Aquatic Botany 61 (1998) 165±180
Growth and root oxygen release by Typha latifolia
and its effects on sediment methanogenesis
Dorthe N. Jespersen, Brian K. Sorrell1, Hans Brix*
Department of Plant Ecology, Institute of Biological Sciences, University of Aarhus, Nordlandsvej 68,
DK-8240 Risskov, Denmark
Received 7 August 1997; accepted 14 February 1998
Abstract
Growth of Typha latifolia L. and its effects on sediment methanogenesis were examined in a
natural organic sediment and a sediment enriched with acetate to a concentration of 25 mM in the
interstitial water. The lower redox potential and higher oxygen demand of the acetate-enriched
sediment did not significantly impede growth of T. latifolia despite some differences in growth
pattern and root morphology. Plants grown in acetate-enriched sediment were ca. 15% shorter than
plants grown in natural sediment, but the former produced more secondary shoots at earlier stages,
which resulted in similar total biomasses after 7 weeks of growth in the two sediment types. Plants
grown in acetate-enriched sediment had thicker and much shorter roots than plants grown in natural
sediment. This difference did not significantly affect the release of oxygen from the roots when
measured under laboratory conditions, which was 0.12±0.20 mmol O2 gÿ1 DW hÿ1. Enrichment
with acetate resulted in much higher sediment methanogenesis rates (643 vs. 90 nmol CH4 gÿ1
sediment DW hÿ1). Growth of T. latifolia significantly reduced methanogenesis in both types of
sediment, but the effect was twice as marked in the natural sediment (34%) as in the acetateenriched sediment (18%), although in absolute terms the reduction was higher in the enriched
sediment. The data suggest that this effect of plant growth was via root oxygen release and its effect
on redox conditions. In the natural sediment, oxygen release resulted in a significantly higher redox
potential and lower sediment oxygen demand, whereas there were no significant changes in the
acetate-enriched sediment. The very high oxygen demand of this sediment probably masked the
effect of root oxygen release so that a significant reduction in methanogenesis occurred without any
significant increase in the redox potential. This demonstrates how root oxygen release from plants
like T. latifolia can significantly alter rates of biogeochemical processes such as methanogenesis,
* Corresponding author. Tel.: 45 8942 4714; fax: 45 8942 4747; e-mail: [email protected]
1
Present address: National Institute of Water and Atmospheric Research Ltd., PO Box 8602, Riccarton,
Christchurch, New Zealand
0304-3770/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved
PII S 0 3 0 4 - 3 7 7 0 ( 9 8 ) 0 0 0 7 1 - 0
166
D.N. Jespersen et al. / Aquatic Botany 61 (1998) 165±180
even in sediments with high oxygen demands where this is not evident from instantaneous
parameters such as redox potential. # 1998 Elsevier Science B.V. All rights reserved
Keywords: Cattail; Aeration; Oxygen stress; Root development; Redox potential; Sediment oxygen demand
1. Introduction
Emergent aquatic macrophytes are intimately involved in the production, oxidation and
release of methane from freshwater wetlands (Cicerone and Oremland, 1988; Chanton
and Whiting, 1995). Most of the large amounts of organic matter produced by these
plants is decomposed anaerobically in the sediment, resulting in high rates of
methanogenesis and saturation of methane in the interstitial water (SchuÈtz et al., 1991;
Chanton and Whiting, 1995). Rates of methane production and consumption in sediments
are controlled by the relative availability of substrates for methanogenesis (acetate,
carbon dioxide and hydrogen) and methane oxidation (methane and oxygen), which
plants can affect through transpiration, organic carbon excretion from roots, and root
oxygen release (Dacey and Howes, 1984; Lambers, 1987; Armstrong et al., 1991).
Furthermore, sites colonized by emergent macrophytes generally have higher rates of
methane release to the atmosphere than unvegetated sites, due to internal gas transport
processes in plants, and therefore retain less interstitial methane (Chanton et al., 1993;
Sorrell and Boon, 1994; Brix et al., 1996).
Determining how, or indeed whether, macrophytes modify any particular aspect of
methane cycling in wetlands is not straightforward because resolving the various, often
conflicting effects they can have is fraught with difficulties. For example, the low
methane concentrations frequently observed in rhizospheres could result from internal
methane transport and release through the macrophytes, but also from effects of root
oxygen release. This rhizosphere oxidation could inhibit methanogenesis by regenerating
electron acceptors used by more energy-efficient bacteria (Conrad, 1989), or alternatively
allow methane oxidation (King, 1996; Lombardi et al., 1997). These issues prompted us
to investigate whether root oxygen released by Typha latifolia L., which has an effective
aeration system based on internal convective through-flow of gases (Bendix et al., 1994),
can reduce methanogenesis in the reducing, organic sediments in which it grows. This has
involved growing T. latifolia under conditions in which roots were the only possible
oxygen source for the sediment, and measuring methanogenesis in vitro on sediment
samples from the rhizosphere under conditions excluding the possibility of methane
oxidation.
