Trees (1991) 5:14-21
9 Springer-Verlag1991
Ammonium and nitrate uptake rates and rhizosphere pH in
non-mycorrhizal roots of Norway spruce [Picea abies (L.) Karst.]
Horst Marschner, Max H~iussling, and Eckhard George
Institute of Plant Nutrition, Universityof Hohenheim,P. O. Box 700562, 7000 Stuttgart 70, FederalRepublicof Germany
Received August 21, 1990/AcceptedOctober25, 1990
Summary. Relationships between root zone temperature,
concentrations and uptake rates of NH~ and NO~ were
studied in non-mycorrhizal roots of 4-year-old Norway
spruce under controlled environmental conditions. Additionally, in a forest stand NH~ and NO3 uptake rates along
the root axis and changes in the rhizosphere pH were
measured. In the concentration (Cmin) range of 100150 g M uptake rates of NH~ were 3 - 4 times higher than
those of NO~ The preference for NH2 uptake was also
reflected in the minimum concentration (Cmin) values.
Supplying NH4NO3, the rate of NO~ uptake was very low
until the NH~ concentrations had fallen below about
100 gM. The shift from NH~ to NO~ uptake was correlated
with a corresponding shift from net H + production to net
H + consumption in the external solution. The uptake rates
of NH~ were correlated with equimolar net production of
H+. With NO, nutrition net consumption of H § was approximately twice as high as uptake rates of NO~ In the forest
stand the NO3 concentration in the soil solution was more
than 10 times higher than the NH~ concentration
(<100 gM), and the rhizosphere pH of non-mycorrhizal
roots considerably higher than the bulk soil pH. The rhizosphere pH increase was particularly evident in apical root
zones where the rates of water and NO~ uptake and nitrate
reductase activity were also higher. The results are summarized in a model of water and nutrient transport to, and
uptake by, non-mycorrhizal roots of Norway spruce in a
forest stand. Model calculations indicate that delivery to
the roots by mass flow may meet most of the plant demand
of nitrogen and calcium, and that non-mycorrhizal root tips
have the potential to take up most of the delivered nitrate
and calcium.
Key words: A m m o n i u m - C a l c i u m - Nitrate - Picea abies
- Rhizosphere pH
Offprint requests to: H. Marschner
Introduction
In many Norway spruce forest locations in Central Europe
large amounts of nitrogen are deposited as ammonium,
nitrate or nitrogen oxides. Nitrogen deposition is considered an important contributor to soil acidification and forest decline (Schulze 1989). In acid mineral soils roots are
exposed to an environment which induces various stresses
on root growth and metabolism (Marschner 1989). However, for solubility and toxicity of mineral elements like
aluminium and for availability and uptake of mineral
nutrients, the pH in the rhizosphere soil and at the rhizoplane (root surface) is more important than the bulk soil
pH. The rhizosphere and rhizoplane pH may differ from
the bulk soil pH by up to 2 units (Marschner and R6mheld
1983; Marschner et al. 1986). For a number of species,
including coniferous trees, it has been shown that the form
of nitrogen taken up (NO3 versus NH~) has the most prominent influence on rhizosphere pH (H~iussling et al. 1985;
Rollwagen and Zasoski 1988; Gijsman 1990a).
In nutrient solutions, NO~ and NH~ uptake rates of
coniferous tree roots differ considerably and depend on
both concentrations and proportions of the nitrogen forms
supplied. At similar external concentrations the uptake
rates of NH~ are usually considerably higher than of NO5
(Boxman and Roelofs 1988; Scheromm and Plassard
1988). Similarly to annual species (Lee and Drew 1989), in
coniferous trees uptake rates of NO5 are severely depressed
by high NH~ concentrations (Scheromm and Plassard
1988). Despite low NO3 uptake rates, in coniferous trees
nitrate reductase activity in roots is high, particularly in
apical zones (Peuke 1987), and most of the NO5 is reduced
in the roots (Scheromm and Plassard 1988). In Norway
spruce, along the root axis variations also occur in rates of
uptake of nutrients (e. g. Ca, K and Mg) and water (H~ussling et al. 1988) as well as in the pH of the rhizosphere soil
and at the rhizoplane when different forms of nitrogen are
supplied (H~ussling et al. 1985; Marschner et al. 1986).
Average values of the bulk soil pH or the rhizosphere pH
therefore provide inadequate information to assess the impact of soil acidification and nitrogen source on tree root
15
g r o w t h and p h y s i o l o g y . F u r t h e r m o r e , e n h a n c e d n i t r i f i c a tion at h i g h e r soil t e m p e r a t u r e s and the g r e a t e r t e m p e r a t u r e
d e p e n d e n c e o f N O ~ as c o m p a r e d to N H ~ u p t a k e ( C l a r k s o n
et al. 1986; T a c h i b a n a 1987) m a y l e a d to a t y p i c a l i n c r e a s e
in r h i z o s p h e r e p H o f c o n i f e r o u s trees d u r i n g the g r o w t h
period.
In the p r e s e n t s t u d y the r e l a t i o n s h i p b e t w e e n c o n c e n trations and u p t a k e rates o f N H ~ and N O 3 and c o r r e s p o n d i n g c h a n g e s in r h i z o s p h e r e p H w e r e s t u d i e d in N o r w a y
s p r u c e w i t h 4 - y e a r - o l d s e e d l i n g s u n d e r c o n t r o l l e d env i r o n m e n t a l c o n d i t i o n s and at d i f f e r e n t r o o t z o n e t e m p e r a tures, and w i t h 6 0 - y e a r - o l d trees in a f o r e s t stand.
Materials and m e t h o d s
Growth chamber experiments. Studies on the uptake rates of NH2 and
NO5 as affected by the external concentration and root temperature were
carried out in a growth chamber under controlled environmental conditions (day/night 16/8 h; light intensity 130 W m -2, Osram Power Star
QI-T lamps; relative humidity 80/90%; room temperature 20~ ~C; root
zone temperature varied between 5 ~ and 20 ~C) using 4-year-old Norway
spruce [Piceaabies (L.) Karst.] trees. Prior to the uptake experiments the
trees were grown in a greenhouse in plastic boxes filled with the subsoil
(C-horizon) of a gleyic Cambisol covered by a 5 cm humus layer from a
Norway spruce stand. Vigorous growth of non-mycorrhizal long roots
was observed (Hfiussling et al. 1988).
