Aquatic Botany 64 (1999) 369–380
Interactive effects of N and P on growth, nutrient
allocation and NH4 uptake kinetics by Phragmites
australis
José Antonio Romero a , Hans Brix b , Francisco A. Comı́n a,∗
a
b
Department of Ecology, University of Barcelona, Diagonal 645, 08028 Barcelona, Spain
Department of Plant Ecology, University of Aarhus, Nordlandsvej 68, DK-8240 Risskov, Denmark
Abstract
The interactive effects of three levels of NH4 –N (50, 500 and 1000 ␮mol l−1 ) and two levels of
phosphate (15 and 50 ␮mol l−1 ) on growth, nutrient allocation and ammonium uptake kinetics by
Phragmites australis (Cav.) Trin. ex Steudel were studied in hydroponic culture in the laboratory.
Nitrogen level in the root solution significantly affected the relative growth rate of the plants, the
rate being lower at low N (0.026 per day) than at intermediate (0.035 per day) and high N (0.037
per day), but phosphorus did not significantly affect growth. The N : P ratio in the root solution
significantly affected the growth rate which was highest at N : P ratios between 10 and 33 on a
molar basis. Nitrogen and phosphorus concentration in the plant tissues generally increased with
N level in the root solution, but P level had no effect. Plant tissue N : P ratios (on a molar basis)
varied between 13.5 in the stems to 28.0 in the leaves and were unaffected by the treatments. Ammonium uptake kinetics were unaffected by N treatment, but Vmax was significantly affected by P
treatment averaging (mean ± 95% confidence limits (CL)) 151 ± 44 ␮mol g−1 root dry weight h−1
in the low-P treatment and 229 ± 70 ␮mol g−1 root dry weight h−1 in the high-P treatment. The
overall mean (±95% CL) NH4 –N uptake kinetic parameters were: Vmax = 190 ± 20 ␮mol g−1 root
dry weight h−1 ; K1/2 = 21.8 ± 1.8 ␮mol l−1 , and Cmin = 1.2 ± 0.2 ␮mol l−1 . Mean (±SD) root respiration rate was 72 ± 22 ␮mol CO2 g−1 dry weight h−1 and was unaffected by the treatments.
The results of the study support the general hypothesis that P. australis is well-adapted for growth
in nutrient-rich habitats. However, P. australis is able to acclimate to low nutrient availability by
increasing the affinity for ammonium uptake. ©1999 Elsevier Science B.V. All rights reserved.
Keywords: Relative growth rate; Nutrient allocation; Root respiration; Die-back; Acclimation; N : P ratio;
Phragmites australis
∗ Corresponding author. Tel.: +34-3-4021510; fax: +34-3-411-1438
E-mail address: [email protected] (F.A. Comı́n)
0304-3770/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 4 - 3 7 7 0 ( 9 9 ) 0 0 0 6 4 - 9
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J.A. Romero et al. / Aquatic Botany 64 (1999) 369–380
1. Introduction
The common reed (Phragmites australis (Cav.) Trin. ex Steudel) grows in a wide range
of habitats, including saltmarshes, the littoral zone of lakes, fens, bogs, and even relatively
dry areas, if competition from other species is low (Haslam, 1970). Although, P. australis
commonly can be found in oligotrophic habitats, it grows best at fertile sites (Hocking et
al., 1983). It is the preferred species in constructed wetlands heavily loaded with domestic
sewage or sewage sludge and generally grows prolifically in these systems (Hofmann,
1990; Brix, 1994; Kadlec and Knight, 1996; Vymazal et al., 1998). Thus, P. australis is a
species with a broad ecological amplitude and able to acclimate to a wide range of growth
conditions.