2. Materials and methods
2.1. Sediment and plant material
Sediment rich in organic material was collected from Lake Brabrand, a natural wetland
near Aarhus, Denmark. After the sediment had been sealed in 10 l buckets for two weeks
D.N. Jespersen et al. / Aquatic Botany 61 (1998) 165±180
167
to ensure anoxic conditions, half of it was enriched with acetate to a concentration of
25 mM in the interstitial water in order to obtain substrate-saturating conditions for
methanogenic bacteria. The natural organic sediment (without acetate enrichment) will
henceforth be termed `low C sediment' whereas acetate-enriched sediment will be termed
`high C sediment'.
Seeds of T. latifolia were germinated in vermiculite flooded with a commercial nutrient
solution (Pioneer, Lyngby, Denmark). When the seedlings were 10±15 cm high, they
were transplanted into 20 cm height individual glass jars of 1565 ml volume filled with
the two sediments.
2.2. Experimental design
The interactive effects of sediment type (low C sediment vs. high C sediment) and
vegetation (planted vs. unplanted) were examined in a two by two factorial experiment
with five replicates per treatment. The planted jars contained one plant each. The jars
were placed in a plastic box, which was subsequently filled with de-oxygenated water to a
height of a few cm above the rim of the jars. Supplements of de-oxygenated water were
added as required throughout the incubation period in order to keep the water surface well
above the jars. To prevent light penetration, a piece of black plastic with holes for the
shoots was placed ca. 2 cm above the water surface. These precautions prevented oxygen
from entering the sediments by diffusing from the overlying water, by plant transpiration,
or by algal photosynthesis so that root oxygen release was the only possible source of
oxygen for the sediment.
The boxes were placed in a growth chamber (Weiss Technik GMBH, Lindenstruth,
Germany) for 11 weeks at 258C, 85% RH, and a light intensity of 300 mmol mÿ2 sÿ1 PAR
at the base of the plants. The chamber provided a 16:8 h light:dark cycle. To maintain
a high acetate concentration in the high C sediment, 30 ml of a 1 M sodium acetate
solution was distributed into the sediments every week using a syringe and a long
needle.
Shoot sizes and methanogenesis rates were measured approximately once a week
during the first seven weeks of plant growth. Sediment characteristics, including pH,
redox potential, and content of organic acids in the pore water, were measured after
seven weeks. To further test the influence of plant growth, the shoots were cut below the
water surface just above the sediment surface after eight weeks of growth in order to
block the gas transporting tissues of the plants. The experiment was continued for a
further three weeks after which methanogenesis rates, pH and redox potentials were
measured again.
2.3. Analyses
2.3.1. Plant morphology
The length of the longest leaf was measured weekly and the number of leaves on each
main shoot counted. Shoot dry weights of main and secondary shoots were determined in
week 8 when the shoots had been cut. Root and rhizome dry weights were determined in
168
D.N. Jespersen et al. / Aquatic Botany 61 (1998) 165±180
week 11 at the end of the incubation period. Dry weights were measured after drying for
24 h at 1058C.
2.3.2. Sediment characteristics
Loss on ignition of dried sediment was determined by combustion at 5508C for at least
9 h. The dried sediment's content of organic carbon was determined titrimetrically after
wet combustion in H2SO4/K2Cr2O7. Redox capacity was measured on fresh sediment by
titration with FeCl3 under anoxic conditions, using platinum electrodes and calomel
reference electrodes. All methods followed Schierup and Jensen (1981). The contents of
dissolved organic acids in the sediment interstitial waters were determined on pore water
samples obtained by pressing 1 ml sediment samples through a 0.2 mm filter. The pore
water samples were then analyzed by HPLC using a UV-VIS detector and a Supelcogel
C610-H column (LC-6A, Shimadzu, Kyoto, Japan).
Since measurements of pH and redox potential caused major disturbance of the
sediment in the jars, they were only performed twice: after seven weeks of plant growth
and at the end of the experiment (week 11). Measurements of pH were taken with a
Radiometer pH electrode at a depth of approximately 4 cm in each jar. A platinum
electrode, referenced against a saturated calomel electrode, was used to measure redox
potentials at 0, 6 and 13 cm depth in each jar (max. depthˆ15 cm). The electrodes were
allowed to stabilize for 10 min before each reading. Quinhydrone solutions with known
Eh were used for calibration of the electrodes (Sùndergaard and Riemann, 1979). All
measurements were converted to E7 to allow comparisons between the sediments.
