification of the beech trees, but

further studies are required to test this possibility.

By contrast, NH%

+ net uptake by the roots was

observed for both, spruce and beech during every

field trial during the growing seasons of 1994 and

1995. Apparently, both species meet their N supply

from the soil almost exclusively by NH%

+ uptake at

the ‘Ho$ glwald’ site. Net uptake of NH%

+ showed a

seasonal course with maximum rates in mid-summer

and significantly lower rates in spring and autumn

(Geßler et al., 1998a). This finding is consistent with

previous observations with other tree species indi-

cating that the demand for N during bud break in

76 H. Rennenberg and others

spring can be satisfied almost completely by

remobilization of N from storage tissues (Millard &

Proe, 1992; Millard, 1994, 1996). Despite con-

siderable differences during the growing seasons

of 1994 and 1995, maximum rates of net uptake

of NH%

+ into beech roots (1410 and

880 nmol g−" root f. wt h−" in 1994 and 1995, re-

spectively) were significantly higher than the maxi-

mum rates into spruce roots (890 and

680 nmol g−" root f. wt h−" in 1994 and 1995, re-

spectively). The differences in net uptake of NH%

+

within and between the growing seasons could

mainly be explained by differences in soil tem-

perature for both species (Geßler et al., 1998a).

Thus, the differences in net uptake of NH%

+ between

the two forest tree species studies are due to

differences in the temperature dependencies of the

processes involved.

From these observations it appears that NO$

−

deposited to the soil of both the spruce and the beech

plots at the ‘Ho$ glwald’ site is almost completely

available for soil microbial processes, i.e. especially

for immobilization and denitrification. However,

from the present studies the possibility cannot be

excluded that roots growing in deeper soil layers take

up some NO$

− from the soil water. Since the uptake

of NH%

+ by the roots proceeds at a lower rate in the

spruce than in the beech plot, an enhanced portion of

the NH%

+ deposited to the soil seems to be available

for nitrification processes in the spruce forest. This

view is supported by the observation that

nitrification is the predominant source of the N#O

and NO emitted from the soil of the spruce forest at

the ‘Ho$ glwald’ site (Gasche, 1997; Papen &

Butterbach-Bahl, unpublished).

The preferential uptake of NH%

+ compared with

NO$

− observed at the ‘Ho$ glwald’ site for the roots of

spruce and beech is a well described phenomenon in

woody plant species (Finlay et al., 1989; Marschner

et al., 1991; Plassard et al., 1991; Flaig & Mohr,

1992; Kronzucker, Siddiqi & Glass, 1996). It is

nevertheless surprising that net uptake of NO$

− by

the roots can be down-regulated completely in

spruce and beech. This phenomenon cannot be

explained by differences in kinetic properties and}or

temperature dependencies between NO$

− and NH%

+

net uptake (Geßler et al., 1998a). From incubation

experiments with constant NO$

− and varying NH%

+

concentrations applied simultaneously, inhibition of

NO$

− net uptake in roots by NH%

+ has been reported

in some studies (Glass, Thompson & Bordeleau,

1985; Lee & Drew, 1989; King et al., 1993; Chaillou

et al., 1994; Kreuzwieser et al., 1997; Geßler et al.,

1998a), but few or no effects were observed in others

(Smith & Thompson, 1971; Schrader et al., 1972;

Oaks, Stulen & Boesel, 1979; Flaig & Mohr, 1992).

Direct effects of NH%

+ on processes of NO$

− uptake

have been assumed by some authors (Lee & Drew,

1989; King et al., 1993), whereas others have

postulated that products of N assimilation such as

amino compounds are responsible for the down-

regulation of NO$

− net uptake in roots simul-

taneously supplied with NH%

+ (Lee et al., 1992;

Imsande & Touraine, 1994). This down-regulation

is thought to be achieved by an internal cyling pool

of amino compounds in the tree that can be expanded

in the presence of excess N in the environment. In

order to obtain a more complete picture of the N

nutritional status of the spruce and beech trees at the

‘Ho$ glwald’ site, N composition and contents of the

potentially cycling N pool were analysed in different

compartments of the trees.

Tree internal nitrogen allocation

For characterization of the potentially cycling N

pool in spruce and beech trees, total soluble non-

protein N (TSNN) contents were analysed in

leaves}needles, root and shoot xylem saps, root and

shoot phloem exudates, and fine roots during the

growing seasons of 1994 and 1995 (Schneider et al.,

1996; Geßler et al., 1998b). TSNN was defined as

the N content in proteinogenic and non-

proteinogenic amino compounds plus NH%

+ and

NO$

−. From the data obtained in these studies a

whole-plant model for allocation and cycling of

TSNN in spruce and beech trees exposed to high

loads of N has been proposed (Geßler et al., 1998b).

During the entire growing season, Arg and}or Gln

are the most abundant TSNN constituents in all

compartments of the spruce trees studied. In the

roots the origin of Arg and Gln might change during

the growing season. In spring during bud break,

when N uptake by the roots is supposed to be

negligible (Millard, 1994, 1996; Geßler et al.,

1998a), the contribution of N uptake and assimi-

lation in the roots might be minute, and Arg and Gln

might originate from remobilization of N from

storage tissues in the roots, and}or phloem transport

of Arg and Gln remobilized from storage tissues in

the shoot. Assimilation of inorganic N in the roots

might become more important when the N pools of

the storage tissues have been depleted. Arg, Gln, and

Asp were loaded into the xylem of spruce roots

during the entire growing season. During most of

the growing season Gln and Asp, but not Arg were

present in the xylem sap of spruce twigs. Apparently,

Arg loaded into the root xylem was removed during

its long-distance transport from the roots to the

twigs. This finding indicates that Arg transported in

the xylem could serve as a N source for stem and root

growth. Since the phloem of the spruce trees was

enriched with Arg from the twigs to the roots, Arg

unloaded from the xylem might also be reloaded into

the phloem, and might circulate within the stem and

the roots of spruce trees. During N storage in

September, Arg was found in both the xylem of the

roots and the twigs. It might therefore be concluded

Effects of high nitrogen loads on forests 77

that Arg loaded into the root xylem is no longer

removed in the root and}or stem region during long-

distance transport at this stage of the growing season,

and is used for the storage of N in the needles.

Since NO$

− and NH%

+ contents of the xylem of

spruce are extremely low, assimilation in the needles

of inorganic N taken up by the roots does not appear

to contribute significantly to N nutrition of the

spruce trees at the ‘Ho$ glwald’ site. The Arg present

in the previous year’s needles throughout the

growing season might originate either from Gln and

Arg transported in the xylem, or from assimilation of

inorganic N absorbed from the atmosphere. It might

constitute a major part of the N storage pool

remobilized during initial growth of current-year

needles (Nambiar & Fife, 1991; Flaig & Mohr, 1992;

Gezelius & Na$ sholm, 1993; Geßler et al., 1998b).

The accumulation of Arg also in current-year needles

of spruce trees at the ‘Ho$ glwald’ site might be

indicative of excess N supply (Na$ sholm & Ericson,

1989; Geßler et al., 1998b).

TSNN allocated in the phloem from the twigs to

the roots is considered to represent the N pool

exceeding the N demand of spruce needles in growth

and storage. Therefore, the size of this N pool has

been considered to be a regulatory factor of NO$

−

uptake by plant roots (Imsande & Touraine, 1994).