The die-back of P. australis recorded in recent years in many European wetlands has
frequently been associated with eutrophication (Ostendorp, 1989; Čı́žková-Končalová et
al., 1992). It has been shown experimentally that extremely high nutrient levels (Dykyjová,
1978) and high loadings of sludge, particularly anaerobic digested sludge (Hofmann, 1986),
inhibit growth and induce typical stress symptoms such as stunted growth and chlorotic
leaves in P. australis. However, this is probably not an effect of the high nutrient levels per se, but rather an effect of high salt concentrations or reducing soil conditions
generating high concentrations of toxic compounds such as organic acids and sulphides
in the sediment (Armstrong et al., 1996; Čı́žková et al., 1999). Eutrophication has also
been reported to stimulate the die-back process via accumulation of litter and allogenous organic matter (van der Putten, 1997; Brix, 1999), as well as via disturbed carbohydrate and nutrient cycling in the plant (Kühl and Kohl, 1992; Čı́žková-Končalová et al.,
1992; Kubin and Melzer, 1996). It is, however, possible that an imbalanced supply of the
main nutrients, N and P, will adversely affect the metabolism and hence the growth of
P. australis.
Many studies have dealt with the effects of eutrophication on growth (Rintanen, 1996;
Kubin and Melzer, 1997), mechanical strength (Ostendorp, 1995), carbohydrate dynamics
(Kubin et al., 1993; Kubin and Melzer, 1996), decomposition (Andersen, 1978), anatomy
and gas exchange properties (Votrubova et al., 1997) of P. australis, but to our knowledge, no studies have been conducted on the possible interactive effects of nitrogen and
phosphorus availability on growth and the physiology of nutrient uptake. In this study, we
investigated the interactive effects on ammonium and phosphorus availability on growth,
biomass, nutrient allocation and ammonium uptake kinetics by P. australis in order to esTable 1
Concentrations of NH4 –N and PO4 –P (␮mol l−1 ) and the corresponding N : P ratios in the root solutions in the
growth experiments with P. australis
Treatment
NH4 –N (␮mol l−1 )
PO4 –P (␮mol l−1 )
N : P ratio
Low N; low P
Low N; high P
Intermediate N; low P
Intermediate N; high P
High N; low P
High N; high P
150
150
500
500
1000
1000
15
50
15
50
15
50
3.3
1
33
10
67
20
J.A. Romero et al. / Aquatic Botany 64 (1999) 369–380
371
tablish to what extent the plant responds to, and acclimates to, the nutrient availability in
the surroundings, and in order to elucidate how imbalanced N and P supplies affect the
plant. We hypothesise that P. australis acclimates to low nutrient availability by increasing
the affinity for nutrient uptake and conversely to high nutrient availability by increasing
uptake capacity. As nutrient uptake is controlled by the activity of membrane-bound ATPases it is argued that root respiration might be affected in concert with uptake metabolism.
2. Materials and methods
2.1. Experimental design
The experimental design was a two by three factorial set-up with two levels of P (high
and low) and three levels of NH4 –N (high, intermediate and low) with five replicates in
each treatment (Table 1). The N : P ratios in the treatments were selected to cover conditions
with balanced availability of N and P in relation to the requirements of the plant as well as
conditions where either P or N occurs in surplus. All other nutrients were kept at the same
level in the treatments.
2.2. Plant material and growth conditions
Seeds of P. australis were collected from a polyclonal stand at Vejlerne Nature Reserve,
Denmark (57◦ 050 N, 9◦ 040 E), germinated in trays in a greenhouse and propagated in peat
for four months in 0.7–l pots. Then the seedlings were rinsed and transferred to hydroponic culture in 30–l vessels in indoor growth chambers (Weiss Umwelttechnic GMBH,
Lindensruth, Germany) at a temperature of 25◦ C, 85% relative air humidity, a photon flux
density of 300 ␮mol m−2 s−1 (PAR) at the base of the plants and a light : dark cycle of
16 : 8 h. The vessels contained a nutrient solution prepared in tap water using 1 g l-1 of a
commercial fertiliser (Pioner NPK Makro 19-2-15 + Mg, Brøste, Denmark) and 0.1 ml l−1
of a micronutrient solution (Pioner Mikro Plus, Brøste, Denmark). In addition, Fe2+ was
added as FeSO4 each day and the pH adjusted to pH 6.5 by 1 mol l−1 NaOH. In order
to avoid depletion of nutrients the nutrient solution was replaced every four days. The
root vessels were continuously bubbled by atmospheric air 10 min every hour in order to
secure proper mixing. After three weeks the plants had developed new roots and leaves.