2.3.3. Methanogenesis
Three sediment subsamples (ca. 5 ml) were taken from various depths in each jar by
inserting a 7 mm diameter plastic tube connected to a syringe into the sediment. The
sampling was carried out carefully in order to minimize disturbance of the sediment and
avoid damaging the roots. The subsamples were transferred to 30 ml incubation flasks
and moved as quickly as possible to an anaerobic chamber (COY Laboratory Products,
Michigan, USA) where 2 ml of de-oxygenated cysteine solution (0.03%) were added. The
flasks were then flushed for 1 min with gaseous N2 in order to remove pre-existing
methane and ensure anoxic conditions. They were sealed with plastic caps fitted with
teflon-lined septa and incubated in darkness in a shaking water-bath at 258C for about
24 h. Headspace samples (1.5 ml) were withdrawn by syringe and needle over time to
monitor the development of methane in the headspace. One ml of these samples were
injected into and analyzed on a gas chromatograph equipped with a flame ionization
detector (GC-8A, Shimadzu, Kyoto, Japan). Before sampling, the incubation flasks were
shaken thoroughly to ensure equilibrium between headspace and sediment methane
concentration. After each sampling, 1.5 ml N2 gas were injected into the incubation flasks
to compensate for the amount of headspace removed. Following a lag phase of about
5±7 h, the methane concentration in the flasks increased linearly with time (r2 generally
>0.98), and rates were estimated from the slope of methane concentration against time
during this linear period.
On completion of the incubations, headspace volumes were determined and sediment
dry weights measured after drying for 24 h at 1058C. Methanogenesis rates were
D.N. Jespersen et al. / Aquatic Botany 61 (1998) 165±180
169
expressed on dry weight bases. The methanogenesis rate for each single jar was an
average of the three sediment subsamples taken from each.
2.3.4. Root oxygen release
To confirm that oxygen was released from the roots and examine whether this release
was influenced by sediment type, additional plants were grown in separate jars in low C
and high C sediments for four weeks. Plants of similar size were selected from the two
sediment types. Oxygen release from the roots was examined using a titanium(III) citrate
buffer, which allows root oxygen release measurements in a reducing, oxygen-scavenging
solution with a low redox potential (Kludze et al., 1993; Sorrell and Armstrong, 1994;
Kludze and DeLaune, 1996). After the plant roots had been carefully washed clean of
sediment, they were immersed in 300 ml nutrient solution (Pioneer, Lyngby, Denmark) in
a conical flask. The solution was sparged with N2 gas for 45 min to remove any oxygen
dissolved in the solution, and a 10 mm thick layer of paraffin oil was placed on top of the
solution to prevent re-aeration from the atmosphere. The basal part of the shoot was
wrapped with Parafilm to prevent the oil from infiltrating the aerenchyma. Sparging with
N2 gas continued while 15 ml of a titanium(III) citrate stock solution (89.9 mM, pH 5.9)
were injected with a syringe, the stirring from the sparging being necessary for complete
mixing. Blank flasks without plants were prepared similarly. The flasks, which were
covered with foil to prevent light penetration, were incubated for 12 h in a growth
chamber at 208C, 85% RH, and 300 mmol mÿ2 sÿ1 PAR. As the blue titanium(III) citrate
solution gradually became clear during oxidation, small samples of the solutions in the
incubation flasks were taken regularly with a small syringe and the absorbance at 527 nm
measured immediately on a spectrophotometer (UV-1201, Shimadzu, Kyoto, Japan). The
flasks were gently shaken immediately before sampling to even out any colour gradients
around the roots. The absorbances of the samples were compared to those of solutions
with a known concentration of Ti3‡. After an equilibration phase of 4 h, the blanks were
stable for the following 4±5 h during which the concentration of Ti3‡ in the flasks with
plants decreased linearly (r2 on average 0.90). Because the oxidation of Ti3‡ is
stoichiometric, rates of root oxygen release could be calculated from the rate of decrease
in the concentration of Ti3‡ in the incubation flasks. After the incubation, length and
mean diameter of each root were measured and the surface area of the main roots of the
root systems calculated. The percentage of root length that was covered with laterals was
also measured, but the surface area of laterals was not estimated. Root dry weights were
determined after drying for 24 h at 1058C.
2.4. Statistical analyses
Plant characteristics and data on root oxygen release were tested in two-tailed t-tests.
Homogeneity of variance was tested with Cochran's C tests, which showed that
heterogeneity of variance was significant in the data on methanogenesis, redox potential
and organic acids. In these cases, a two-way ANOVA was applied to logarithmically
transformed data. For the remaining data, ANOVA was applied directly. Significant
differences between individual means were distinguished by Tukey tests. Overall effects
of time, sediment type and presence/absence of plant (plants) on methanogenesis were
tested by a three-way ANOVA on logarithmically transformed data.