For the studies on temperature effects on NH~ and NO5 uptake, the
trees were preincubated in the growth chamber for 5 days at different root
temperatures by placing the plastic boxes into temperature-controlled
chambers. Thereafter, intact individual long roots were carefully recovered from the substrate without separating them from the plant, washed
free of soil, and each placed in a Plexiglas container (H~iussling et al.
1988) filled with 70 ml nutrient or soil solution. For adaptation, the
individual long roots in the containers were pretreated for 12 h in nutrient
solution (pH 3.8) of the following composition (p.M): NH4, 150; K, 250;
Ca, 300; Mg, 150; NO3, 150; C1, 800; SO4, 450; PO4, 50; Mn, 3; Fe, 5;
Cu, 0.3; Zn, 0.3; Mo, 0.03; B, 10. Ion uptake for each root was determined over a 16-day period as calculated from the decrease in ion
concentration and volume of the nutrient solution. Samples from the
nutrient solution were taken and analysed at regular intervals, and after
10 days the solution was replenished with adequate amounts of water and
nutrients. Control containers without roots were kept to quantify evaporation losses and to check whether nitrification takes place.
For studies on the interactions between concentrations of NH] and
NO~, uptake rates and external pH the nutrient solution was modified by
addition of Ca(NO3)2 to give NO~ concentrations of 100, 500, and
1000 p-M. Calcium concentrations were kept at constant level by adjustment with CaSO4. In this experiment, relative humidity was kept between 90% and 100%. Ion uptake was determined over a 7 day period.
The solutions were analysed for NH~, NO3 and total nitrogen with an
autoanalyser (Technicon II autoanalyser), and pH measured with micro
glass combination electrodes (Metrohm).
During the uptake experiments (16 and 7 days) the root length
increased depending on temperature (5 ~C, 0.2 mm day-~; 10~C, 0.3 mm
day-I; 15~C, 0.8 mm day-l; 20 ~C, 1.1 mm day -l) or NO3 concentrations
(100 p-M, 2.3 mm day-l; 500 p.M, 1.1 mm day-I; 1000 p.M, 0.8 mm
day-I). For calculations of uptake rates, constant linear root growth rates
during the experiment were assumed.
Field experiments. Experiments were carried out in a 60-year-old Norway spruce stand at Mauzenberg, Baden-W/irttemberg (Evers et al.
1986). The soil is a podzol with a moder/mor humus. At regular intervals
during the 1986 vegetation period samples of soil solutions were collected by suction cups (p 80 ceramic; Staatliche Porzellanmanufaktur,
Berlin) at a depth of 5 - 1 0 cm below the soil surface (Ahe-horizon).
For the experiment on ion uptake rates along the root axis, intact
individual long roots of trees showing slight symptoms of forest decline
were recovered from the Oh-Ahe layers, gently washed with distilled
water and placed in Plexiglas containers with compartments, each 2.5 cm
in length. The compartments were sealed with silicon putty, and the roots
pretreated with nutrient solution for 12 h. The pH and nitrogen concentrations in the nutrient solution were adjusted m those in the soil solution
collected at this forest site. The nutrient solution (pH 4.0) had the following composition (gM): NH4, 100; K, 600; Ca, 500; Mg, 150; NO3, 1000;
C1, 800; SO4, 200; PO4, 50; Mn, 3; Fe, 5; Cu, 0.3; Zn, 0.3; Mo, 0.03; B,
10. The experiment was conducted for 108 h. The average air (under
canopy) temperature was 9 ~ ranging from 11 ~C (day) to 6 ~C (night).
The temperature of the nutrient solution was about 8 ~C.
In October, additional long roots were recovered from the humus
layer, cut into segments of 2 cm each (root tip 1 cm), immediately frozen
in liquid nitrogen, and stored at -70 ~C. In these root segments nitrate
reductase activity was determined according to Peuke (1987).
For observations of the root growth at this forest site and for measurement of the pH in the rhizosphere soil and at the rhizoplane, inclined
soil profiles were installed in 1984 at a distance of 1.5 m from the trees.
The soil surface was tightly sealed by a Plexiglas lid, and the profile
protected by an insulating cover. In 1985, new roots grew along the
Plexiglas lid, which was removed at different times during the vegetation
period for pH measurements along intact roots using antimony microelectrodes (H~iussling et al. 1985).
Soil samples were collected at the end of the vegetation period
(October) at different distances from the root surface [rhizoplane soil
(0-1 ram), rhizosphere soil ( 1 - 2 mm), and bulk soil (>2 ram)] and in
different zones along the root axis. Water extractable ions were determined by shaking 1 g soil with 50 ml distilled water for 1 h (Van der
Paauw 1971), and pHH2o was measured in a 1 : 2.5 soil water suspension.
Results
G t v w t h chamber experiments
U p t a k e rates o f N H ~ b y n o n - m y c o r r h i z a l roots, i n d i c a t e d
b y the d e p l e t i o n in the n u t r i e n t s o l u t i o n , i n c r e a s e d w i t h
t e m p e r a t u r e (Fig. 1). T h e m i n i m u m c o n c e n t r a t i o n s o f N H ~
w h e r e net u p t a k e c e a s e s (Cmin) w e r e d i s t i n c t l y l o w e r at
h i g h t e m p e r a t u r e s . Initial u p t a k e rates o f N O 3 w e r e l o w ,
a n d m u c h o f the N O ~ u p t a k e d u r i n g this p e r i o d
(30%-90%)
can be explained by transpiration-driven
m a s s f l o w o f n u t r i e n t s o l u t i o n into t h e roots. O n s e t o f
h i g h e r u p t a k e rates o f NO~ w a s d e l a y e d b y 5 0 - 1 5 0 h until
Table 1. Relationship between concentrations and uptake rates of NIL]
and NO~ supplied simultaneously to non-mycorrhizal roots of 4-year-old
Norway spruce at 20~ (day)~16~C (night)
Nitrate
concentration
(gM)
Ammonium concentration range (p-M)
150-100
80-40
<10
Ammonium uptake rate (mol 10-14 mm -2 s-1)
100
6.9
4.4
500
7.5
3.1
1000
5.3
5.6
SE
0.8
0.6
0.01
0.09
0.03
0.08
Nitrate uptake rate (mol 10 14 ram-2 s-t)
100
-0.3
1.9
500
0.03
1.4
1000
0.2
2.2
SE
0.1
0.5
1.7
1.7
2.8
0.4
Uptake rates were calculated for defined time intervals during NH~
depletion, and expressed per unit root surface area; three replicates
16
&mmonium depletion]
INitrate depletion I
IB0
160"
;x..x
140"
~,,•
t~0
x"x
~'~ ,oo.