Thirty plants were chosen, dead roots and rhizomes were removed, and the plants each
mounted in 1.5–l darkened glass vessels containing 1.4 l nutrient solution made up from
ultra pure deionized water. The composition of the nutrient solution used in the experiments was as follows (mmol l−1 ): K+ 1.05; Ca2+ 1; Mg2+ 1; Na+ 1.03; SiO3 2− 0.0125;
Fe2+ as Fe-EDTA 0.01; SO4 2− 1.5; HCO3 − 2; and Cl− 0.5 (pH 6.5). The basic composition of the nutrient solution was kept constant in all treatments except for the concentrations of N and P which was added separately as (NH4 )2 SO4 and KH2 PO4 3− . The
nutrient solutions were changed daily and aerated for 15 min every hour to secure efficient
mixing.
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2.3. Plant growth and mineral composition
Fresh weights of all plants were determined weekly by a standardised weighing procedure. The relative growth rate (RGR), based on total plant dry weights, were estimated
from fresh weights and the ratio between dry and fresh weights. At the final harvest the
plants were fractionated into stems, leaves, roots and rhizomes, the fractions weighed,
and then dried to constant weight at 80◦ C. Total nitrogen and carbon were analysed by
gas chromotography after combustion of 200 ␮g ground material (Fisons Instruments,
Model NA2000, Italy). The concentrations of P, K, Ca, Mg, Na, Mn and Fe were analysed by plasma emission spectrometry (Perkin Elmer Plasma II Emission Spectrometer,
USA) after digestion of ground material in HNO3 –H2 O2 according to Schierup and Jensen
(1981).
2.4. NH4 –N uptake kinetics
The uptake kinetics of ammonium were studied during two weeks prior to the final harvest
of the plants. During that period the plants had reached a constant relative growth rate. The
following modified Michaelis-Menten model was used (Brix et al., 1994):
In =
Vmax (Cs − Cmin )
K + (Cs − Cmin )
(1)
where In is the inflow rate at substrate concentration Cs , Cs the substrate concentration
in the root medium, Vmax the maximum influx rate at saturating substrate concentration,
Cmin substrate concentration at which there is no net inflow, i.e. In = 0, K = Cs− Cmin when
In = 1/2Vmax .
From the above equation it can be seen that the half saturation constant, K1/2 , i.e. the
substrate concentration when In = 1/2Vmax , is equal to K + Cmin . The uptake kinetics of five
individual plants were estimated simultaneously. The plants were transferred to five 1–l
root chambers each containing 700 ml nitrogen-free basic nutrient solution for 12 h before
the start of the uptake studies. The nutrient solutions in the root chambers were stirred by
a magnetic stirrer and aerated continuously to secure mixing. After the 12 h preincubation
period, duplicate samples of the root solution were taken and analysed for NH4 –N by flow
injection analyses (Lachat, Quick Chem Instuments, Milwaukee, USA) to estimate the Cmin
value. Then the NH4 –N concentrations in the root solutions were increased in steps to a
maximum of 250 ␮mol l−1 . At each concentration level a series of solution samples were
taken at 5 min intervals for 20 min to estimate NH4 uptake rate. Linear regression analyses
of NH4 –N concentration versus time were used to calculate uptake rates. The uptake kinetic
parameters, Vmax and K1/2 , were estimated based on corresponding uptake rates (In ) and
NH4 –N concentrations (Cs ) using non-linear curve-fitting of Eq. (1) (Curve-fitter, Fig-P,
Biosoft, UK). The model estimated the kinetic parameters satisfactorily in all experiments.