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D.N. Jespersen et al. / Aquatic Botany 61 (1998) 165±180
3. Results
3.1. Plant morphology
There were significant differences between plants grown in the two types of sediment.
Leaves of high C plants were generally narrower, thinner, and darker green than leaves of
low C plants. Maximum leaf lengths of low C and high C main shoots were similar after
two and three weeks of incubation, but thereafter leaves of low C main shoots were
significantly longer than leaves of high C main shoots (P<0.01; Table 1). The numbers of
live leaves were similar on low C and high C plants throughout the seven weeks of plant
growth. After the third week of incubation, the plants started producing secondary shoots,
which were more frequent in the high C sediment at earlier stages (Table 1). The biomass
of low C main shoots was significantly higher than that of high C main shoots, but
biomasses of secondary shoots, total aboveground biomass, and biomass of roots and
rhizomes did not differ significantly between low C and high C plants (Fig. 1).
Observations made on plants from the root oxygen release experiment showed that the
highly reducing high C sediment affected root morphology (Table 2). The surface area of
the main roots did not differ significantly between low C and high C plants and neither
Table 1
Characteristics of Typha latifolia main shoots grown for 7 weeks in a natural organic sediment (low C plants)
and in a sediment enriched with acetate (high C plants)
Time
(week)
Max. leaf length (cm)
Low C plants
2
3
4
5
7
54.6
67.4
74.8
77.6
79.0
a
(15.0)
(11.3) a
(6.9) b
(4.4) b
(3.2) b
# of leaves per shoot
High C plants
48.0
55.2
59.8
65.6
68.8
a
(8.8)
(9.7) a
(6.5) a
(1.5) a
(2.8) a
Low C plants
7.6
8.4
7.8
7.6
8.0
a
(1.8)
(1.3) a
(1.1) a
(0.5) a
(0.7) a
Proportion of plants with
secondary shoots
High C plants
8.8
9.2
7.4
7.6
7.6
a
(0.8)
(0.8) a
(0.9) a
(1.1) a
(0.5) a
Low C plants
High C plants
0/5
1/5
1/5
1/5
4/5
0/5
2/5
3/5
4/5
4/5
Numbers in brackets are 1 SD; nˆ5; for each parameter, means within row with different letter superscripts are
significantly different (P<0.05).
Table 2
Root morphology of Typha latifolia grown for 4 weeks in a natural organic sediment (low C plants; nˆ4) and in
a sediment enriched with acetate (high C plants; nˆ5)
Parameter
Low C plants
ÿ1
Number of roots (# plant )
Mean root length (cm)
Mean root diameter (mm)
Surface area of main roots (cm2 plantÿ1)
Lateral root coverage (% of root length)
Root biomass (mg DW plantÿ1)
a
49 (18)
16.2 (7.0) b
0.87 (0.21) a
223 (107) a
70 (30) b
131 (77) a
High C plants
58 (16) a
4.4 (3.0) a
1.16 (0.43) b
97 (31) a
34 (36) a
153 (48) a
Numbers in brackets are 1 SD; means within row with different letter superscripts are significantly different
(P<0.05).
D.N. Jespersen et al. / Aquatic Botany 61 (1998) 165±180
171
Fig. 1. Biomass of main and secondary shoots, total aboveground biomass, and biomass of roots and rhizomes
of Typha latifolia grown for 7 weeks in a natural organic sediment (low C plants; open bars) and in a sediment
enriched with acetate (high C plants; solid bars). Error bars are 1 SD; nˆ5. Significant differences within plant
part are indicated by $ (P<0.05).
did the number of roots per plant, but there were significant differences in mean root
length and mean root diameter of plants grown in the two types of sediment. The main
roots of low C plants were longer and thinner than the roots of high C plants (P<0.001),
and roots of low C plants bore laterals on twice as high a proportion of each root as high
C plants (P<0.001). However, the laterals on high C plants tended to be longer and more
densely packed than laterals on low C plants.
3.2. Root oxygen release
Roots from both low C and high C plants released oxygen into the medium, as the Ti3‡
was oxidized only in the flasks that contained plants. The root weight specific oxygen
release rates tended to be higher for the low C plants than the high C plants, whereas the
opposite was observed for rates expressed on a root surface area basis (Table 3). These
differences were, however, not statistically significant. It should be noted that the root
surface area specific release rates were overestimated, as the surface areas of laterals were
not included in the calculations.