In spruce trees at the ‘Ho$ glwald’ site Arg and Gln

are the most abundant TSNN constituents in the

phloem during the entire growing season. Arg is

enriched in the phloem of the root compared with

that of the twig, most likely as a consequence of (a)

xylem-to-phloem exchange of Arg in the root and}or

stem and (b) excessive transport of Arg from the

twigs to the roots. The high accumulation of Arg in

the root phloem may be considered to be an indicator

of excessive N nutrition, but seems not to be suitable

as a regulatory device for NO$

− uptake (Geßler et al.

1998a). Glutamine that is also present in high

concentrations in all sections of the spruce trees at

the ‘Ho$ glwald’ site analysed, including the twig and

root phloem, seems to be a more likely candidate for

this function (Geßler et al., 1998a). Since Gln is

enriched in the twig compared with the root phloem,

probably as a consequence of phloem-to-xylem

exchange in the twig and stem region, it may be

assumed that the amount of Gln that escapes this

exchange processes regulates NO$

− uptake by the

roots.

Nitrogen cycling and allocation in beech trees at

the ‘Ho$ glwald’ site seems to be more complex, and

there are changes during the growing season (Geßler

et al., 1998b). Apparently, the differences in TSNN

composition and contents mainly reflect differences

in the sites of N storage and remobilization between

conifers, i.e. previous year needles and roots, and

deciduous trees, i.e. stem and roots (Millard, 1994,

1996). Nevertheless, several common features of

TSNN composition and contents were found be-

tween spruce and beech trees at the ‘Ho$ glwald’ site :

xylem sap TSNN content of beech trees is

dominated by organic N rather than by inorganic N.

Thus, assimilation of inorganic N in beech and

spruce trees seems to take place mainly in the roots.

Arginine seems to be the predominant storage

compound and accumulates in different parts of

beech trees during the growing season. In contrast to

spruce, Arg does not accumulate in root phloem

during autumn. Therefore, the Arg content of root

phloem exudate is not an appropriate indicator of

excess N supply to beech at this time of the year. As

in spruce, Gln is present in beech trees in high

amounts in all tissues and transport systems during

the entire growing season and shows the charac-

teristics of a N compound circulating between the

shoot and the roots. Also in beech this compound is

likely to adapt N uptake by the roots to the N

demand of the entire tree (Geßler et al., 1998a, b).

The high amounts of organic TSNN compounds

in different plant sections of both, spruce and beech

trees at the ‘Ho$ glwald’ site indicate that, despite

down-regulation of NO$

− uptake by the roots,

storage pools of N are highly expanded as a

consequence of NH%

+ uptake by the roots, and N

uptake by the shoot from wet and dry deposition.

Nevertheless, total N contents of half-year-old

spruce needles are remarkably low (Table 1)

indicating that accumulation of N in storage pools of

the current year needles has not been initiated at this

stage of needle development. For a proper evaluation

of the TSNN analyses performed with plant material

from the ‘Ho$ glwald’ site a comparison with trees

growing under N-limiting conditions is required

(see below). For a quantitative estimate of the

significance of the N fluxes to spruce and beech trees,

a detailed analysis of N turnover in other com-

partments of the ‘Ho$ glwald’ ecosystem has to be

considered.

Ecosystem fluxes of nitrogen

The turnover of N in different compartments of the

ecosystem has been studied on all plots of the

‘Ho$ glwald’ site, but is not yet completed. Figure 3

provides current estimates for the spruce control-

plot. External input of N into the canopy from

atmospheric sources results from bulk precipitation

and interception including dry deposition. An un-

known fraction of the atmospheric N is retained in

the canopy by uptake and assimilation. This re-

tention includes wet deposition of NO$

− and NH%

+ as

well as dry deposition of NOx

and NH$. Rough

calculations on the basis of preliminary data indicate

a dry deposition of NH$

of !5 kg N ha−" yr−"

(Huber, 1997). Most of the external input seems to

be deposited on the soil surface as throughfall

(30 kg N ha−" yr−"). The gaseous N input into the

soil has only been quantified for NOxand it amounts

78 H. Rennenberg and others

Bulk precipitation12

Uptakea

Interception18 + a

Requirement10

Litterfall50

Throughfall30

Gaseousemission

7 + d

Adsorption ofN trace gases

3 + b

Root decay25

Change inN

org store±c

Uptake85 - a

Leaching30

Figure 3. Fluxes of nitrogen in a spruce forest stand

exposed to high loads of N (‘Ho$ glwald’). The numbers

given are N fluxes in kg ha−" yr−" between different

compartments of the ecosystem. Letters indicate fluxes

that have not been quantified as follows: (a), uptake and

assimilation of atmospheric nitrogen by the canopy; (b),absorption of atmospheric trace gases by the soil (without

NOx) ; (c), changes in the turnover of organic nitrogen in

the soil ; (d), emission of N#

originating from denitri-

fication.

to 3 kg N ha−" yr−" (Papke-Rothkamp, 1994;

Gasche, 1997). The total flux of gaseous N into the

soil might be considerably greater, since NH$

concentrations inside the canopy are significant and

the NH$

absorption capacity of the surface humus

layer is high due to acidic water films. The emission

of NO and N#O from the soil amounts to

7 kg N ha−" yr−" (Butterbach-Bahl et al., 1997, 1998;

Gasche, 1997). However, part of the NO emitted

might not be released from the ecosystem because it

is converted to NO#inside the canopy in the presence

of ozone. The NO#

produced in this conversion

reaction might be taken up by the canopy in

appreciable amounts. In addition, N#

produced

during denitrification may contribute considerably

to the release of gaseous N compounds from the soil.

At the ‘Ho$ glwald’ site this flux might be significant

owing to transient water saturation of the subsoil.

Preliminary investigations indicate an N#

emission

of c. 5 kg N ha−" yr−"(Butterbach & Papen, unpub-

lished). Mass-balance estimates indicate that c.

85 kg N ha−" yr−" are taken up by the roots of the

trees. From this estimate has to be subtracted

atmospheric N retained and assimilated in the

canopy. Similar estimates are obtained, when the

rates of N uptake measured with the depletion

technique at the field site are used for the calculations

(Geßler, unpublished). A major uncertainty in these

estimates is that changes in the storage of organic N

in the soil due to increased}decreased mineralization

cannot be excluded. As a consequence,

mineralization might exceed or might be lower than

the annual input of organic N by needle and root

litter. For example, the decay of lignoid compounds

can be retarded in the presence of high NH%

+ levels

that repress the activity of microbial lignolytic

enzymes (Berg & Matzner, 1997).

The N influx by bulk precipitation into the

‘Ho$ glwald’ ecosystem varies between 10 and

12 kg ha−" yr−", with a slight predominance of NH%

+.