I \ '~ " '
120-
+---+
15
~,~ t00,
\'~~\ 'x .,
d
E 80.
~i!~
(2
'~ ~0 \ !
i
<
2(3
x
~ i
Z
x
EE 4o \ i i
"x
Cmi n
o,
:
:
~o
~oo
'ko.o.o.o.o_o2.5
=~r~,
~so
2oo
2so
30o
35o
X-x.
C rain
~i'i3., ..0 Xx..x 54
3O
22
20.
~.,5 ,,
_.~.~,
"~
60.
40-
9
~\ '8-a-~
X "X.
I
450
~oo
0
1
0
I
I
I
[
I
~
I "-----!
t00 150 200 250 300 3 O 400 450
Time [h)
Time (h)
Fig. 1. Time-course of NH~ and NO3 uptake by
long roots of 4-year-old Norway spruce supplied
with 150 gMNH4NO3 at different root zone
temperatures. Uptake measured by depletion of
the external solution. Three replicates
4-4.0-
-~
v
+5.0-
E
+2.0-
\
g '7
0.0
~
-I .0-
.L+T
g
O
g
Root zone temperature
+1.o.
o
57,
Table 2. Influence of root zone temperature on the maximal rates of NI-I~
and NO~ uptake and of H + production and H§ consumption in non-mycorrhizal roots
O
-2,0-
~-~.o.
--
NH~ uptake
H + production
20
15
10
5
SE
6.0
5.6
3.0
2.5
0.9
6.7
5.3
3.4
2.6
1.0
NO3 uptake
1.4
0.7
0.6
0.5
0.3
H+ consumption
20
15
10
5
SE
--4.0--5.0-
12.
-6.0
0
Rates (mol 10-~4 mm -2 s I)
(~
20
40
60
80
100
120 140
160
Time (h)
2.3
1.4
1.2
1.0
0.4
Fig. 2. Time course of H + production (+) and consumption (-) by the
roots of Norway spruce supplied with 150 gMNH~] and different concentrations of NO5 (100, 500, and 1000 gM) at 20~ (day)/16~ (night).
Three replicates per treatment, only averages across NO3-concentrations
given. Vertical bars, SD
Uptake rates based on the concentration ranges of 150- 100 gMNHa and
150- 100 gMNO~ + <10 gMNH.; (see Fig. 1); three replicates
the NH~ concentration in the nutrient solution had declined
below about 100 g M (Fig. 1). Thereafter the uptake rates
of NO5 were similarly temperature dependent as the NH~
uptake rates, but lower and ceased at much higher Cmin
values.
The uptake rates of NH~ were not affected by increasing the NO5 concentrations from 100 to 1000 g M
(Table 1). On the other hand, the inhibition of NO3 uptake
by NH~ was still effective even at NO5 concentrations of
1000 gM. Irrespective of the external NO3 concentration
net uptake of NO3 was very low until the NH~ concentration had declined to below 100 g M (Table 1). Even after
NH~ depletion the net uptake rate of NO5 was much lower
than of NH~ before depletion. In this and the other experiments in the control containers (without roots) nitrification
was absent and organic nitrogen did not accumulate, indicating that microbial nitrogen transformation was low in
these solutions.
The shift from preferential NH~ uptake to NO~ uptake
during the uptake experiments with solutions containing
both NH2 and NO, is reflected in the time-course of proton
production and consumption in the external solution
(Fig. 2). During the first 80 h nitrogen uptake is confined to
NH~ with corresponding high H+ net production (pH
decrease). As NH4 depletion proceeds, the net uptake of
NO~ increases, and this change is correlated with a shift
17
2.5
70.4T!
/pH
rhizoplane
.x
oc
J Water uptake]
9/
0,1
'm,. . . . . . . . . ~" . . . . . . . . . . ~m. . . . .
/abundant
, "......... ~. . . . . . . . . r----m ..... i--'.'_"_1~f e w
J,~ o.o/
I
~
:: "
."'~
/ p H b:]k soil
"'0
"~
~1.5
iO- 2.8
2.6
._c 1.0/~
......... ~ ' , ,
.m.
I
./ "
E 20 } m """--m-.,.- ...... ~ .......... .m"
.... e - ' " " ' "
-~
10
v~
o
2.4
d
''m--abundant
~'~few
am
0.5-
2.0
. . . . . . "I . . . . . . . . . . 9 . . . . . .
,
,
,
~
,
_AI
,
5o
I NLup take I
1.5,86
12-
~'E
~o.
oO
64-
2E
-3.2
-3.0
N i t r a t e r e d u c t a s e a c t i v i t y (NRA)I
4-o
pH
15- -3.4
-od "
~0
-3.6
~,
o Nitrate
',,
* Ammonium
',,
,abundant
',
/,few
b-..,::.-:::-:o . . . . . . . . . . . o . . . . . . . . : 8 ~ / a b u n d a n t
~-.._.~.-'~
" ~ : ~ f e w
0
I
i
2
I
4
Distance
I
6
from
i~2
110
8
the a p e x ( c m )
Fig. 3. Water uptake rates, nitrate reductase activity, and NH,] and NOs
uptake rates along individual roots of 60-year-old Norway spruce (location Mauzenberg) with abundant laterals ( 4 - 6 per cm root length) or few
laterals ( 0 - 2 per cm root length). Average uptake rates from a nutrient
solution with 100 p M NH~ and 1000 p~M NO3 without replenishment of
nutrient solution during an uptake period (108 h). Eight replicates
ioo
2.7.86
~5o
2oo days
10.9.85
Fig. 5. Time-course of the bulk soil solution concentrations of NH~, NO3
and AI, of the pH in the bulk soil and at the rhizoplane, and of the soil
temperature during the vegetation period 1986 (location Mauzenberg).