Oxygen release from the roots was also visible as oxygenated rhizospheres in the
planted sediments. After four weeks of growth, the black high C sediment was slightly
grey in the areas just around the roots, but the oxidation was more noticeable in the
greyish-black low C sediment where the oxygenated zones were up to four to six times as
wide, with conspicuous reddish-brown areas in the otherwise light grey rhizospheres.
172
D.N. Jespersen et al. / Aquatic Botany 61 (1998) 165±180
Table 3
Oxygen release rates from roots of Typha latifolia previously grown for 4 weeks in a natural organic sediment
(low C plants; nˆ4) and in a sediment enriched with acetate (high C plants; nˆ5)
Sediment type
Low C
High C
O2 release rate
Root weight specific
(mmol O2 gÿ1DW hÿ1)
Root surface area specific
(mmol O2 mÿ2 hÿ1)
0.20 (0.16) a
0.12 (0.03) a
1.26 (1.08) a
1.79 (0.30) a
Numbers in brackets are 1 SD; means within column with different letter superscripts are significantly different
(P<0.05).
3.3. Sediment characteristics
Sediment characteristics and the results of the analysis of variance performed on the
data are presented in Tables 4 and 5, respectively. Loss on ignition did not differ between
treatments, and the content of organic carbon was not influenced by the type of sediment
but was significantly increased by plant growth. Redox capacity was about 14% higher in
the high C than in the low C sediment (Table 4). Overall, plant growth did not affect
redox capacity significantly, but the Tukey test showed that there was a significant
difference between low C sediment with and without plants, the capacity being lower in
sediment with plants.
Redox potentials differed markedly between the two sediment types. The high acetate
concentration in the high C sediment led via increased microbial activity to a highly
reducing sediment with an E7 of ÿ170 to ÿ220 mV (Table 4). The redox potentials in
high C sediments were significantly lower than those in low C sediments in both week 7
and 11. After 7 weeks of plant growth, there was significant interaction between the
effects of plants in the two types of sediment. Plant growth significantly increased the
redox potentials at 6 and 13 cm depth, on average from ÿ7 to ‡160 mV in low C
sediment and from ÿ190 to ÿ160 mV in high C sediment, but the increase in the high C
sediment was not significant in the Tukey test. In week 11, three weeks after the shoots
had been cut, the effect of the former plant growth was no longer significant and neither
was the interaction.
After seven weeks of plant growth, pH differed significantly between treatments, and
the interaction between sediment type and plants was significant (Tables 4 and 5).
Generally, pH was higher in the high C sediment, and in both sediments plant growth led
to a pH decrease, probably because of root respiration and proton release connected with
uptake of cations and/or microbial nitrification in the rhizosphere. Three weeks after the
shoots had been cut, in week 11, the difference in pH between the two sediment types still
prevailed, as did the effect of the earlier plant growth, but now the pH decrease in the low
C sediment was not significant in the Tukey test. The interaction between sediment type
and plants was still significant showing that the effect of the earlier growth of plants on
pH was dependent on the type of sediment.
The total concentration of organic acids in the interstitial water was significantly
higher in the high C sediment probably mainly because of the addition of acetate and the
209 (48) b
ÿ55 (63) a,
ÿ42 (42) b
6.82 (0.11) a
7.01 (0.05) a
208 (15) b
ÿ9 (8) b
ÿ34 (15) b
7.20 (0.05) b
7.20 (0.05) a
131 (73) b
98 (58) a
25 (37) a
ND
ND
0 cm depth
6 cm depth
13 cm depth
Redox potential, E7 (mV), week 11
pH, week 7
pH, week 11
Lactic acid (mmol lÿ1)
Formic acid (mmol lÿ1)
Acetic acid (mmol lÿ1)
Propionic acid (mmol lÿ1)
Butyric acid (mmol lÿ1)
27 (28) a
446 (676) a
88400 (38700) b
474 (61) a
163 (41) a
8.46 (0.06) d
8.72 (0.15) c
ÿ74 (55) a
ÿ188 (26) a
ÿ217 (11) a
ÿ42 (23) a
ÿ171 (11) a
ÿ206 (14) a
20.8 (0.7)
8.2 (1.2) a
5.66 (0.25) c
b
22 (32) a
109 (33) a
60800 (17300) b
416 (171) a
83 (57) a
7.89 (0.07) c
7.89 (0.17) b
23 (45) a, b
ÿ132 (22) a,
ÿ173 (13) a
29 (118) b
ÿ146 (12) a
ÿ174 (8) a
20.4 (0.7) a
10.3 (0.5) b
5.66 (0.08) c
Planted
Parameters were generally measured after seven weeks of growth of Typha latifolia (week 7). Sediment pH and redox potential were also measured three weeks after
harvest of the shoots (week 11). Numbers in brackets are 1 SD; nˆ5; NDˆnot detectable. Means within row with different letter superscripts are significantly different
(P<0.05).