In throughfall, the N influx has increased to more

than twice the bulk precipitation with a much

stronger predominance of NH%

+. In the humus layer,

the efflux of dissolved N by drainage exceeds the

influx by throughfall by c. 30 kg N ha−" yr−". There

is also an inverse change in the fluxes of NH%

+ and

NO$

− in the humus layer, indicating that NO$

−

production by nitrification exceeds its consumption

by denitrification, microbial immobilization, and

root uptake. This surplus nitrification varies between

2±2 and 4±0 kmol NO$

− ha−" yr−". In the soil layer

between 0 and 20 cm depth, significant changes in

the flux of NO$

− are generally not observed. But

there are significant differences between the years. In

warm summers enhanced nitrification can result

in a surplus nitrification in this soil layer of

4 kmol NO$

− ha−" yr−" (e.g. in 1992). In other years,

e.g. 1988 and 1994, NO$

− consumption prevails,

indicating denitrification, microbial immobilization

or root uptake of NO$

−. The subsequent soil layer

between 20 and 40 cm depth, is characterized by

persistent NO$

- consumption. This might result

mainly from denitrification, as transient water satu-

ration is rather frequent in this layer. However, it

also has to be considered that this layer is still rooted,

and uptake by the roots might contribute to NO$

−

consumption. In the soil layer between 40 and

175 cm depth, which is below the main rooting zone

of spruce, significant changes in N turnover are

usually not observed.

As an indication of the degree of N saturation of

the ecosystem, the amount and the time course of N

loss by leaching have to be considered. As shown in

Figure 4a, depletion of Ninorg

in the soil solution has

not been observed in any month of the growing

season during almost a decade despite some re-

duction of N fluxes in the soil during summer (Fig.

4b). The finding of high amounts of NO$

− in the

drainage water leaving the main rooting zone at

40 cm depth clearly demonstrates oversaturation of

Effects of high nitrogen loads on forests 79

4

3

2

1

0

90

60

30

0

1986 1987 1988 1989 1990 1991 1992 1993 1994

(b)

(a)

NO3–

NH4+

NO3–

NH4+

253

2·9

325

3·8

141

6·9

126

3·1

150

4·2

188

9·8

358

12·5

323

3·0

144

2·6

Flu

xes

(meq

m–2)

An

nu

al fl

ux

(meq

m–2)

Co

ncen

trati

on

(meq

l–1)

Figure 4. Concentrations and fluxes of inorganic nitrogen in the soil of a spruce plantation (‘Ho$ glwald’) at

40 cm depth. (a) Time course of NO$

− and NH%

+ concentrations. (b) Time course of NO$

− fluxes. In addition,

annual fluxes of NO$

− and NH%

+ (meq m−#) are given.

the ecosystem with N. This oversaturation could

affect ground water quality severely, since the NO$

−

concentrations found in the drainage water by far

exceed the drinking water threshold level of 25 mg l−"

or 0±4 meq l−".

-

Site and stand description

The results obtained in the spruce plantation at the

‘Ho$ glwald’ site that is exposed to high loads of N in

the atmosphere were compared with data collected in

a spruce forest with limiting N supply. For this

purpose field experiments were performed during

1994–1996 in a spruce stand (‘Villingen’ site) near

the city of Villingen (Baden-Wu$ rttemberg,

Germany) at 8° 22« E longitude and 48° 3« N latitude

in the Black Forest. The spruce stand at the

‘Villingen’ site is well characterized and located in

the experimental watershed areas of the ARINUS

project (Zo$ ttl, Feger & Brahmer, 1987; Feger, Zo$ ttl& Brahmer, 1988; Zo$ ttl, Brahmer & Feger, 1989).

Owing to the cold-air-hollow of the river Baar, the

climate at the ‘Villingen’ site is subcontinental

with a relatively high mean annual precipitation of

1200 mm and mean annual air temperatures of 6 °C.

Elevation of the experimental site is between 810 and

945 m above sea level. At the time of the present

study the stand at the ‘Villingen’ site was fully

stocked with 85–125-yr-old spruce trees and had a

closed canopy. The soil is a sandy-loamy acidic

brown earth with bedrock of Mesozoic quartz-rich

sandstone layers (Feger, 1992).

Nitrogen input from wet deposition into the soil

measured as throughfall is c. 10 kg N ha−" yr−" (Zo$ ttlet al., 1989; Feger, Brahmer & Zo$ ttl, 1993). Despite

high N contents of the soil, plant-available N is low,

most probably because of forest pasture and extreme

litter}wood utilization until the 19th century. Nitrate

and NH%

+ contents in the soil water were usually !3

and 5 µmol l−", respectively (Armbruster & Feger,

1998). These inorganic-N contents are more than

one order of magnitude lower than observed in the

soil water at the ‘Ho$ glwald’ site (Geßler et al.,

1998a). As a consequence of low plant-available N in

the soil and low atmospheric N input, the ‘Villingen’

spruce stand is N-limited and shows N starvation

(Feger, 1992; Feger, Brahmer & Zo$ ttl, 1992). In

order to get information about the impact of N

deposition on N uptake by the roots of spruce trees,

tree internal cycling of N compounds, and fluxes of

N#O between the soil and the atmosphere, N

deposition was simulated by application of a single

dosage of 150 kg N ha−" in the form of dry

(NH%)#SO

%in early May 1994, directly onto the

surface of the soil of one of the two experimental

watershed areas studied at this site. This N ap-

plication was equivalent to c. 15 times the annual N

input into the soil by throughfall (Zo$ ttl et al., 1989).

The treatment was the third N fertilization, since

(NH%)#SO

%had already been applied in 1988 and

1991 at the same dosage.

80 H. Rennenberg and others

10

5

0

30

20

10

0

July1994

Oct./Nov. Feb./Mar.1995

May July Oct./Nov. May1996

July Oct./Nov.

0

10

20

30

0

5

10

N2O

flu

x r

ate

(µg

N2O

-N m

–2 h

–1)

NH

4+-c

on

cen

trati

on

(µm

ol N

H4+ g

–1 S

DW

)

Figure 5. Mean N#O flux rates and NH

%

+ contents of the organic layer as affected by nitrogen fertilization of

a N-limited spruce forest (‘Villingen’ site). Nitrogen was provided by a single application of 150 kg N ha−" in

the form of (NH%)#SO

%in spring 1994. N

#O fluxes between the soil and the atmosphere were determined by

the closed-chamber technique with a mobile measuring system (Papen et al., 1993). The N#O flux rates given

are means calculated from individual fluxes obtained with 2-hourly resolution from eight closed chambers

installed at each site. Each measuring trial lasted for at least 14 d. Ammonium contents of the organic soil layer

represent means of three independent determinations and are based on soil d. wt (SDW). Bars on symbols

represent . +, mean N#O flux rates from soil of the untreated control site ; *, mean N

#O flux rates from soil

of the N-fertilized site ; E, NH%

+ contents of the organic layer of the untreated control site ; D, NH%

+ contents

in the organic layer of the (NH%)#SO

%fertilized site (data from: Daum, Ph.D. thesis, University of Freiburg,

in preparation).

Exchange of N#O with the atmosphere

During 1994–1996 nine field trials were conducted

for the determination of N#O-fluxes between the soil

and the atmosphere at the ‘Villingen’ sites. Flux

rates were determined using a mobile, fully

automated measuring system consisting of eight

closed chambers, automatic sampling devices for

sample air from the chambers’ atmospheres, and

automatic N#O-analysis by a gas chromatograph

equipped with an electron-capture detector (Papen

et al., 1993). Time resolution of the N#O-flux

measurements was 2 h. Measuring trials lasted for at

least 2 wk and were distributed over the growing

seasons.