Uptake rates were based on root dry weights.
J.A. Romero et al. / Aquatic Botany 64 (1999) 369–380
373
Table 2
Results of ANOVA (P-values) for growth and biomass partitioning of P. australis grown at two P and three N
levels for 5 weeksa
Source of variation
RGR
RWR
RhWR
LWR
N
P
N×P
0.006b
0.911
0.102
0.892
0.408
0.403
0.233
0.609
0.085
0.149
0.816
0.220
a
b
RGR: Relative growth rate; RWR: Root weight ratio; RhWR: Rhizome weight ratio; LWR: Leaf weight ratio.
Figures in bold indicate statistically significant values at the 0.05 probability level.
2.5. Root respiration
Root respiration was measured by incubating three roots from each plant (approx. 0.2 g
fresh weight) in 33.5 ml incubation flasks mounted on a rotating wheel in a thermostated
(25◦ C) water bath. Respiration was estimated as production of dissolved inorganic carbon
(DIC) over time in the incubation flasks. Concentrations of DIC were measured on 100 ␮l
subsamples that were acidified and sparged with N2 gas to carry all CO2 into an infrared
gas-analyser (ADC 225-MK-3, UK) connected to an integrator (Chromatopac C-R3A, Shimadzu Corp., Kyoto, Japan).
2.6. Statistics
Data were analysed by analysis of variance (ANOVA) using Type III sum of squares
with the software Statgraphics version 3.1 (Statistical Graphics Corp., USA). Multiple
comparisons of means were performed using Fisher’s LSD procedure at the 0.05 significance
level. All data were tested for normal distribution and for homogeneity of variance by
Cochran’s test.
3. Results
3.1. Plant growth and biomass allocation
The estimated growth rates were erratic during the initial two weeks of the incubation
period probably because of dying of roots and acclimation to the experimental conditions.
However, after two weeks the growth rates became stable and averaged 0.033 ± 0.008 per
day (RGR, mean ± SD) for all treatments. Nitrogen level in the root solution significantly
affected RGR, the rate being lower at low N (0.026 per day) than at intermediate (0.035
per day) and high N (0.037 per day). Phosphorus level had no effect on the RGR, and no
significant interaction term between N and P was found (Table 2). The N : P ratio in the
growth medium significantly affected RGR, the rates being highest at molar N : P ratios
between 10 and 33 (Fig. 1). The relative allocation of biomass to roots (11.1 ± 2.6%),
rhizomes (27.3 ± 12.6%), stems (33.4 ± 6.1%) and leaves (28.2 ± 7.3%) was unaffected
by treatment. The root–shoot ratios were very variable between plants and therefore no
statistically significant difference could be detected between the treatments (Table 2).
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J.A. Romero et al. / Aquatic Botany 64 (1999) 369–380
Fig. 1. Relative growth rate (mean ± SD) based on total plant dry weight of P. australis at three levels of NH4 + –N
(50, 500 and 1000 ␮mol l−1 ) and two levels of P (15 and 50 ␮mol l−1 ). Identical letters above bars indicate groups
of means with no statistically significant differences at the 95% confidence level.
Fig. 2. Tissue concentration (mean ± SD) of (a) nitrogen, (b) phosphorus and (c) the N : P ratio (on a molar basis)
of different fractions of P. australis grown at three levels of NH4 + –N (50, 500 and 1000 ␮mol l−1 ) and two levels
of P (15 and 50 ␮mol l−1 ). Identical letters above bars indicate groups of means with no statistically significant
differences at the 95% confidence level.
J.A. Romero et al. / Aquatic Botany 64 (1999) 369–380
375
Table 3
Tissue concentrations (␮mol g−1 dry weight) of Fe, Mn, Mg, Ca, Na and K in stems, leaves, roots and rhizomes
of P. australis grown for five weeks at two levels of P and three levels of N (mean ± SD)c
Element Fraction
Fe
P-low
P-high
N-low
N-interm.