150 (31) b
93 (35) a
ND
ND
ND
348 (47) c
188 (78) c
127 (94) c
323 (76) c
5 (11) b
ÿ20 (11) b
0 cm depth
6 cm depth
13 cm depth
Redox potential, E7 (mV), week 7
b
20.7 (1.7)
10.1 (0.9) b
4.78 (0.30) a
22.0 (1.0)
9.8 (0.8) a, b
5.15 (0.06) b
Loss on ignition (% DW)
Total organic carbon (% DW)
Redox capacity (meqFe3‡ gÿ1DW)
a
Unplanted
a
Unplanted
Planted
High C sediment
Low C sediment
a
Parameter
Table 4
Sediment characteristics and concentrations of organic acids in the interstitial water of a natural organic sediment (low C sediment) and of a sediment enriched with
acetate (high C sediment)
D.N. Jespersen et al. / Aquatic Botany 61 (1998) 165±180
173
174
D.N. Jespersen et al. / Aquatic Botany 61 (1998) 165±180
Table 5
Results of two-way ANOVAs for sediment characteristics measured in two types of sediment (low C and high C
sediment) with and without growth of Typha latifolia
Parameter
Source of variance
Main effects
Interaction
Sediment
Plants
Sediment Plants
0.1601
0.1053
0.0000
0.0959
0.0107
0.0566
0.3630
0.0368
0.0608
0.0000
0.0000
0.0000
0.0000
0.0021
0.0000
0.0548
0.0000
0.0000
0.0849
0.8264
0.4972
0.0637
0.0000
0.0000
0.0817
0.5182
0.9081
pH, week 7
pH, week 11
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
Total organic acids
0.0000
0.5604
0.2770
Loss on ignition
Total organic carbon
Redox capacity
Redox potential, week 7
Redox potential, week 11
0 cm depth
6 cm depth
13 cm depth
0 cm depth
6 cm depth
13 cm depth
All measurements were taken after seven weeks of growth except for pH and redox potential, which were also
measured three weeks after harvest of the shoots (week 11). P-values are the probability of a greater F-value.
Bold indicates P<0.05.
associated microbial processes. Concentrations of formic acid were similar in the two
types of sediment, whereas lactic acid concentrations were higher in low C than in high C
sediment. The concentrations of all other analyzed acids were significantly higher in the
high C sediment, with acetate accounting for far the greatest part. Plant growth had no
effect on the concentrations of any of the organic acids analyzed (Table 5).
3.4. Methanogenesis
Methanogenesis rates were significantly influenced by incubation time, type of
sediment, and presence/absence of plants, and all interaction terms were highly
significant (Table 6). Methanogenesis was seven-fold higher in the acetate-enriched
sediment than in the low C sediment (643 vs. 90 nmol CH4 gÿ1DW hÿ1) when averaged
over the entire incubation period (Fig. 2). Generally, methanogenesis rates in planted
sediments were lower than in unplanted sediments (Fig. 2). After two weeks of
incubation, no effect of plants could be detected in either of the sediments, but in weeks 3
and 4, plant growth significantly reduced the rate of methanogenesis in the low C
sediment but not in the high C sediment, as also indicated by the significant interaction
terms (Table 6). After 5 weeks of incubation, plant growth also reduced the rate of
methanogenesis in the high C sediment, and this effect prevailed in both sediments until
the plants were harvested. In the periods when plant growth influenced methanogenesis
significantly, rates were on average reduced by 34% (from 93 to 61 nmol
CH4 gÿ1DW hÿ1) in the low C sediment (weeks 3 to 7) and by 18% (from 818 to
D.N. Jespersen et al. / Aquatic Botany 61 (1998) 165±180
175
Table 6
Three-way analysis of variance (ANOVA) for methanogenesis rates measured during an incubation period of
seven weeks in two types of sediment (low C and high C sediment) with and without growth of Typha latifolia
(plants)
Source of variance
d.f.
Sum of squares
F-ratio
P
Main effects
Time
Sediment
plant
4
1
1
3.518
26.616
0.754
48.1
1454.7
41.2
0.0000
0.0000
0.0000
Interactions
Time sediment
Time plant
Sediment plant
Time sediment plant
4
4
1
4
3.282
0.679
0.582
0.974
44.8
9.3
31.8
13.3
0.0000
0.0000
0.0000
0.0000
80
1.464
Residual
Fig. 2. Methanogenesis rates in a natural organic sediment (low C sediment) and a sediment enriched with
acetate (high C sediment) with and without growth of Typha latifolia. Rates were measured during seven weeks
of growth and three weeks after harvest of the shoots (week 11). Low C sediment without plants (*), low C
sediment with plants (*), high C sediment without plants (&), high C sediment with plant (&). Means1 SD;
nˆ5.