Measurements at 10 cm soil depth (Abe horizon)
from H + production to H + consumption, i.e. pH increase in
the nutrient solution.
At the different root zone temperatures the net uptake
of NH~ was correlated with equimolar net production of H+
(Table 2). In contrast, the net uptake of NO~ was lower by
a factor of about 2 than the corresponding net consumption
of H+.
Field experiments
*- - -*
+- - -+
o
o
Rhizoplane soil
Rhizosphere soil
Bulk soil
22.7.B6
Mycorrhiza]
root t i p s
3.4 ~
3.2 4- *............. ~-............ ~ ......
q}
~.o
2.8
2.6
2
E~
C
O
-6
"r
EL
o
o
i
i
o
i
3.2 T
i
..... ~ .............
i
i
i
i
14.9.86
3.0 t * ............. ~+......
2.8 ~............. ~ . . . . . . . . . . . ~........
2.6
,
,
,
,
,
,
~..........
,
,
0
,
3"4
5.1t.86-w...... ....... w.............
3.2 t w............. ~ ............
3.0
2.8 o............. ~ ............ ~ . . . . . . . . . . ~ ...........
2.6
,
,
,
,
,
,
,
,
0
1
2
3
4
5
6
7
B
Distance from the apex (cm)
,
g
Fig. 4. pH at the rhizoplane and in the rhizosphere soil along individual
long roots of 60-year-old Norway spruce, and bulk soil pH at different
times during the vegetation period 1986 (location Mauzenberg). Five
roots each of six trees were measured, pH measured with antimony
microelectrodes
The uptake rates of water, NO~ and NH~ as well as nitrate
reductase activity (NRA) vary along the root axis (Fig. 3).
The uptake rates of NO~ and water in particular are much
higher in apical than in basal root zones. In basal zones the
uptake rates depend upon the abundance of lateral roots.
The pattern of NH~ uptake along the root axis is similar to
that of the NO3 uptake, except of a much less distinct peak
in the apical root zone (Fig. 3). In the soil solution at this
forest site the NO3 concentration was 10 times higher
(1000 gM) than the NH~ concentration (100 gM). Corresponding NO5 and NH~ concentrations were used in the
nutrient solution. The uptake rates of NO~ were higher than
of NH~ In this experiment, the inhibitory effect of NH~ on
NO3 uptake (Fig. 1, Table 2) was masked by depletion of
NH~ in the nutrient solution during the course of the
measurements, since nutrient solutions were not replenished in this case.
Along the root axis the NRA is higher in the apical
zone and rises again in basal zones, particularly when
abundant lateral roots are present (Fig. 3).
During the growing season the pH of rhizosphere soil,
and particularly the pH at the rhizoplane of soil-grown
roots at this forest site was considerably higher than the
bulk soil pH (Fig. 4). These differences were more distinct
in apical root zones and were maintained despite a drop in
18
~- - -~
§ - -+
c
o
(n
0,4
w. . . . . . .
0.2
-~
Rhizoplane so~l
RhJzosphere soil
Bulk soil
0.1
1
r
i
J
i
i
-'d
~'~I
~:2
0
0
0
0.0
I~
42t
4.0
0
+.
"~w. . . . . . . . . . . . . . . . . . . .
i
i
i
1
E
3
i
4
i
5
i
B
Distance from the apex (cm)
Fig. 6. Water extractable NI~ and NO5 and pH (H20) of different soil
fractions along the root axis of 60-year-old Norway spruce at the end
of the vegetation period 1986 (location Mauzenberg). Rhizoplane soil
= 0 - 1 ram; rhizosphere soil = 1 - 2 mm from the root surface. Four
replicates
bulk soil pH between July 22 and September 14. The pH at
mycorrhizal root tips (lateral roots) was similar to that of
the apical zone of the non-mycorrhizal long roots (Fig. 4).
The changes in bulk soil pH and rhizosphere pH (not
shown) during the growing season were closely related to
changes in the NO~ concentration of the soil solution
(Fig. 5). With an increase in soil temperature nitrification
was enhanced and NO~ concentrations increased and thus
also NO~ supply to the roots. As the NH88concentrations in
the soil solution remained at a very low level (<100 gM)
uptake of NO~ dominated, with a corresponding increase in
rhizoplane pH. Concentrations of A1 in the soil solution
were slightly increased in summer when the bulk soil pH
fell below 3.
Mechanical separation of rhizoplane and rhizosphere
soil along the axis of long roots confirmed the existence of
pH gradients both in a radial direction and along the root
axis (Fig. 6). Compared to the bulk soil the concentration
of water-extractable NO3 was lower in the rhizoplane and
rhizosphere soil, except in rhizosphere soil of apical root
zones. Depletion of NH~ was confined to the rhizoplane
soil, reflecting the low mobility of NH~ and the restricted
depletion zone around the roots. The relatively high concentrations of water extractable NH~]compared to the low
concentrations of NH~ in the soil solution (Fig. 5) are presumably caused by both the contribution of exchangeable
NH~] and destruction 9 of soil
aggregates. during extraction
.
+
leading to an overestlmatton of the available N H ,
Discussion
The preferential uptake of NH~ as compared to NO3 by
non-mycorrhizal roots of Norway spruce (Fig. 1, Table 1)
confirms previous results of studies with other coniferous
tree species (Scheromm and Plassard 1988; Boxman and
Roelofs 1988). Higher Cmin values for NO~ as compared to
NH~ (Fig. 1) may reflect either a lower capacity of the roots
to metabolize NO3 or a lower affinity of the binding sites in
the plasma membrane of root cells for NO~ However, as
only net uptake rates have been determined, the observed
lower uptake rates of NO3 could also be the result of higher
efflux rates of NO3 compared to NH~ Relatively high
efflux rates of NO~ from roots have been demonstrated in
pine (Martin et al. 1981) and barley (Lee and Clarkson
1986).