Fluxes of N#O between the atmosphere and the

soil of the ‘Villingen’ control site were generally low

compared to the fluxes observed at the ‘Ho$ glwald’

site, and varied between emission and deposition

during the seasons (Fig. 5). Emission fluxes were

usually observed in summer and autumn, deposition

fluxes in winter and spring. Integrated over the

entire observation period (May 1994–October}November 1996) the soil of the control site

functioned as a net sink for atmospheric N#O

(®1±0³0.6 µg N#O®N m−# h−", i.e. equivalent to c.

0±1 kg N#O-N ha−" yr−"). Since net NO

$

− production

in the soil was marginal at the ‘Villingen’ control site

(Papen & Daum, unpublished), it appears that N-

limited forest soils contain a microbial population

which obviously is able to consume atmospheric

N#O. At present it is not clear whether the uptake of

atmospheric N#O into the soil is catalysed by a

denitrifier-population, which is able to use atmos-

pheric concentrations of N#O (310 ppbv) as an

electron acceptor instead of NO$

− under conditions

of NO$

− limitation, or whether the uptake of

atmospheric N#O is catalysed by soil microbial

processes not yet identified.

As a consequence of N application, NH%

+ contents

of the soil initially showed a dramatic increase, then

declined exponentially (Weber et al., 1995b ;

Armbruster & Feger, 1998). During the observation

period of the present study, NH%

+ contents of the soil

Effects of high nitrogen loads on forests 81

were usually higher at the N-fertilized than at the

control site (Fig. 5). Unfortunately, determination of

N#O fluxes started c. 2 months after N application,

when extremely high NH%

+ contents in the soil were

no longer observed. With two exceptions (May 1995

and July 1996) there was still a significant (P!0±01)

positive effect of N application on N#O-flux rates.

This effect was most pronounced until October

1994, i.e. until 6 months after N application (Fig. 5).

During this time the soil of the fertilized site was a

source of atmospheric N#O. The rate of N

#O

emission declined with increasing time after N

application and, hence, with decreasing NH%

+ con-

tents of the soil (Fig. 5). The positive effect of N

fertilization on N#O-flux rates could also be demon-

strated, though less pronounced, until October}November 1996, even when the soil of the N-

fertilized site became a sink for atmopheric N#O

(Fig. 5). At such times the soil of the N-fertilized site

was a significant (P!0±01) weaker sink for atmos-

pheric N#O than the control site. Integrated over the

entire observation period (May 1994–October}November 1996) the soil of the N-fertilized site was

a weak but significant (P!0±01) net source of

atmospheric N#O (0±9³0±9 µg N

#O-N m−# h−", i.e.

equivalent to c. 0±1 kg N#O-N ha−" yr−"). This result

demonstrates that N application to a N-limited

forest soil in the form of (NH%)#SO

%alters the

dynamics of N#O-production}consumption within

the soil and, as a consequence, can cause a change in

the direction of the N#O-flux between the soil and

the atmosphere from net deposition to net emission.

From these results it can be concluded that the high

rates of N#O emission from the soil of the ‘Ho$ glwald’

site are caused by the high loads of atmospheric N to

this forest.

Uptake of nitrogen by the roots

Uptake of inorganic N compounds from the soil by

the roots of spruce trees in the field was determined

at the ‘Villingen’ site by the modification of the

depletion technique (Rennenberg, Herschbach &

Polle, 1996; Geßler et al., 1998a) described by

Weber et al. (1995b). Despite low NO$

− and NH%

+

contents in the soil water of this N-limited field site,

both inorganic N compounds were taken up by

spruce trees during the entire growing season (Weber

& Rennenberg, 1998). The NH%

+ uptake rate was

one order of magnitude lower than observed for the

roots of spruce trees at the ‘Ho$ glwald’ site, which

corresponded to a one order of magnitude lower

NH%

+ content of the soil solution (Armbruster &

Feger, 1998; Geßler et al., 1998a). In contrast to the

‘Ho$ glwald’ site, where NO$

− was not taken up by

spruce roots despite high NO$

− contents in the soil

solution, the ratio of the NO$

− to NH%

+ uptake by

spruce roots at the ‘Villingen’ site reflected the ratio

of the NO$

− to NH%

+ content in the soil solution (c.

0±7). As at the ‘Ho$ glwald’ site, a seasonal pattern

with low rates of uptake in spring and autumn and

high rates of uptake in summer was observed at the

‘Villingen’ site (Weber & Rennenberg, 1998). Ap-

parently, inorganic N uptake at both sites is largely

dependent on soil temperature (Geßler et al., 1998a).

Total inorganic N uptake from the soil amounted to

30 –70 kg N ha−" yr−" at the ‘Villingen’ site (Weber

et al., 1996). This estimate is consistent with the

uptake rates reported by Feger (1993) from mass-

balance studies (c. 30 kg N ha−" yr−"). Fertilization

of the soil with (NH%)#SO

%caused a dramatic

inundation of the soil with NH%

+ (Armbruster &

Feger, 1998). Because NH%

+ uptake by spruce roots

shows a linear increase over a wide range of

increasing NH%

+ concentrations (Stoermer, unpub-

lished), fertilizer application also dramatically stimu-

lated NH%

+ uptake of the roots by up to two orders

of magnitude. With decreasing NH%

+ availability in

the soil, uptake of NH%

+ ceased and was no longer

observed 1 yr after fertilizer application for almost

an entire growing season. Because NH%

+ fertilization

also enhanced the NO$

− content of the soil water,

most probably as a consequence of nitrification

processes, NO$

− uptake by the roots was slightly

enhanced by (NH%)#SO

%fertilization (Weber et al.,

1996). However, with a delay of 1 yr NO$

− uptake

was also extinguished completely. From these obser-

vations it can be concluded that (NH%)#SO

%

fertilization initially caused a high rate of N uptake

by the roots that was used to fill up the storage pools

of the trees. Expansion of these storage pools might

have blocked further uptake of inorganic N by the

roots. In order to test these assumptions, TSNN

composition and content were analysed in different

sections of spruce trees at the fertilized site and the

unfertilized control site. A more detailed analysis

than at the ‘Ho$ glwald’ site was achieved by

collecting xylem sap and phloem exudates at

different height of the stems in addition to the root

and twig samples.

Internal nitrogen allocation in trees

As was observed at the ‘Ho$ glwald’ site, NO$-and

NH%

+ were found in the xylem sap of spruce trees at

the ‘Villingen’ site in trace amounts only, irres-

pective of (NH%)#SO

%fertilization (Weber et al.,

1996). Apparently, NO$

−and NH%

+ taken up by the

roots from the soil were assimilated almost com-

pletely in the roots (Fig. 6). In trees from the N-

limited control site transport of N in the xylem sap

from the roots to the twigs was dominantly Gln and

Asp (Weber et al., 1995b). The ratio between Gln

and Asp in the xylem sap changed with tree height.

With increasing height the Gln content in the xylem

sap increased while the Asp content decreased. This

finding can be explained by xylem-to-phloem

exchange, with preferential loading of Gln in the

82 H. Rennenberg and others

Protein

other AA

Arg Gln Asp Gln Asp

Needles

Gln Asp

Gln Asp

ArgGln Asp

ArgGln Asp

ArgGln Asp

ArgGln Asp

Arg Gln Asp

Gln other AA

Protein

Fine roots

Roots

Stem

Branches

Xylem

Phloem

NO3–

NO3–

NH4+

NH4+

Figure 6. Allocation of amino compounds in needles, fine

roots, xylem sap and phloem exudates of branches, stem

segments and roots of spruce trees at the ‘Villingen’ site.