667 nmol CH4 gÿ1DW hÿ1) in the high C sediment (weeks 5 to 7). There was no
significant relationship between the methanogenesis rates from week 7 and root biomass
for low C plants (Pˆ0.52), but for high C plants there was a highly significant decrease in
methanogenesis rate of ca. 170 nmol CH4 gÿ1DW hÿ1 per g increase of root DW
(P<0.001; data not shown).
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D.N. Jespersen et al. / Aquatic Botany 61 (1998) 165±180
Three weeks after the shoots had been cut, in week 11, the methanogenesis rates in
high C sediment remained significantly higher than the rates in low C sediment (P<0.001;
Fig. 2). The prior reduction of the methanogenesis rates by plant growth had disappeared
(Pˆ0.10), and there was no interaction between sediment and former plant growth
(Pˆ0.10).
4. Discussion
Although aquatic macrophytes survive in flooded soils by virtue of their internal gas
transport and the ability of their rhizomes to survive prolonged hypoxia and anoxia
(Armstrong et al., 1991; Crawford, 1992), these strategies do not provide unlimited
tolerance of highly reducing sediments. The growth of many aquatic plants is inhibited as
sediments become increasingly reducing (Kludze and DeLaune, 1996; Pezeshki et al.,
1996), and even where total biomass accumulation is unaffected, changes in morphology
provide clear evidence of the stress suffered. The shorter, smaller shoots and increased
production of secondary shoots in the high C plants in this study almost certainly result
from effects of the more reducing sediment. They were accompanied by differences in
root morphology known to be caused by oxygen stress in highly reducing sediments, such
as shorter, thicker roots and reduced lateral root formation (Kludze et al., 1993;
Armstrong et al., 1996). Short, thick roots are favoured when plants are under oxygen
stress because they have low axial resistances to oxygen diffusion, whereas lateral roots
are more difficult to support, having limited oxygen transport capacities due to their
narrow diameter and low porosity (Armstrong et al., 1990; Sorrell, 1994). Hence,
although T. latifolia produced similar total biomass in the low C and high C sediments,
the smaller individual shoots and roots are a similar negative response to sediment
oxygen demand to that seen in other emergent macrophytes (e.g. Kludze and DeLaune,
1996; Pezeshki et al., 1996).
Whilst inhibition of growth is a general response of most aquatic macrophytes to
increasing sedimentary decomposition rates, some taxa are far more sensitive than others,
due to poorer internal oxygen transport. Relative to many genera of wetland plants, Typha
species have well-developed internal gas transport pathways with low internal resistances,
and effective gas transport physiology (Brix et al., 1992; Bendix et al., 1994), and they
tolerate deep water and reducing sediments well (Grace, 1988; Callaway and King,
1996). In particular, high rates of convective gas flow in the shoot systems of emergent
macrophytes are associated with the ability to colonize deep water and reducing
sediments, and all Typha species have efficient convective gas flow (Brix et al., 1992;
Chanton et al., 1993; Bendix et al., 1994). This also indirectly benefits root aeration
(Armstrong et al., 1992), which may partly explain the ability of T. latifolia here to
maintain its root biomass in the high C sediment. Our data therefore agree with earlier
studies suggesting a high ability to avoid sediment oxygen stress by internal gas transport
in Typha species. However, T. latifolia does appear to have less efficient convective gas
flow and root aeration than the more narrow-leaved Typha species (T. angustifolia L. and
T. domingensis Pers.), which is one of the factors thought to restrict it to shallower water
when competing with them (Grace, 1988; Tornbjerg et al., 1994).
D.N. Jespersen et al. / Aquatic Botany 61 (1998) 165±180
177
Differences in the extent to which aquatic macrophytes affect sediment biogeochemistry are related primarily to the extent of their root development, with only those species
with extensive root systems affecting sediments to any degree (Chanton and Whiting,
1995; Wigand et al., 1997). Large emergent macrophytes such as T. latifolia, with their
prolific root development, often dramatically alter nutrient concentrations and
biogeochemical parameters through a range of physiological processes (Dacey and
Howes, 1984; Boon and Sorrell, 1991). Their effects on redox-dependent processes are
complicated by the fact that oxygen can enter sediments via mechanisms other than root
oxygen release (Dacey and Howes, 1984). By excluding other possible factors in this
study, we have demonstrated the ability of T. latifolia's root oxygen release to affect
biogeochemical processes, supporting claims that root oxygen release can be a significant
oxygen input into sediments colonized by emergent plants (Boon and Sorrell, 1991;
Armstrong et al., 1990; 1992).