T h e Cmin values of 22-54 g M for NO3 in Norway
spruce (Fig. 1) are high as compared to 1-10 gMfound in
annual species (Deane-Drummond and Chaffey 1985). In
Norway spruce in the temperature range of 15-20~ the
Cmin values of NH~ were more than 10 times lower than
those of NO3, at lower temperatures these differences became smaller (Fig. 1). This demonstrates that Cmin values
are not fixed characteristics of a given plant species but
depend on root activity (Fig. 1) and demand, i.e. the
nutritional status of the plants (Drew et al. 1984). A much
higher temperature dependence of NO~ than of NH~ uptake
as in annual species (Clarkson et al. 1986; Tachibana 1987)
has also been found in Norway spruce, however, only in
the temperature range between 20 ~C and 15~C (Table 2).
When NH~ was present in the external solution at concentrations of about 100 g M and higher, net uptake of NO~
was very low, even at NO~ concentrations up to 1000 g M
(Table 1). Strong inhibition of NO~ uptake by NH~ is a
well-known phenomenon caused either by a decrease in
NRA (Peuke 1987), NH~ induced NO~ effiux (DeaneDrummond and Glass 1983) or direct inhibition of NO3
influx (Lee and Drew 1989), probably via the inducible
carrier system for NO3 uptake (Siddiqi et al. 1989).
The nitrogen form (NH~ or NO~) has a prominent influence on the cation-anion balance in plants (Van Beusichem
et al. 1985) and the net production or net consumption of
H + of roots (Table 2), accounting for a corresponding
decrease or increase in substrate pH (Fig. 2). Uptake of
nitrogen as NH] decreases and uptake of NO3 increases the
pH of rhizosphere soil not only in annual species but also
in coniferous trees such as Norway spruce (Marschner et
al. 1986; Hfiussling et al. 1985; Figs. 2, 5), Scots pine
(Boxman and Roelofs 1988) and Douglas fir (Gijsman
1990a). This pH increase in the rhizosphere of NO3-fed
plants is in part caused by H+/NO5 cotransport (1/1 or >1/1)
at the plasma membrane (McClure et al. 1990). The pH
increase is further accentuated in plant species which preferentially reduce NO3 in the roots such as pine (Martin et
al. 1981; Scheromm and Plassard 1988) or Norway spruce
(Peuke 1987). Accordingly, the net consumption of H + by
roots of Norway spruce is considerably higher than the
corresponding net uptake rates of NO~ (Table 2).
In most well-aerated agricultural soils NO3 is the dominant nitrogen species in the soil solution. In forest soils, in
the soil solution of the humus layer the NH~ concentrations
may be as high as, or even higher than, the NO, concentrations (Meyer et al. 1988; Marschner et al. 1989; Schneider
et al. 1989), but in the mineral soil NO~ is the dominant
nitrogen species in the soil solution (Schierl and Kreutzer
19
DIFFUSION
. MASS FLOW
Nitrogen and calcium uptake of a spruce stand
9 uWater
pto..,
k_e .'_ -300 L
NH~" K"
c
Total uptake
Delivery bymasss flow
h~ t Yr- 1
hff. 1 Yr- 1
56 k N
-~
IF-
/
35 kg Ca
..........
4.5
24
[ NO3- Ca** H20
H" Mg§
Fe Mn
~
x
~t:i ~~ o
1
Mg'" K ~
NH~* H"
At
soil solution
....... 30 kg NO3-N ~
(kL~T~NER1988)-. . . . . .
Aver, conc. in
H2Pq-~g
NO3- Ca"
(650 pM NO3-N)
kg NH4-N
~
(100 ~JM NH4-N )
kg Co
~
(200 pu co)
{MURACH pert, comm.)
&
li
L
.
.,,<:f.,
Lateral root
-=IV
.
.
.
.
SOlLk.~!
Potential contribution of
n o n - m y c o r r h l z a l roof tips to NO 3-- and
. . . Ca
. .2§. uptake
. .
No, of non-mycorrhlzol tips: - 1 2 x 10 6 ha -1
#m~PARTICLES.,
(Uuroch lg84)
Water uptake rote of root tip: ~0,4 x 10-9L s -1 mm - 2
Delivery
Uptake rate
Total uptake
by mass flow
to root tips
(real s - l m m -2 )
of root tips,
measured
(real s - I turn - 2 )
by root tips.
calculated
(ha-1 yr -1)
14 x 10- 14
~20 kg NO3--N
6.6 x 10- 1 4
~29 kg Ca
26 x 10 -14
8 x 1 0 - 14
hairs
delivery by mass flow, and potential contribution of non-mycorrhizal
root tips to nitrogen and calcium acquisition. Data from Fig. 3; Fig. 5;
Murach 1984; Matzner 1988; Hfiussling et al. 1988
Root tip
Fig. 8. Model of nutrient and water supply by mass flow and diffusion,
and of nutrient and water uptake rates and net change in H + and OH- in
the rhizosphere of non-mycorrhizal long roots of Norway spruce growing
in the mineral soil horizon (NRA, nitrate reductase activity)
1989; Fig. 5). At sites with high atmospheric input of nitrogen and a proportion of NO3 to NH4 in the canopy drop of
up to 1 (Schierl and Kreutzer 1989) the proportion of NO5
should also increase in the soil solution of the humus layer.
In healthy forest stands most of the fine root biomass of
Norway spruce is located in the top 10 cm of the mineral
soil (Schulze 1989). In this soil layer, with an increase of
soil temperature and nitrification rate during the vegetation
period the NO~ concentration in the soil solution rises and
bulk soil pH decreases, whereas the NH~ concentration
remains at a low level (Fig. 5). This pattern of NI-I~ and
NO5 concentrations shown in Fig. 5 for the Mauzenberg
site is also typical of other sites in Baden-Wtirttemberg (M.
Haussling, in preparation). The dominant nitrogen supply
as NO3, at least in the mineral soil, leads to an increase in
rhizosphere pH, particularly in apical root zones (Fig. 4;
Gijsman 1990a). The gradient along the root axis is presumably a consequence of gradients in metabolic activity,
uptake capacity and anatomy of the various root zones.
Compared to basal zones (approx. 2 - 5 cm behind the
apex) in the apical zone the uptake rates of water and
mineral ions such as Ca and Mg (H~iussling et al. 1988), of
NO3 (Fig. 6), and NRA (Fig. 6; Peuke 1987) are considerably higher. The gradient in these parameters from apical to
basal root zones depends on the abundance of lateral roots
(H~iussling et al. 1988; Fig. 3).