N-high
N-low
N-interm.
N-high
7.6 ± 2.8ab
3.5 ± 3.9a
9.4 ± 3.7b
10.5 ± 3.0b
8.2 ± 3.3b
12.6 ± 3.2ab
12.1 ± 3.6ab
4.8 ± 3.6a
6.7 ± 2.8ab
10.9 ± 4.3a
10.4 ± 4.4a
4.9 ± 4.4a
Stems
Leaves
20.4 ± 6.5ab
Roots
19.7 ± 2.1ab
Rhizomes 3.8 ± 3.7a
12.6 ± 3.4ab 18.5 ± 11.2ab
23.1 ± 17.5b 13.0 ± 6.4ab
8.6 ± 4.1a
6.2 ± 3.8a
24.3 ± 17.9b
22.5 ± 9.8b
8.4 ± 8.5a
Mn
Stems
Leaves
Roots
Rhizomes
0.46 ± 0.14a
0.66 ± 0.24a
4.28 ± 0.56b
0.67 ± 0.16a
0.53 ± 0.12a
0.91 ± 0.35a
2.50 ± 0.90b
0.53 ± 0.12a
0.81 ± 0.63a
1.20 ± 0.72a
1.73 ± 0.46a
0.61 ± 0.35a
0.98 ± 0.77a
1.58 ± 1.79a
4.79 ± 0.96b
1.36 ± 0.80b
0.67 ± 0.14a
1.32 ± 0.57a
2.46 ± 1.86b
0.62 ± 0.16a
0.61 ± 0.13a
1.07 ± 0.61a
1.43 ± 0.26a
0.47 ± 0.04a
Mg
Stems
Leaves
Roots
Rhizomes
43.2 ± 6.0a
109.3 ± 12.3a
77.6 ± 22.7ab
40.2 ± 15.5a
46.3 ± 8.1a
114.6 ± 9.1a
78.2 ± 5.9ab
34.8 ± 3.2a
54.1 ± 15.8a
123.2 ± 19.9a
70.2 ± 30.5a
40.2 ± 20.6a
51.2 ± 10.6a
145.6 ± 66.3a
97.4 ± 22.4b
41.1 ± 5.2a
54.0 ± 11.8a
126.9 ± 23.9a
81.4 ± 8.0ab
40.5 ± 6.5a
46.2 ± 4.8a
108.0 ± 16.9a
71.2 ± 5.1a
33.9 ± 2.2a
Ca
Stems
Leaves
Roots
Rhizomes
24.0 ± 6.0a
97.7 ± 6.9ab
31.9 ± 3.9bc
11.2 ± 2.9a
20.7 ± 2.8a
88.7 ± 5.8a
28.2 ± 4.7abc
10.9 ± 1.2a
24.9 ± 7.4a
111.3 ± 17.4ab
21.8 ± 9.4a
20.4 ± 25.7a
27.4 ± 7.29a
132.9 ± 59.5b
33.7 ± 5.8c
16.3 ± 7.8a
28.9 ± 8.5a
104.3 ± 24.6ab
28.4 ± 5.6abc
11.5 ± 1.77a
22.4 ± 2.4a
83.9 ± 8.1a
25.0 ± 5.5ab
9.1 ± 2.8a
Na
Stems
Leaves
Roots
Rhizomes
87.7 ± 26.5a
63.6 ± 49.6a
42.5 ± 10.5a
29.7 ± 9.9a
79.8 ± 25.6a
49.2 ± 34.7a
50.6 ± 11.6a
39.9 ± 4.2a
84.9 ± 41.1a
47.2 ± 37.2a
46.1 ± 13.3a
39.5 ± 19.8a
78.2 ± 27.8a
43.5 ± 19.2a
56.3 ± 22.5a
39.1 ± 24.9a
100.8 ± 11.6a
80.2 ± 21.6a
46.4 ± 22.7a
34.6 ± 3.8a
94.1 ± 19.7a
46.1 ± 23.8a
43.1 ± 13.2a
48.2 ± 29.6a
K
Stems
Leaves
Roots
Rhizomes
670 ± 224a
904 ± 110a
718 ± 67ab
532 ± 125a
776 ± 117a
976 ± 49a
786 ± 64b
548 ± 51a
683 ± 92a
975 ± 171a
767 ± 60b
461 ± 51a
686 ± 139a
931 ± 98a
674 ± 62a
494 ± 72a
612 ± 109a
876 ± 109a
750 ± 83ab
475 ± 58a
663 ± 121a
958 ± 81a
737 ± 34ab
545 ± 74a
c
Figures with equal letter superscript within rows are not statistically different at the 0.05 probability level.