Our measurements of root oxygen release by T. latifolia were at the upper end of
comparable data on whole plants from earlier studies (Kludze et al., 1993; Sorrell and
Armstrong, 1994; Kludze and DeLaune, 1996). These interspecific differences in rates of
root oxygen release can result from differences in root development, morphology and
porosity (Smits et al., 1990; Kludze and DeLaune, 1996; Wigand et al., 1997).
Environmental control of root morphology can also cause considerable intraspecific
variation in root oxygen release, as in this study, with the high C plants having shorter
roots that released less oxygen per unit dry weight than the low C plants (Table 3),
although this difference was not statistically significant. Laterals, with their high surface
area:volume ratios, are important sites for root oxygen release (Armstrong et al., 1991;
Sorrell, 1994). The greater similarity of root oxygen release rates between the low C and
high C plants when expressed on a surface area basis therefore provides further evidence
that the amount of permeable surface area of the root system is the major morphological
factor limiting root oxygen release, although the surface area of laterals were not included
in our calculations.
Some plants, however, show noticeably higher root oxygen release rates when grown in
more reducing sediments, as their root porosity increases in response to the higher
external oxygen demand (Kludze and DeLaune, 1996). In contrast, root porosity is little
affected by external oxygen demand in many wetland species that produce roots with
high porosities under all conditions (Justin and Armstrong, 1987), and our data suggest
that T. latifolia may be one of these.
It is a common observation that root oxygen release by aquatic macrophytes can effect
large changes in Eh and concentrations of oxidized compounds in oligotrophic sediments,
whereas these effects are rarely as evident in fertile, reducing sediments (Barko et al.,
1991). Rather than suggesting that roots do not release oxygen into more reducing
sediments, which is highly unlikely given that the oxygen demands of root and sediment
are competitive (Armstrong et al., 1991), this is normally interpreted as rapid
consumption of the oxygen released by the external oxygen demand (Sorrell and
Armstrong, 1994). Our data strongly support this, given that our high C plants released as
much oxygen as our low C plants, but had relatively little apparent effect on Eh.
Measurements of Eh and other indicators of soil aeration status, which provide
instantaneous measurements of relative concentrations of redox-active substances, reveal
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little about fluxes of materials and rates of processes in the rhizosphere. High rates of
oxygen flux from roots into reducing media can provide a narrow but active zone
favourable for aerobic processes (Roden and Wetzel, 1996; Lombardi et al., 1997), and
the reduction in methanogenesis in the vegetated sediment here is clear evidence of this.
These reductions were unlikely to have resulted from other effects of roots: the
methanogenesis rates in this sediment were apparently little affected by pH differences in
the jars, whilst the plants had no apparent effect on sediment organic acid contents.
The importance of root oxygen release in highly reducing sediments is evident from
the degree of reduction in methanogenesis in our data: although the reduction in the high
C sediment only averaged 18% whereas it was 34% in the low C sediment, this
corresponds to a total loss of 151 nmol CH4 gÿ1DW hÿ1 in the high C sediment but only
32 nmol CH4 gÿ1DW hÿ1 in the low C sediment. Given that the organic carbon and
acetate concentrations in our high C sediment were the same in both vegetated and
unvegetated treatments, this represents a substantial flux of organic carbon passing
through other, less anaerobic microbial pathways ± fluxes that are otherwise cryptic,
because of the high turnover rates and their physical restriction to the narrow, highly
active rhizosphere.
The high root to sediment ratios that develop in greenhouse studies of this type can
favour the oxidizing effect of roots in sediments more than may occur in the field
(Schipper and Reddy, 1996; Lombardi et al., 1997). This, together with the decrease in
oxygen release from roots as they age (Armstrong et al., 1990; Gilbert and Frenzel,
1995), suggests that our results are maximum estimates of the degree of methanogenic
inhibition effected by T. latifolia roots. This ought, however, to be viewed in the context
of the growth of this species as a rhizomatous perennial, where new rhizomes are forming
with dense, young roots in some areas of the sediment, whilst elsewhere older material is
decomposing and stimulating anaerobic processes (Sorrell et al., 1997). Sediments under
these plants are therefore likely to be extremely heterogeneous, and our understanding of
their nutrient cycling may be improved by better recognition of the mosaic of aerobic and
anaerobic sites they generate.
Acknowledgements
This study was funded by the Environment and Climate Programme of the European
Commission, contract no. ENV4-CT95-0147: ``EUREED: Dynamics and stability of
reed-dominated ecosystems in relation to major environmental factors that are subject to
global and regional anthropogenically-induced changes''.
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