Whether these different capacities along the root axis
can be fully utilized in soil-grown roots depends on the
concentration and mobility of ions in the bulk soil solution,
transport by mass flow and diffusion to the root surface and
replenishment of ions in the rhizosphere. Model calculations are summarized in Fig. 7 for nitrogen and calcium in
a Norway spruce stand. Based on a transpirational water
loss of about 1 - 2 1 m -2 day -1 during the vegetation period,
or 300 1 m -2 year-1 (Schulze et al. 1989) and average
concentrations in the bulk soil solution of 650 p.M NO3 and
100 ~tM NH~ (Fig. 4) and 200 ~tM Ca 2+ (M. Hfiussling, in
preparation), the delivery by mass flow to the roots can be
calculated and compared to the total uptake. It is evident
that delivery of NO3-N by mass flow may be equivalent to
about 50% of the total nitrogen demand compared to only
about 8% in the case of NH4-N. In view of the preferential
NH~ uptake (Fig. 1) a depletion zone of NH~ in the rhizosphere is to be expected.
Uptake rates of water, NO5 and particularly Ca 2+, however, differ along the root axis and are very high in apical
zones of non-mycorrhizal roots. This pattern holds true
under different environmental conditions (H~iussling et al.
1988; Fig. 3), although the actual water uptake rates at the
root tip may vary by a factor of 3 - 5 depending on root
growth rate or soil moisture conditions in other root zones
of the same tree (data not shown). In periods of low root
growth, substantial amounts of water may be taken up also
via suberized woody regions of the root (MacFall et al.
1990).
Irrespective of these variations in water uptake rates,
however, the potential contribution of non-mycorrhizal
root tips to Ca 2+ and NO~ uptake of Norway spruce can be
calculated (Fig. 7) using data from previous uptake experiments with individual roots (H~iussling et al. 1988), from
Fig. 7. Yearly nitrogen and calcium uptake of a Norway Spruce stand,
20
the present data and with data from the literature. The
number of non-mycorrhizal root tips in a Norway spruce
forest stand is in the range of 12 x 106 ha -1 (Murach 1984)
and the water uptake rate of the root tips may be as high as
0.4 • 10-9 1 s-1 mm -2 (Fig. 3).
Accordingly, at a concentration in the bulk soil solution
of 200 g M Ca 2+ and 650 g M NO3 the delivery by mass
flow to the non-mycorrhizal root tips can be compared with
the potential uptake rates, which are in the range of
6 . 6 x 10-14 mol mm -2 s-l for Ca 2+ (recalculated from
1.7 x 10-6 mol cm-2 72 h-l, H~iussling et al. 1988) and in
the range of 14 x 10-14 mol mm -2 s-1 for NO3 (Fig. 3).
Thus, non-mycorrhizal root tips have the potential to contribute up to 80% of the Ca demand of the trees. For NO3-N
this contribution is less, but still substantial. According to
this calculation NO~ delivery exceeds uptake rate. Thus,
depletion of NO~ in the rhizosphere soil of root tips should
not have occurred, an assumption which is supported by
the data in the forest stand (Fig. 6).
In contrast to Ca 2+ and NO3, for mineral nutrients like
K + and phosphate - but also for NH~ - delivery by mass
flow is low and thus delivery by diffusion dominates, particularly in the mineral soil. These inherent differences in
delivery and the corresponding consequences for gradients
of mineral elements and pH in the rhizosphere and along
the axis of non-mycorrhizal roots are summarized in a
model in Fig. 8, based on results presented in this paper
and elsewhere (Marschner et al. 1986; Hfiussling et al.
1988; H~iussling and Marschner 1989; Marschner 1989).
In the mineral soil, the supply of NH~ to the roots
depends largely on diffusion, leading to a depletion zone
confined to the rhizoplane soil ( 0 - 1 mm from the root
surface; Fig. 6). The depletion zone of NH~ is similar to
that of K + or phosphate in the rhizosphere of annual species
(Jungk and Claassen 1989). Because of the higher mobility
of NO~ in the soil the depletion zone of NO3 may extend
even beyond the rhizosphere soil ( 1 - 2 mm from the root
surface; Fig. 6). Due to the differences in water uptake
rates this general pattern of concentration gradients of NO~
and NH~ may be modified along the root axis (Fig. 8).
Additionally, a shift in the proportion of NH~]to NO~ in the
soil solution in favour of NH4, for example in the humus
layer (Meyer et al. 1988; Marschner et al. 1989; Schneider
et al. 1989), may also modify uptake and depletion
gradients of NH2 and NO}~
The pH in the rhizosphere and rhizoplane soil is mainly
determined by the supply of NH~ and NO3 to the roots. For
given NO3 concentrations, at NH~ concentrations below
100 gM, rhizoplane and rhizosphere pH are higher than the
bulk soil pH (Fig. 4) whereas at NH~ concentrations above
100 g M rhizoplane and rhizosphere pH are lower than the
bulk soil pH (Marschner et al. 1986). However, in apical
root zones of Norway spruce rhizosphere and rhizoplane
pH values are typically high (Fig. 4; H~iussling et al. 1985),
even at higher NH~ supply (Leisen et al. 1990). Similar
results have been obtained in Douglas fir (Gij sman 1990 a).
These results, summarized in Fig. 8, demonstrate that
average values for rhizosphere pH and nutrient concentrations in the rhizosphere are of limited value for predicting
the nutritional status or the risk of A1 toxicity of trees at
forest sites (Gijsman 1990 a). Likewise, average values of
soil solution composition and the concentrations of exchangeable mineral nutrients in homogenized soil samples
may be misleading when the macropore system as a preferential pathway of solute fluxes is not taken into account
(Kaupenjohann and Hantschel 1987). Conclusions on nitrogen nutrition of forest trees based on studies in solution
culture and on chemical soil extractions in homogenized
soil samples (e.g. determination of exchangeable NH~)
necessarily overestimate the role of NH;in forest stands, as
they ignore the key role of the soil solution concentrations
and of the spatial availability of mineral nutrients for uptake by roots of soil-grown plants (Fig. 8; Gijsman 1990b).