3.2. Nutrient and mineral composition of the plant tissue
The concentrations of nitrogen and phosphorus in the plant tissues were significantly
affected by the treatments (Fig. 2). Nitrogen concentration generally increased with nitrogen
level in the root solution, but phosphorus level had no effect (Tables 3 and 4). The tissue
concentration of phosphorus was affected by both N and P level, the effect of nitrogen being
most pronounced. Looking at the other analysed elements, the N-treatment significantly
affected the concentrations of Fe and Mn (Table 4), the concentrations generally decreasing
with N-level, especially at the high-P treatments (Table 3). The phosphorus treatment did not
affect the mineral composition of the plant tissues (Table 4). However, we found significant
N–P interactions for C, Mn, Mg and Ca.
The concentrations of all analysed elements were significantly different between the plant
fractions (Table 4). Nitrogen concentrations were generally highest in leaves followed by
rhizomes, roots, and stems. Also, tissue concentrations of K, Ca, Mg and to some extent Fe
were highest in the leaves, whereas Na concentrations were generally highest in the stems.
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J.A. Romero et al. / Aquatic Botany 64 (1999) 369–380
Table 4
Results of 3-way ANOVA (P-values) for tissue nutrient concentrations in different fractions (stems, leaves, rhizomes and roots) of P. australis grown at two P and three N levels for 5 weeks
Source of variation
N
P
N/P ratio
C
Fe
Mn
Mg
Ca
Na
K
N
P
Fraction
N×P
N × Fraction
P × Fraction
N × P × Fraction
0.000a
0.000
0.049
0.033
n.s.
n.s.
n.s.
n.s.
0.095
0.080
0.000
n.s.
n.s.
n.s.
n.s.
0.952
0.709
0.009
0.014
n.s.
n.s.
n.s.
0.016
0.517
0.000
n.s.
n.s.
n.s.
n.s.
0.000
0.109
0.000
0.025
0.000
n.s.
n.s.
0.234
0.123
0.000
0.024
n.s.
n.s.
n.s.
0.081
0.344
0.000
0.008
0.409
0.682
0.037
0.616
0.367
0.000
n.s.
n.s.
n.s.
n.s.
0.539
0.103
0.000
n.s.
n.s.
n.s.
n.s.
a
b
0.513
0.000
n.s.b
n.s.
n.s.
n.s.
Figures in bold are statistically significant values at the 0.05 probability level.
n.s.: Not significant.
Table 5
Results of ANOVA (P-values) for NH4 –N uptake kinetic parameters and root respiration rates of P. australis grown
at two P and three N levels for five weeks
Source of variation
Vmax
K1/2
Cmin
Root respiration
N
P
N×P
0.992
0.049a
0.190
0.159
0.099
0.094
0.136
0.678
0.597
0.442
0.436
0.954
a
Figures in bold indicate statistically significant values at the 0.05 probability level.