Between 1993 and 1995 twelve field trials were performed

at the ‘Villingen’ site in order to characterize the nitrogen

budget of spruce trees. During each trial two trees were

felled at the (NH%)#SO

%treated and the untreated control

area. From each tree needle samples as well as xylem sap

and phloem exudates from twigs, two different stem

segments, and from roots with a diameter of up to 1±5 cm

were collected. In addition fine-root samples were col-

lected. In all samples TSNN (total soluble non-protein

nitrogen) composition and contents were determined (for

details see Schneider et al. (1996); Geßler et al. (1998b)).The predominant TSNN compounds in individual tree

sections and their possible allocation inside the tree are

shown.

phloem and preferential loading of Asp in the xylem

and}or by remobilization of Gln from storage tissues

and the use of Asp for growth processes in stem

tissues (e.g. of cambial cells). In root, but not in

stem, xylem sap, considerable amounts of Arg were

detected (Weber & Rennenberg, 1998). Apparently,

Arg was completely withdrawn from the xylem sap

during transport through the roots. From the

increased Arg contents in root compared with stem

phloem exudates it can be assumed that Arg

withdrawn from the xylem sap was reloaded into the

phloem and, hence, circulated inside the roots.

During spring and summer most of the Gln and

Asp transported in xylem from the roots to the twigs

might be used for protein synthesis. Under these

conditions Gln was not only the predominant N

compound in the xylem, but also in phloem. Part of

the Gln transported in xylem and phloem to current-

year sprouts might originate from remobilization of

N in older generations of needles. In autumn the

dominance changed from Gln to Arg in the phloem,

but not in the xylem (Weber et al., 1994, Weber &

Rennenberg, 1998). At this time of the growing

season N assimilated in the roots and transported to

the needles seems to be converted into this N storage

compound with a favourable C:N ratio of 4:3.

Phloem transport of Arg out of needles in autumn

might be required to refill N storage tissues in those

parts of the tree previously drained during growth

and development of the current-year sprouts and

roots. If xylem and phloem transport constitute part

of a cycling pool of organic N, metabolic inter-

conversion of amino compounds is required in the

roots and the needles in autumn, since in the xylem

Gln and Asp are the main forms transported,

whereas in the phloem Arg is predominant (Weber et

al., 1996). Similar conclusions were drawn from

experiments with young spruce trees exposed to

different levels of N nutrition under controlled

environmental conditions (Stoermer et al., 1997).

The increased uptake of NH%

+ from the soil

subsequent to (NH%)#SO

%fertilization caused an

immediate increase in TSNN contents in all sections

of the spruce trees analysed, i.e. needles, roots,

xylem and phloem of roots, different stem sections

and twigs. In root xylem the Arg and Gln content

increased, in stem and twig xylem the Gln content

increased. The Asp content in the xylem sap of these

tree segments were almost unaffected. In addition,

Gln content increased in current-year and the

previous year’s needles, and Arg content in phloem

exudates. However, with a delay of 1 yr TSNN

content started to decrease in twig and stem sections,

but still further increased in the root sections

analysed. This change in TSNN accumulation

coincided with a complete inhibition of the uptake of

NO$

− and NH%

+ by the roots. This finding is

consistent with the view that excess N in the soil can

initially be used to fill up storage pools of N, but

organic N accumulating in the roots might finally

prevent further N uptake by the roots. Laboratory

experiments were performed to identify the organic

N compound(s) in the roots responsible for the

down-regulation of NO$

− uptake.

External feeding of amino compounds

In the laboratory NO$

− uptake by the roots of young

spruce and beech trees was inhibited, when they

were exposed to various concentrations of NH%

+ at

constant NO$

− supply (Geßler et al., 1998a). At a

NO$

− to NH%

+ ratio of 4 this inhibition amounted to

66 and 75% of controls not exposed to NH%

+ for

Effects of high nitrogen loads on forests 83

spruce and beech, respectively. A further increase of

the NH%

+ supply did not enhance inhibition. This

result was surprising, since complete inhibition of

NO$

− uptake by the roots was observed with adult

spruce and beech trees at the ‘Ho$ glwald’ site exposed

to high loads of N (Geßler et al., 1998a) and also

with spruce trees of the N-limited ‘Villingen’ site

1 yr after (NH%)#SO

%fertilization (Weber &

Rennenberg, 1998). Since the roots of the fertilized

spruce trees at the ‘Villingen’ site were initially

exposed to NH%

+ concentrations in the soil orders of

magnitude higher than those used in the laboratory

experiments, yet inhibition of NO$

− uptake by the

roots was not observed (Weber & Rennenberg,

1998), a direct effect of NH%

+ on NO$

− uptake by the

roots can be excluded.

Ammonium taken up by the roots is rapidly

assimilated in the roots into organic amino com-

pounds supposed to be allocated in the trees.

Therefore, organic amino compounds were fed to

the roots of young spruce and beech trees in the

presence of NO$

− in order to study effects of the

applied compounds on NO$

− uptake by the roots. In

these experiments the organic amino compounds

chosen for feeding to the roots were those previously

found to be present in phloem exudates of adult

spruce and beech trees in the field (Schneider et al.,

1996; Geßler et al., 1998b ; Weber & Rennenberg,

1998). The phloem transport of these compounds is

thought to signal the N nutritional status of the shoot

to the roots. Inhibition of NO$

− uptake by the roots

of spruce trees was observed as a consequence of Gln

and Glu application, but not when Ala, Asp, or Arg

were fed. Aspartate and Gln reduced NO$

− uptake

by beech roots. Significant effects of Arg, Asn, or

Gaba on NO$

− uptake by beech roots were not

observed (Geßler et al., 1998a). From these results it

appears that Arg, the predominant organic amino

compound in root phloem exudates of spruce and,

during most of the growing season, also of beech

(Geßler et al., 1998b), is not involved in the

regulation of NO$

− uptake by the roots. Since the

organic amino compounds fed to the roots can be

rapidly metabolized into other amino compounds

inside the roots (Lee et al., 1992), the effects observed

as a consequence of external supply of amino

compounds to the roots cannot immediately be

attributed to the compounds fed. Therefore, the

composition and content of amino compounds were

analysed in roots fed with various amino compounds

for comparison with controls. In beech roots only

Asp and Gln were exclusively enriched in feeding

experiments that resulted in inhibition of NO$

−

uptake. Exclusive enrichment of one amino com-

pound in feeding experiments with spruce that

resulted in inhibition of NO$

− uptake was not

observed. Linear correlation analysis revealed

highest correlations between NO$

− uptake and Glu,

Gaba, Gln, and Asn contents of the roots. These

amino compounds that are all transported in the

phloem of adult beech and spruce trees in the field

(Schneider et al., 1996; Geßler et al., 1998b), might

be involved in regulating NO$

− uptake to the N

demands of the entire tree.