In forest stands most of the root tips of coniferous trees
are mycorrhizal. In Norway spruce the proportion of ectomycorrhizal root tips may vary between 30% and 70%
(Meyer et al. 1988) and over 90% (Haug and Feger 1990).
The proportion varies greatly with location and time, and is
much depressed as mineral nutrient supply is increased
(Ahlstr6m et al. 1988). Uptake of water and mineral
nutrients by ectomycorrhizal roots is at least in part mediated by the external mycelium (Read et al. 1985), whereas
the water uptake by mycorrhizal root tips may be severely
restricted by the fungal sheath (Ashford et al. 1988). Fungal ectoenzymes are particularly important for the acquisition of organically bound nutrients, for example acid
phosphatases for organically bound P (H~iussling and Marschner 1989; Fig. 7) and proteases for protein-N (Read et
al. 1989). Supplied with inorganic nitrogen, mycorrhizal
roots can absorb both NH2 and NO~ (Finlay et al. 1989), but
usually show a preference for NH~] (K~ihr and Arveby
1986), in the same way as the non-mycorrhizal roots. Thus,
in soil-grown roots of coniferous trees similar effects of the
dominant form of nitrogen in the soil solution on rhizosphere pH can be expected in ectomycorrhizal and in nonmycorrhizal roots. The amplitudes of these pH changes,
however, may be less in the ectomycorrhizal roots (Rygiewicz et al. 1984a, b).
Acknowledgements. The research program was financiallysupportedby
the German Federal Ministryfor Research and Technology.The authors
thank Mr. E. A. Kirkby for correction of the English text, and Ms. E.
Gorgus for preparationof the drawings.
References
Ahlstr6m K, Persson H, BOrjesson I (1988) Fertilization in a mature
Scots pine (Pinus sylvestris L.) stand - effects on fine roots. Plant
Soil 106: 179-190
Ashford AE, Peterson CA, Carpenter JL, Cairney JWG, Alloway WG
(1988) Structureand permeabilityof the fungal sheath in the Pisonia
mycorrhiza.Protoplasma 147:149 - 161
Boxman AW, Roelofs JGM (1988) Some effects of nitrate versus ammonium nutritionon the nutrientfluxes in Pinus sylvestris seedlings.
Effects of mycorrhizalinfection.Can J Bot 66:1091 - 1097
Clarkson DT, HopperMJ, Jones LHP (1986)The effect of root temperature on the uptake of nitrogen and the relative size of the root system
in Lolium perenne. I. Solutionscontainingboth NH~ and NO~.Plant
Cell Environ9:535-545
Deane-DrummondCE, ChaffeyNJ (1985) Characteristics of nitrate uptake into seedlings of pea (Pisum sativum L. cv. Feltham First).
Changes in net NO3 uptake following inoculation with Rhizobium
and growth in low nitrate concentrations. Plant Cell Environ 8:
517-523
21
Deane-Drummond CE, Glass ADM (1983) Short term studies of nitrate
uptake into barley plants using ion-specific electrodes and 36C103. II.
Regulation of NO~ efflux by NH~. Plant Physio173: 105-110
Drew MC, Saker LR, Barber SA, Jenkins W (1984) Changes in the
kinetics of phosphate and potassium absorption in nutrient-deficient
barley roots measured by a solution depletion technique. Planta 160:
490 - 4 9 9
Evers FH, Hildebrand EE, Kenk G, Kremer WL (1986) Boden-, ern~rungs- und ertragskundliche Untersuchungen in einem stark gesch~idigten Fichtenbestand des Buntsandstein-Schwarzwaldes. Mitt
Ver Forstl Standortskd Forstpflanzenztichtung 32: 7 2 - 80
Finlay RD, Ek H, Odham G, S/3derstr6m B (1989) Uptake, translocation
and assimilation of nitrogen from 15N-labelled ammonium and nitrate sources by intact ectomycorrhizal systems of Fagus sylvatica
infected with Paxillus involutus. New Phytol 113: 4 7 - 55
Gijsman AJ ( 1990 a) Rhizosphere pH along different root zones of Douglas-fir (Pseudotsuga menziesii), as affected by source of nitrogen.
Plant Soil 124:161 - 167
Gijsman AJ (1990b) Nitrogen nutrition of Douglas-fir (Pseudotsuga
menziesii) on strongly acid sandy soil. II. Proton excretion and
rhizosphere pH. Plant Soil 126:63 - 70
Hang I, Feger KH (1990) Effects of fertilization with MgSO4 and
(NH4)2SO4 on soil solution chemistry, mycorrhiza and nutrient content of fine roots in a Norway spruce stand. Water Air Soil Pollut (in
press)
H~iussling M, Marschner H (1989) Organic and inorganic soil phosphates
and acid phosphatase activity in the rhizosphere of 80-year-old Norway spruce [Picea abies (L.) Karst.] trees. Biol Fertil Soils 8:
128-133
H~iussling M, Leisen E, Marschner H, R6mheld V (1985) An improved
method for non-destructive measurements of the pH at the root-soil
interface (rhizosphere). J Plant Physiol 117:371 -375
H~iussling M, Jorns CA, Lehmbecker G, Hecht-Buchholz C, Marschner
H (1988) Ion and water uptake in relation to root development in
Norway spruce [Picea abies (L.) Karst.]. J Plant Physiol 133:
486-491
Jungk A, Claassen N (1989) Availability in soil and acquisition by plants
as the basis for phosphorus and potassium supply to plants. Z Pflanzenernahr Bodenkd 152:151 - 157
K~ihrM, Arveby AS (1986) A method for establishing ectomycorrhiza on
conifer seedlings i n steady-state conditions of nutrition. Physiol
Plant 6 7 : 3 3 3 - 3 3 9
Kanpenjohann M, Hantschel R (1987) Die kurzfristige pH-Pufferung
von gest6rten und ungest6rten Waldbodenproben. Z Pflanzenern~ihr
Bodenkd 150: 156- 160
Lee RB, Clarkson DT (1986) Nitrogen-13 studies of nitrate fluxes in
barley roots. I. Compartmental analysis from measurements of 13N
efflux. J Exp Bot 37:1753 - 1767
Lee RB, Drew MC (1989) Rapid, reversible inhibition of nitrate influx in
barley by ammonium. J Exp Bot 4 0 : 7 4 1 - 7 5 2
Leisen E, H/iussling M, Marschner H (1990) Einflug von StickstoffForm und -Konzentration und saurer Benebelung auf pH-Ver~inderungen in der Rhizosph~e von Fichten [Piceaabies (L.) Karst.].