Manganese concentrations were generally highest in the roots. The carbon content of the
tissues was fairly constant varying from 42.5% of dry weight in the leaves to 44.3% of dry
weight in the rhizomes. The plant tissue N : P ratios (on a molar basis) varied between 13.5
in the stems to 28.0 in the leaves (roots and rhizomes intermediate) and were unaffected by
the treatments (Fig. 2(c)). The ratio tended to increase slightly (especially in the stems) in
concert with the nutrient solution N : P ratio, however the difference was not statistically
significant (P = 0.38).
3.3. Ammonium uptake kinetics and root respiration
The overall mean values (±95% CL) of the ammonium uptake kinetic parameters were:
Vmax : 190 ± 20 ␮mol g−1 root dry weight h−1 ; K1/2 : 21.8 ± 1.8 ␮mol l−1 , and Cmin : 1.2 ± 0.2
␮mol l−1 . Vmax was significantly (see Table 5) affected by P-treatment, averaging (mean ±
95% CL) 151 ± 44 ␮mol g−1 h−1 in the low-P treatment and 229 ± 70 ␮mol g−1 h−1 in the
high-P treatment. Nitrogen treatment had no clear effect on Vmax . The mean K1/2 values
tended to increase and the Cmin values tended to decrease with both N and P level although
the differences were not statistically significant (Table 5). The mean (±SD) root respiration
rate was 72 ± 22 ␮mol CO2 g−1 dry weight h−1 and was not significantly affected by the
treatments (Table 5). There was no significant relation between root respiration rate and
any of the uptake kinetic parameters (Fig. 3).
J.A. Romero et al. / Aquatic Botany 64 (1999) 369–380
377
Fig. 3. Uptake kinetics parameters (mean ± SD) of P. australis grown at three levels of NH4 + –N (50, 500 and
1000 ␮mol l−1 ) and two levels of P (15 and 50 ␮mol l−1 ). Vmax values are based on root dry weight. Identical
letters above bars indicate groups of means with no statistically significant differences at the 95% confidence level.
4. Discussion
Nitrogen supply significantly affected the growth of the plants whereas phosphorus did
not have any effect. This shows that, within the concentration range tested, nitrogen was the
main nutrient limiting the growth of P. australis. The relative growth rates were highest in
treatments with molar N : P ratios between 10 and 33, and tended to decrease (although not
statistically significantly) at higher N : P ratios in spite of the fact that the N supply increased
in these treatments. This indicates that imbalanced supply of N and P suppresses growth of
P. australis. The concentration of nitrogen in the plant tissue increased with N supply, and
P tissue concentration increased in concert with N, and not P supply, resulting in a constant
N : P ratio in the tissues across treatments. Koerselman and Meuleman (1996) studied the
N : P ratios of various types of wetland vegetation that were known to be limited by either N
or P supply. They suggested that the N : P ratio of the vegetation directly indicates which of
the two nutrients, N or P, are limiting. An N : P ratio greater than 16 on a weight basis (>35.4
on a molar basis) should indicate P limitation on a community level, whereas an N : P ratio
less than 14 (<31 on a molar basis) should be indicative of N limitation. Duarte (1992)
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J.A. Romero et al. / Aquatic Botany 64 (1999) 369–380
compiled data on nutrient concentrations in a wide range of aquatic macrophytes and found
that N and P tended always to be present in the plant tissue at a ratio of ∼12 on a weight basis
(26.6 on a molar basis). In the present study, the molar N : P ratio of the aboveground tissues
varied between 17 and 22, which according to the relationship above would be indicative
of N limitation in all treatments. However, the growth of the plants was not increased by
increasing the N supply from 500 ␮mol l−1 to 1000 ␮mol l−1 . Koerselman and Meuleman
(1996) stated that the relationship is only valid when either N or P controls plant growth.
Therefore, it can be concluded that neither N nor P limited growth in the intermediate and
high-N treatments, and judging from the contents of other nutrient minerals in the plant
tissues, these were not limiting either (Dykyjová, 1978; Ho, 1981; Gries and Garbe, 1989).
