Introduction
Since peatlands store approximately one-third of all terrestrial carbon (C),
they are important in the global C cycle (Gorham, 1991), and their C
dynamics have been studied throughout the world (Gorham et al., 2003;
Bortoluzzi et al., 2006; Golovatskaya and Dyukarev, 2009; Rowson et al.,
2010). Although it is well known that degraded and drained peatlands
generally are net C sources due to increased decomposition rates (Alm et
al., 1999; Waddington et al., 2001; Moore, 2002) – with net emissions
ranging from +80 to +880 g C m-2 yr-1
(Lamers et al., 2015; for all presented values
of C fluxes, positive values represent net C losses to the atmosphere,
whereas negative values represent net storage of C in growing peat
throughout the manuscript) – pristine, growing peatlands (mires) accumulate
C and are therefore considered to be C sinks (Belyea and Malmer, 2004).
The full greenhouse gas budget is, however, more complex. First, almost all
peatlands are sources of methane (CH4) (Moore and
Roulet, 1995; Saarnio et al., 2007), and second, not all pristine peatlands
appear to be sinks of carbon dioxide (CO2) (Waddington and Roulet,
2000; Riutta et al., 2007). For groundwater- or surface-water-fed
(minerotrophic) fens, CO2 fluxes have been reported to range from -208
to +190 g C m-2 yr-1 (Martikainen et al., 1995; Carroll and
Crill, 1997; Bubier et al., 2003), whereas for transitional mires, fluxes of
-124 to +58 g C m-2 yr-1 have been reported (Moore and
Knowles, 1987; Koch et al., 2008; Salm et al., 2009).
Transitional mires are examples of intermediate systems that display
characteristics of both minerotrophic fens and ombrotrophic bogs (Wheeler
and Proctor, 2000; Sjörs and Gunnarsson, 2002). Other examples include
edges of bog systems (lagg zones) influenced by surrounding surface water
and local patches influenced by percolating water (Giller and
Wheeler, 1988). Transitional mires often consist of floating peat
infiltrated by moderately base-rich water, which determines species
composition and stimulates buoyancy, through its effect on decomposition and
subsequent gas production (Lamers et al., 1999; Smolders et al., 2002).
Since they increase habitat heterogeneity at various scales, these
intermediate peatland systems often form hotspots of biodiversity
(Verberk et al., 2010). Transitional, floating mires are mainly
characterised by Cyperaceae and a moss layer of different Sphagnum species, whose dominance
strongly increases during succession (Du Rietz, 1954; Vitt and Chee, 1990;
Wheeler and Proctor, 2000). Sphagnum growth in transitional mires is, however, not as
straightforward as in bogs, since most Sphagnum species are sensitive to both high
pH and increased concentrations of calcium (Ca) and bicarbonate
(HCO3-) in pore water and surface water (Clymo, 1973). As
Sphagnum spp. lack stomata, water conducting tissue and roots, their growth,
nutrition and vitality depend on the chemical composition of the surrounding
water (Robroek et al., 2009). Despite Ca- and HCO3--rich
conditions, floating rafts in transitional mires may, however, still form
suitable habitats for Sphagnum species, since they are always water-saturated and
are fed by rainwater, which accumulates in the top (moss) layer and dilutes
the buffered surface water (Lamers et al., 1999; Smolders et al., 2003)
Sphagnum spp. strongly influence their environment and are thus important ecosystem
engineers in peatlands (Van Breemen, 1995). They are capable of
actively acidifying their habitat by exchanging cations for protons
(Clymo, 1963; Hajek and Adamec, 2009) and releasing organic acids
(Van Breemen, 1995). Furthermore, Sphagnum spp. keep their environment moist
due to the high water holding capacity of their hyaline cells (Clymo,
1973) and compact growth structure. By increasing the acidity and moisture
content of their habitat, Sphagnum spp. also slow down decomposition rates, thus
providing optimal conditions for the accumulation of organic material.
Moreover, the high concentration of phenolic compounds in their tissues,
including antibiotics (Verhoeven and Toth, 1995), further
decreases decomposition rates (Yavitt et al., 2000; Freeman et al.,
2001). This combination of traits results in a strong contribution of
Sphagnum mosses to C sequestration and peat formation worldwide (Coulson and
Butterfield, 1978; Limpens and Berendse, 2003).
Due to differences in habitat preference among Sphagnum species, they inhabit
different successional stages in peatlands (Vitt and Chee, 1990).
Since biomass production (Gerdol, 1995), acidification rates
(Kooijman and Bakker, 1994), decomposition rates (Rochefort et
al., 1990; Limpens and Berendse, 2003) and drought-tolerance (Nijp et
al., 2014) are species-specific, the species composition of the Sphagnum layer in
turn may strongly influence the biogeochemistry and C balance of their
habitat. This means that the C sequestration potential of the different
successional stages of peatlands may strongly depend on which Sphagnum species is
dominant at a particular stage. In transitional mires, the species composition will
strongly depend on pH, buffering components and water content. How the
Sphagnum species composition influences the biogeochemistry and C balance in
transitional mires, however, remains largely unknown.