Forstwiss Centralbl 109:275 - 286
MacFall JS, Johnson GA, Kramer PJ (1990) Observation of a waterdepletion region surrounding loblolly pine roots by magnetic resonance imaging. Proc Natl Acad Sci USA 87: 1203- 1207
Marschner B, Stahr K, Renger M (1989) Potential hazards of lime
application in a damaged pine forest ecosystem in Berlin, Germany.
Water Air Soil Pollut 4 8 : 4 5 - 5 7
Marschner H (1989) Effect of soil acidification on root growth, nutrient
and water uptake. In: Ulrich B (ed) Forest decline research: state of
knowledge and perspectives. Friedrichshafen, pp 381 - 404
Marschner H, R6mheld V (1983) In-vivo measurement of root-induced
pH changes of the soil-root interface: effect of plant species and
nitrogen source. Z Pflanzenphysiol 111: 241 - 251
Marschner H, H~iussling M, Leisen E (1986) Rhizosphere pH of Norway
spruce trees grown under both controlled and field conditions. In:
Proceeding of the CEC Workshop, Jialich, 1985 on "Indirect effects
of air pollution on forest trees". Kernforschungsanlage, Jtilich,
pp 113-118
Martin F, Chemardin M, Godal P (1981) Nitrate assimilation and nitrogen circulation in Australian pine. Physiol Plant 53:105 - 110
Matzner E (1988) Der Stoffumsatz zweier Wald-Okosysteme im Solling.
Berichte des Forschungszentrums Wald6kosysteme/Waldsterben,
Reihe A, vo140, G6ttingen, FRG
McClure PR, Kochian LV, Spanswick RM, Shaft JE (1990) Evidence for
cotransport of nitrate and protons in maize roots. II. Measurement of
NO~ and H + fluxes with ion-selective microelectrodes. Plant Physiol
93:290-294
Meyer J, Schneider BU, Werk K, Oren R, Schulze E-D (1988) Performance of two Picea abies (L.) Karst. stands at different stages of
decline. V. Root tip and ectomycorrhiza development and their relations to above ground and soil nutrients. Oecologia 77: 7 - 1 3
Murach D (1984) Die Reaktion der Feinwurzeln von Fichten (Picea
abies Karst.) auf zunehmende Bodenversauerung. G6ttinger
Bodenkundliche Berichte 77
Peuke AD (1987) Der Effect yon Schwefeldioxid-, Ozon- und Stickstoffdioxid-Begasung auf den Stickstoff-Metabolismus steril kultivierter
Fichtenkeimlinge [Piceaabies (L.) Karst.]. Ph.D. Thesis, University
of G6ttingen
Read D J, Frances R, Finlay RD (1985) Mycorrhizal mycelia and nutrient
cycling in plant communities. In: Fitter AH (ed) Ecological interactions in soil. Special Publication of the British Ecology Society.
Blackwell, Oxford, pp 193-218
Read DJ, Leake JR, Longsdale AR (1989) The nitrogen nutrition of
mycorrhizal fungi and their host plant. In: Boddy L, Marchant R,
Read DJ (eds) Nitrogen, phosphorus and sulphur utilization by fungi.
Cambridge University Press, Cambridge, pp 181 - 204
Rollwagen BA, Zasoski RJ (1988) Nitrogen source effects on rhizosphere pH and nutrient accumulation by Pacific Northwest conifers.
Plant Soil 105: 7 9 - 86
Rygiewicz PT, Bledsoe CS, Zasoski RJ (1984 a) Effects of ectomycorrhizae and solution pH on (15N)ammonium uptake by coniferous seedlings. Can J For Res 14: 8 8 5 - 892
Rygiewicz PT, Bledsoe CS, Zasoski RJ (1984b) Effects of ectomycorrhizae and solution pH on (15N)nitrate uptake by coniferous seedlings. Can J For Res 14:893 - 899
Scheromm P, Plassard C (1988) Nitrogen nutrition of non-mycorrhized
pine (Pinus pinaster) grown on nitrate or ammonium. Plant Physiol
Biochem 2 6 : 2 6 1 - 2 6 9
Schierl R, Kreutzer K (1989) Dolomitische Kalkung eines Fichtenbestandes auf saurer Parabraunerde: Auswirkungen auf Bodenchemie und Vegetation. Kali-Briefe 19:417 - 423
Schneider BU, Meyer J, Schulze E-D, Zech W (1989) Root and mycorrhizal development in healthy and declining Norway spruce
stands. In: Schulze E-D, Lange OL, Oren R (eds) Forest decline and
air pollution (Ecological studies, vol 77) Springer, Berlin Heidelberg
New York, pp 370-391
Schulze E-D (1989) Air pollution and forest decline in a spruce (Picea
abies) forest. Science 2 4 4 : 7 7 6 - 7 8 3
Schulze E-D, Hantschel R, Werk KS, Horn R (1989) Water relations of
two Norway spruce stands at different stages of decline. In: Schulze
E-D, Lange OL, Oren R (eds) Forest decline and air pollution.
(Ecological studies, vol 77) Springer, Berlin Heidelberg New York,
pp 341-351
Siddiqi MY, Glass ADM, Ruth TJ, Fernando M (1989) Studies of the
regulation of nitrate influx by barley seedlings using 13NO3. Plant
Physio190:806-813
Tachibana S (1987) Effect of root temperature on the rate of water and
nutrient absorption in cucumber cultivars and figleaf gourd. J Jpn
Soc Hort Sci 5 5 : 4 6 1 - 4 6 7
Van Beusichem ML, Baas R, Kirkby EA, Nelemans JA (1985) lntracellular pH regulation during NO~ assimilation in shoot and roots of
Ricinus communis. Plant Physio178:768-773
Van der Paauw F (1971) An effective water extraction method for the
determination of plant-available soil phosphorus. Plant Soil 34:
467 - 481