Generally photosynthesis of P. australis is not saturated at the light levels in the growth
cabinet used for the experiment, and so the light regime used, maybe in combination with
temperature, probably limited growth in the high nutrient supply treatments in our set-up.
Hence, when N and P are not limiting, N : P ratios in the tissues seem to be regulated, not by
the external N and P supply, but by some other growth factors, or combination of factors.
Ammonium uptake kinetic parameters for P. australis have not, to our knowledge, been
previously determined, and only few uptake kinetic studies have been performed on aquatic
emergent macrophytes (Brix et al., 1994; Dyhr-Jensen and Brix, 1996). In the present study,
the ammonium uptake kinetics were significantly affected by the treatments. In particular
the treatment with low N and low P supply deviated from the rest. The maximum uptake
capacity, Vmax , was lower, and the affinity for ammonium was higher, as indicated by the
lower K1/2 value. While a high capacity for nutrient uptake, Vmax , is considered a valuable adaptation to nutrient rich conditions, a high affinity for nutrient uptake (i.e. low K1/2
and Cmin values) is an important adaptation to nutrient limited conditions. The results of
this study indicate that P. australis at the studied supply rates of N and P had a high uptake
capacity and low affinity for ammonium, conforming with adaptation to a high nutrient supply rate. However, the data also shows that ammonium uptake kinetics of P. australis are
somewhat plastic and can be modified in response to nutrient availability. The Vmax values
recorded for P. australis were much higher than values reported for other wetland species:
Glyceria maxima: 4.6–10.3 ␮mol g−1 h−1 ; Phalaris arundinacea: 24.7–29.6 ␮mol g−1 h−1 ;
Typha latifolia: 19.9–28.0 ␮mol g−1 h−1 (Brix et al., 1994; Dyhr-Jensen and Brix, 1996).
This may partly be explained by the different nutrient levels used in the different studies, but
it may also indicate true differences between the species. The K1/2 values for P. australis
were higher than the values recorded for the other species, and the Cmin values at the same
level, corresponding with a lower affinity for P. australis than for the species cited above.
This would suggest that P. australis has a competitive advantage against the other wetland
species at high nutrient levels whereas the other species would be better competitors for ammonium at low nutrient levels. However, the distribution of wetland species in littoral zones
and other wetland habitats is not only determined by the nutrient uptake kinetics, but also,
and probably more so, with the individual species’ ability to cope with the prevailing water regime and the anoxic sediment conditions, among others (see e.g. Čı́žková-Končalová
et al., 1996). The relative importance of water regime, soil anoxia and nutrient supply on
growth and species distribution in wetlands is an important issue that needs further research.
We did not find any relation between root respiration and any of the uptake kinetic
parameters. As nutrient uptake is controlled by the activity of membrane-bound ATPases it
J.A. Romero et al. / Aquatic Botany 64 (1999) 369–380
379
could be argued that root respiration would be positively related to uptake capacity (Vmax ).
However, the overall range encountered in uptake capacity in the present study was probably
not sufficient to detect such a relation (if it exists), given the large variability in the data.
In conclusion, the results of the present study support the general impression of P. australis
being a species well-adapted for growth in nutrient-rich habitats. Growth responds to the
nutrient levels in the surroundings and is fastest at high and balanced nutrient supply rates
(molar N : P ratios between 10 and 33). However, the species is also somewhat plastic as
ammonium uptake kinetics is modified in response to growth at low nutrient levels. The
nutrient levels used in the present set-up did not cover either low extremes representative
of oligotrophic lake littoral zones nor high extremes representative of constructed wetlands
loaded with high-strength sewage. Therefore, in order to fully establish the ecological niche
of the plant and to what extent eutrophication and/or imbalanced nutrient conditions are
involved in the die-back phenomenon, additional studies are needed.
Acknowledgements
This study was funded by the Environment and Climate Programme of the European
Commission, contract No. ENV4-CT95-0147: ‘EUREED II’. We thank Dr. B.K. Sorrell
for constructive comments on the manuscript.
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