The rates of NO$

− uptake as measured by the

depletion technique describe a net flux that is the

balance between NO$

− influx and efflux. A decrease

of this net flux of NO$

− in the presence of NH%

+ or

organic amino compounds might be mediated by an

inhibition of NO$

− influx, a stimulation of NO$

−

efflux, or both (Aslam, Travis & Huffaker, 1994). To

address this question, "&N}"$N double labelling

experiments were performed with roots of young

beech trees (Kreuzwieser et al., 1997). Significant

differences between the NO$

− net flux and NO$

−

uptake were not observed at low NO$

− concen-

trations and, therefore, efflux was negligible under

these conditions. With increasing NO$

− supply the

efflux of NO$

− increased and amounted to 78% of

influx at 1 m NO$

− concentration in the treatment

solution. Both NH%

+ and Glu inhibited NO$

− influx,

but as a consequence of a reduced NO$

− influx, NO$

−

efflux also declined (Kreuzwieser et al., 1997). From

these results it appears that inhibition of NO$

− net

uptake of beech roots by NH%

+ or Glu is mediated

mainly by a reduced NO$

− influx rather than an

enhanced NO$

− efflux.

Interaction of atmospheric and pedospheric nitrogen

influx

If the assumption is correct that amino compounds

transported in the phloem from the shoot to the roots

act as a signal that controls NO$

− uptake by the roots,

atmospheric N taken up by the leaves should interact

with NO$

− uptake by the roots, since atmospheric N

is rapidly incorporated in amino compounds in the

leaves and allocated to the roots (Nussbaum et al.,

1993; Weber et al., 1995a). Such an interaction has

previously been reported for influx of atmospheric

sulphur into leaves, and sulphate uptake by the roots

(Herschbach, De Kok & Rennenberg, 1995a, b ;

Rennenberg et al., 1996). Therefore, the effect of

NO#

fumigation on N uptake of the roots was

analysed with spruce seedlings (Muller et al., 1996).

When supplied with NO$

− and NH%

+ as N sources,

NO$

− uptake by the roots was reduced by NO#

exposure to an extent that was equal to NO#-N influx

into the leaves. Thus, total N influx into the seedlings

via the shoot and the roots remained constant. With

seedlings exposed to N starvation, NO#exposure did

not affect NO$

− uptake and, hence, NO#

influx into

the shoot was used as an additional N source. These

findings support the idea that the N demand of the

shoot is signalled to the roots so that N uptake by the

roots matches the actual N demand of the entire tree.

In order to get a more detailed view on the

interaction between atmospheric N influx and N

84 H. Rennenberg and others

uptake by the roots, transport and accumulation of N

in various fractions of spruce seedlings was studied

in NO#-fumigated and non-fumigated plants (Muller

et al., 1996). From these investigations it appears

that root and shoot NO$

− and NH%

+ contents are

largely unaffected by NO#

influx into the leaves,

although both, NO$

− and NH%

+ uptake by the roots

declined. As a consequence of this decline, the

organic N pool in the roots slightly decreased. The

organic N pool of the shoot also declined despite an

enhanced flux of N into this pool from the NO#taken

up by the leaves. However, the most pronounced

effect of NO#

influx was found on transport of

organic N. Whereas xylem transport of organic N

from the roots to the shoot remained unaffected,

phloem transport of organic N from the shoot to the

roots increased by a factor of about four as a

consequence of NO#

uptake by the leaves. These

findings are consistant with the assumption that an

expansion of the organic N pool transported in the

phloem to the roots signals the N demand of the

shoot to the roots and mediates inhibition of NO$

−,

and most likely also of NH%

+ uptake by the roots of

plants that experience influx of atmospheric N into

the leaves.

The studies performed at the ‘Ho$ glwald’ site as well as the

laboratory experiments reported are part of an ecosystem

analysis financially supported by the Bundesminister fu$ rBildung, Wissenschaft, Forschung und Technologie

(BMBF). Investigations at the ‘Villingen’ site were carried

out in the frame of the ARINUS project (Auswirkungen

von Restabilisierungsmassnahmen und Immissionen auf

den N- und S-Haushalt der O$ ko- und Hydrospha$ re von

Schwarzwaldstandorten) financially supported by PEF

(Projekt Europa$ isches Forschungszentrum fu$ rMaßnahmen zur Luftreinhaltung, Karlsruhe, Germany)

and the German climate research program ‘Spuren-

kreisla$ ufe’ supported by the BMBF. The authors thank

M. Daum, Fraunhofer-Institut fu$ r Atmospha$ re Umwelt-

forschung, Garmisch-Partenkirchen and H. Stoermer,

Institut fu$ r Forstbotanik und Baumphysiologie,

Universita$ t Freiburg for providing unpublished data from

studies performed for their Ph.D. theses.

Armbruster M, Feger K-H. 1998. Zusammensetzung der

Bodenlo$ sung und Vera$ nderung durch Du$ ngung. In: Raspe S,

ed. Wasser- und Na$ hrelementhaushalt von Waldo$ kosystemen

im Schwarzwald. Ergebnisse interdisziplina$ rer O$ kosystem-

forschung in den ARINUS-Versuchsgebieten. Heidelberg:

ECOMED-Verlag. (In press.)

Aslam M, Travis RL, Huffaker RC. 1994. Stimulation of

nitrate and nitrite efflux by ammonium in barley (Hordeum

vulgare L.) seedlings. Plant Physiology 99 : 1124–1133.

Berg B, Matzner E. 1997. Effect of N deposition of plant litter

and soil organic matter in forest systems. Environmental Reviews

5 : 1–25.

Brumme R, Leimke U, Matzner E. 1992. Interception and

uptake of NH%

+ and NO$

− from wet deposition by above

ground parts of young beech (Fagus sylvatica L.) trees. Plant

and Soil 142 : 273–279.

Butterbach-Bahl K, Gasche R, Breuer L, Papen H. 1997

Fluxes of NO and N#O from temperate forest soils : impact of

forest type, N deposition and of liming on the NO and N#O

emissions. Nutrient Cycling in Agroecosystems 48 : 79–90.

Butterbach-Bahl K, Gasche R, Huber Ch, Kreutzer K, Papen

H. 1998. Impact of N-input by wet deposition on N-trace gas

fluxes and CH%

oxidation in spruce forest ecosystems of the

temperate zone in Europe. Atmospheric Environment}. 32 :

559–564.

Chaillou S, Rideout JW, Raper CD, Morot-Gaudry J-F. 1994.

Responses of soybean to ammonium and nitrate supplied in

combination to the whole root system or separately in a split

root system. Plant Physiology 90 : 259–268.

Cole DW, Rapp M. 1981. Element cycling in forest ecosystems.

In: Reichle DE, ed. Dynamic Proporties of Forest Ecosystems.

Cambridge, UK: Cambridge University Press, 341–409.

Crutzen PJ. 1979. The role of NO and NO#

in the chemistry of

the troposphere and stratosphere. Annual Review of Earth

Planetation Science 7 : 443–472.

Dickson RE. 1989. Carbon and N allocation in trees. In: Dreyer

E, ed. Forest Tree Physiology, Annales de Sciences Forestie[ res.Paris : Elsevier INRA 46 : 631–647.

Erisman JW, Heij GJ. 1991. Concentration and composition of

acidifying compounds. In: Heij GJ, Schneider T, eds. Acidi-

fication Research in The Netherlands. Final Report of the Dutch

Priority Programme on Acidification. Amsterdam, The

Netherlands: Elsevier, 37–96.

Fangmaier A, Hadwiger-Fangmeier A, van der Eerden L,

Ja$ ger H-J. 1994. Effects of atmospheric ammonia on veg-

etation–a review. Environmental Pollution 86: 43–82.

Feger K-H. 1992. Nitrogen cycling in two Norway spruce (Picea

abies) ecosystems and effects of a (NH%)#SO

%addition. Water,

Air and Soil Pollution 61 : 295–307.

Feger K-H. 1993. Bedeutung von oX kosysteminternen UmsaX tzen und

Nutzungseingriffen fuX r den Stoffhaushalt von Waldlandschaften.

Freiburger Bodenkundliche Abhandlungen, Bd. 31. University

of Freiburg, Germany.

Feger K-H, Zo$ ttl HW, Brahmer G. 1988. Projekt ARINUS: II.

Einrichtung der Meßstellen und Vorlaufphase. KfK}PEF

Berichte 35 : 27–38.

Feger K-H, Brahmer G, Zo$ ttl HW. 1992. Projekt ARINUS VI:

Stickstoffumsatz und Auswirkungen der experimentellen

Ammonsulfatgabe. KfK}PEF-Berichte 94 : 199–211.

Feger K-H, Brahmer G, Zo$ ttl HW. 1993. Projekt ARINUS

VII. Zwischenbilanz und Perspektiven. KfK}PEF-Berichte

104 : 23–40.

Finlay RD, Ek H, Odham G, So$ derstro$ m B. 1989. Uptake

translocation and assimilation of nitrogen from "&N labelled

ammonium and nitrate sources by internal ectomycorrhizal

systems of Fagus sylvatica infected with Paxillus involutus. New

Phytologist 113 : 47–55.

Flaig H, Mohr H. 1992. Assimilation of nitrate and ammonium

by Scots pine (Pinus sylvestris) seedlings under conditions of

high nitrogen supply. Physiologia Plantarum 84 : 568–576.

Gasche R. 1997. GanzjaX hrige Messungen zur Quantifizierung der

NO}NO#-FluX sse in einem Stickstoff uX bersaX ttigten WaldoX kosystem

(HoX glwald) und Identifizierung der an der N-Oxid-Emission

beteiligten mikrobiellen Prozesse. Ph.D. thesis, University of

Gießen, Germany.

Geßler A, Schneider S, von Sengbusch D, Weber P,

Hanemann U, Huber C, Rothe A, Kreutzer K,

Rennenberg H. 1998a. Field and laboratory experiments on

net uptake of nitrate and ammonium by the roots of spruce

(Picea abies) and beech (Fagus sylvatica) trees. New Phytologist

138 : 275–286.

Geßler A, Schneider S, Weber P, Hanemann U, Rennenberg

H. 1998b. Soluble N compounds in trees exposed to high loads

of N: a comparison between the roots of spruce (Picea abies) and

beech (Fagus sylvatica) trees grown under field conditions. New

Phytologist 138 : 385–399.

Gezelius K, Na$ shlom T. 1993. Free amino acids and protein in

Scots pine seedlings cultivated at different nutrient avail-

abilities. Tree Physiology 13 : 71–86.

Glass ADM, Siddiqi MY. 1995. Nitrogen absorption by plant

roots. In: Srivastava HS, Singh RP, eds. Nitrogen Nutrition in

Higher Plants. New Delhi : Associated Publishing, 21–56.

Effects of high nitrogen loads on forests 85

Glass ADM, Thompson RG, Bordeleau L. 1985. Regulation of

NO$

− influx in barley. Studies using "$NO$

−. Plant Physiology

77 : 379–381.

Go$ ttlein A, Kreutzer K. 1991. Der Standort Ho$ glwald im

Vergleich zu anderen o$ kologischen Fallstudien. In: Kreutzer

K, Go$ ttlein A, eds. O$ kosystemforschung HoX glwald : BeitraX ge zur

Auswirkung von saurer Beregnung und Kalkung in einem

Fichtenaltbestand. Hamburg, Germany: Paul Parey, 22–29.

Hermann H. 1995. Untersuchung der Stickoxidemissionen in

Abha$ ngigkeit von Stickstoffdu$ ngung und Kalkung auf zwei

verschiedenen Waldstandorten im su$ dlichen Schwarzwald.

Ph.D. thesis, University of Freiburg, Germany.

Herschbach C, De Kok LJ, Rennenberg H. 1995a. Net uptake

of sulfate and its transport to the shoot in spinach plants

fumigated with H#S or SO

#: does atmospheric sulfur affect the

interorgan regulation of sulfur nutrition? Botanica Acta 108 :

41–46.

Herschbach C, De Kok LJ, Rennenberg H. 1995b. Net uptake

of sulfate and its transport to the shoot in tobacco plants

fumigated with H#S or SO

#. Plant and Soil 175 : 75–84.

Huber C. 1997. Untersuchungen zur Ammoniakimmission und zum

Stoffhaushalt auf ungekalkten und neugekalkten FlaX chen in einem

stickstoffuX bersaX ttigten FichtenoX kosystem (HoX glwald). Ph.D.

Thesis, LM University of Munich, Germany.

Imsande J, Touraine B. 1994. N demand and the regulation of

nitrate uptake. Plant Physiology 105 : 3–7.

Johansson C. 1987. Pine forest : a negligible sink for atmospheric

NOx

in rural Sweden. Tellus 39B : 426–438.

King BJ, Siddiqi MY, Ruth TJ, Warner RL, Glass ADM. 1993.

Feedback regulation of nitrate influx in barley roots by nitrate,

nitrite and ammonium. Plant Physiology 102 : 1279–1286.

Kreutzer K. 1992. Forest response to a changing environ-

ment–Central and Northern European aspects. In: Teller A,

Mathy P, Jeffers JNR, eds. Responses of Forest Ecosystems to

Environmental Changes. Oxford: Alden Press, 279–297.

Kreutzer K. 1995. Effects of forest liming on soil processes. Plant

and Soil 168–169 : 447–470.

Kreutzer K, Go$ ttlein A. 1991. O$ kosystemforschung HoX glwald :

BeitraX ge zur Auswirkung von saurer Beregnung und Kalkung in

einem Fichtenaltbestand. Hamburg: Paul Parey.

Kreuzwieser J, Herschbach C, Stulen I, Wiersema P,

Vaalburg W, Rennenberg H. 1997. Interactions of NH%

+

with NO$

− transport processes of non-mycorrhizal Fagus

sylvatica roots. Journal of Experimental Botany 48 : 1431–1438.

Kronzucker HJ, Siddiqi MY, Glass ADM. 1996. Kinetics of

NH%

+ influx in spruce. Plant Physiology 110 : 773–779.

Lee RB, Drew MC. 1989. Rapid reversible inhibition of nitrate

influx in barley by ammonium. Journal of Experimental Botany

40 : 741–752.

Lee RB, Purves JV, Ratcliffe RG, Saker LR. 1992. Nitrogen

assimilation and nitrate absorption by maize roots. Journal of

Experimental Botany 43 : 1385–1396.

Logan JA. 1983. Nitrogen oxides in the troposphere: global and

regional budgets. Journal of Geophysical Research 88 :

10785–10807.

Marschner H, Ha$ ussling M, George E. 1991. Ammonium and

nitrate uptake rates and rhizosphere pH in non-mycorrhizal

roots of Norway spruce [Picea abies (L.) Karst.]. Trees 5 : 14–21.

Millard P. 1994. Measurement of the remobilization of nitrogen

for spring leaf growth under field conditions. Tree Physiology

14 : 1049–1054.

Millard P. 1996. Ecophysiology of internal cycling of nitrogen for

tree growth. Zeitschrift fuX r PflanzenernaX hrung und Bodenkunde

159 : 1–10.

Millard P, Proe MF. 1992. Storage and internal cycling of N in

relation to seasonal growth of sitka spruce. Tree Physiology 10 :

33–43.

Muller B, Touraine B, Rennenberg H. 1996. Interaction

between atmospheric and pedospheric nitrogen nutrition in

spruce (Picea abies L. Karst) seedlings. Plant, Cell and

Environment 19 : 345–355.

Na$ sholm T, Ericson A. 1989. Seasonal changes in amino acids,

protein, and total nitrogen in needles of fertilized Scots pine

trees. Tree Physiology 6 : 267–281.

Nambiar EKS, Fife DN. 1991. Nutrient retranslocation in

temperate conifers. Tree Physiology 9 :185–207.

Nussbaum S, von Ballmoos P, Gfeller H, Schlunegger UP,Fuhrer J, Rhodes D, Brunold C. 1993. Incorporation of

atmospheric "&NO#-nitrogen into free amino acids by Norway

spruce Picea abies (L.) Karst.. Oecologia 94 : 408–414.

Oaks A, Stulen I, Boesel IL. 1979. Influence of amino acids and

ammonium on nitrate reduction in corn seedlings. Canadian

Journal of Botany 57 : 1824–1829.

Papen H, Hellmann B, Papke H, Rennenberg H. 1993.Emission of N-oxides from acid irrigated and limed soils of a

coniferous forest in Bavaria. In: Oremland RS, ed. Biogeo-

chemistry of Global Change, Radiatively Active Trace Gases.

London: Chapman & Hall, 245–260.

Papke-Rothkamp H. 1994. Einfluß von saurer Beregnung und

kompensatorischer Kalkung auf die Emission gasfo$ rmiger

Stickstoffverbindungen aus Bo$ den eines Fichtenbestandes.

Schriftenreihe des Fraunhofer-Instituts fu$ r Atmospha$ rische

Umweltforschung, Bd. 25. Frankfurt}Main, Wissenschafts-

verlag Dr. Wigbert Maraun.

Pearson J, Stewart RG. 1993. The deposition of atmospheric

ammonia and its effects on plants. New Phytologist 125 :

283–305.

Pe! rez-Soba M, Stulen I, van der Eerden LJM. 1994. Effect of

atmospheric ammonia on the nitrogen metabolism of Scots pine

(Pinus sylvestris) needles. Physiologia Plantarum 90 : 629–636.

Plassard C, Scheromm P, Mousain D, Salsac L. 1991.Assimilation of mineral nitrogen and ion balance in the two

partners of ectomycorrhizal symbiosis : data and hypothesis.

Experientia 47 : 340–349.

Rennenberg H, Herschbach C, Polle A. 1996. Consequences

of air pollution on shoot–root interactions. Journal of Plant

Physiology 148 : 296–301.

Rennenberg H, Schneider S, Weber P. 1996. Analysis of

uptake and allocation of nitrogen and sulphur compounds by

trees in the field. Journal of Experimental Botany 47 : 1491–1498.

Ro$ hle H. 1991. Entwicklung der wichtigsten ertragskundlichen

Kenngro$ ßen des Fichtenaltbestandes im Ho$ glwald in der

6ja$ hrigen Beobachtungsperiode zwischen 1983–1988. In:

Kreutzer K, Go$ ttlein A, eds. O$ kosystemforschung HoX glwald :

BeitraX ge zur Auswirkung von saurer Beregnung und Kalkung in

einem Fichtenaltbestand. Hamburg, Germany: Paul Parey,

30–34.

Schneider S, Geßler A, Weber P, von Sengbusch D,Hanemann U, Rennenberg H. 1996. Soluble N compounds

in trees exposed to high loads of N: a comparison of spruce

(Picea abies) and beech (Fagus sylvatica) grown under field

conditions. New Phytologist 134 : 103–114.

Schrader LE, Domska D, Jung PE, Peterson LA. 1972. Uptake

of and assimilation of ammonium-N and nitrate-N and their

influence on the growth of corn (Zea mays L.). Agronomy

Journal 64 : 690–695.

Schulze E-D, Gebauer G, Katz C. 1991. Aufnahme, Abgabe

und Umsatz von Stickoxiden, NH%

+ und Nitrat bei

Waldba$ umen, insbesondere der Fichte (Teil 1),

Abschlußbericht. In: Projektgruppe Bayern zur Erforschung der

Wirksamkeit von Umweltschadstoffen (PBWU).

Smith FW, Thompson JF. 1971. Regulation of nitrate reductase

in excised barley roots. Plant Physiology 48 : 219–223.

Stoermer H, Seith B, Hanemann U, George E, RennenbergH. 1997. Nitrogen distribution in young Norway spruce (Picea

abies) trees as affected by pedospheric nitrogen supply.

Physiologia Plantarum 101 : 764–769.

Weber P, Nußbaum S, Fuhrer J, Gfeller H, Schlunegger UP,Brunold C, Rennenberg H. 1995a. Uptake of atmospheric

"&NO#

and its incorporation into free amino acids in wheat

(Triticum aestivum). Physiologia Plantarum 94 : 71–77.

Weber P, Rennenberg H. 1998. Charakterisierung des N-

Haushalts der Fichte. In: Raspe S, ed. Wasser- und

Na$ hrelementhaushalt von Waldo$ kosystemen im Schwarzwald.

Ergebnisse interdisziplina$ rer O$ kosystemforschung in den

ARINUS-Versuchsgebieten. Heidelberg, Germany:

ECOMED-Verlag. (In press.)

Weber P, Schneider S, Gu$ lpen M, Stoermer H, Geßler A,von Sengbusch D, Hanemann U, Rennenberg H. 1996.Charakterisierung des Stickstoffhaushalts der Fichten auf den

ARINUS-Fla$ chen. III. Einfluß von Ammoniumsulfat-

Du$ ngung auf N-Aufnahme und N-Allokation. KfK}PEF

Berichte 142 : 109–124.

Weber P, Schneider S, Stoermer H, Geßler A, von

86 H. Rennenberg and others

Sengbusch D, Hanemann U, Rennenberg H. 1994.Charakterisierung des Stickstoffhaushalts der Fichten auf den

ARINUS-Fla$ chen. 1. Zwischenbericht. KfK}PEF Berichte

118 : 87–100.

Weber P, Schneider S, Stoermer H, Geßler A, vonSengbusch D, Hanemann U, Rennenberg H. 1995b.

Charakterisierung des Stickstoffhaushalts der Fichten auf den

ARINUS-Fla$ chen. 2. Zwischenbericht. KfK}PEF Berichte

130 : 87–104.

Wellburn AR. 1990. Why are atmospheric oxides of nitrogen

usually phytotoxic and not alternative fertilizers? New Phy-

tologist 115 : 395–429.

Zo$ ttl HW, Feger K-H, Brahmer G. 1987. Projekt ARINUS: I.

Zielsetzung und Ausgangslage. KfK}PEF Berichte 12 :

269–281.

Zo$ ttl HW, Brahmer G, Feger K-H. 1989. Projekt ARINUS.

III. Stoffbilanzen und Du$ ngung der Einzugsgebiete. KfK}PEF-Berichte 50 : 23–34.