Although a vast number of studies have presented field measurements of C
dynamics in all types of peatland systems, including transitional mires,
establishing the origin of the huge variation reported for both CH4 and
CO2 fluxes in these field studies is challenging. Studies on both C
dynamics and the influence of Sphagnum mosses using a controlled laboratory
approach, however, have not yet been performed to our knowledge. The goal of
this study was therefore twofold: first, to investigate the growth of
different Sphagnum species under controlled environmental conditions characteristic
for transitional mires, and second, to study C fluxes and their underlying
mechanisms in these systems. Four different Sphagnum species,
S. squarrosum, S. palustre, S. fallax and S. magellanicum, were grown on
peat floating on Ca- and HCO3--rich water. Besides growth parameters
of these mosses, we studied their contribution to the net C fluxes in these
potentially peat forming systems. We hypothesised that Ca- and HCO3--rich conditions would lead to considerable differences in performance
between the four Sphagnum species, based on differences in their tolerance to these
buffering components and in their growth rates. Furthermore, we expected
more tolerant Sphagnum species to strongly determine the C sequestration of these
systems.
Material and methods
Experimental set-up
Intact floating peat monoliths (25 × 25 cm; height 21.85 ± 2.08 cm;
n= 8) were cut from a floating mire in the southern Netherlands
(51∘24′6.1′′ N, 6∘11′10.5′′ E) in late March 2012. This
floating mire was dominated by helophytes species Typha latifolia and Calla palustris, whereas the moss
layer consisted mainly of Sphagnum fallax. After cutting, all vegetation was removed and
the bare peat was transferred to glass aquaria (25 × 25 × 30 cm; length × width × height)
in the field to minimise damage to the peat structure. The
peat had an organic matter content of 92.7 ± 0.4 % (determined by
loss on ignition; 3 h at 550 ∘C) and contained 3.6 ± 0.4 mmol kg-1
fresh weight (FW) of Ca (determined by digestion of 200 mg of dry
soil with 4 mL of HNO3 and 1 mL of H2O2 using a microwave
oven (MLS 1200 Mega, Milestone Inc., Sorisole, Italy), after which diluted
digestates were analysed by inductively coupled plasma spectrometry (ICP-OES
iCAP 6000; Thermo Fisher Scientific)).
In the laboratory, 6.25 L of Ca- and HCO3--rich treatment water was
added to each aquarium (Table 1), on which the peat floated. The underlying
water layer was subsequently refreshed with treatment water at a rate of 5 L week-1
using peristaltic pumps (Masterflex L/S, Cole-Parmer, Vernon
Hills, IL, USA). All floating peat monoliths received artificial rainwater
(Table 1) five times a week, at a rate corresponding to the Dutch annual
rainfall of 800 mm. During the experiment, the aquaria were kept in a water
bath maintained at 18 ∘C (up to a maximum of 23 ∘C at
the end of the day) using a cryostat (NESLAB, Thermoflex 1400, Breda, the
Netherlands). Furthermore, a light regime of 200 µmol m-2 s-1
(PAR; 16 h light/8 h dark) was maintained (Master Son-T Pia Plus,
Philips, Eindhoven, the Netherlands). This regime of temperature and light
was chosen to mimic summer conditions.
Composition of the infiltrating water and artificial
rainwater used in the experimental set-up. The rainwater composition was
based on the composition of Dutch rainwater. Note that all concentrations
are in µmol L-1, except for the sea salt addition, which is in
mg L-1.
Infiltrating water
Artificial rainwater
HCO3-
3000
–
SO42-
100
–
Cl-
8000
54
Ca2+
2000
17
Mg2+
2000
–
Na+
3000
–
K+
200
20
NH4+
–
36
NO3-
–
36
Sea salt (mg L-1)*
–
5
* Pro Reef, Tropic Marine, aQua united GmbH, Telgte, Germany.
On four floating peat monoliths, four different species of Sphagnum
(Sphagnum. squarrosum, S. fallax, S. palustre and S. magellanicum) were
planted together. S. squarrosum is a species of moderately rich fens and occurs in
environments with pH values up to 7 (Clymo, 1973). S. fallax, on the other
hand, can be quite sensitive to high pH or drought, but is also known for
its high potential growth rate under minerotrophic conditions (Buttler et
al., 1998). S. palustre is a widespread species found in habitats that are neither
highly calcareous nor highly acidic (Daniels and Eddy, 1990). S. magellanicum is a
species associated with poor fens and bogs, and it is restricted to a more
acidic habitat (Vitt and Chee, 1990; Hajek et al., 2006). The first three
species were collected in a peatland area in the north-western part of the
Netherlands (Ilperveld; 52∘26′42.5′′ N, 4∘55′45.1′′ E),
while the latter species was collected in an area in the south of the
Netherlands (Maasduinen; 51∘34′56.3′′ N,
6∘6′13.5′′ E). For all species, a patch of 50 ± 10 g
fresh material (1.6 ± 0.8 g DW; moss length 3 cm) was applied randomly
to one of the corners of the aquarium. Mosses were put upright in a patch of
approximately 50 cm2. The remaining four floating peat monoliths were kept
as non-vegetated controls.
Since soils were floating and not inundated, the “surface water” will be
called infiltrating water throughout this paper. This infiltrating water was
sampled underneath the peat monolith, while pore water was extracted using
10 cm soil moisture samplers (SMS Rhizons, Eijkelkamp, Giesbeek, the
Netherlands), which were inserted vertically into the soil. For each peat
monolith, two SMS rhizons were installed, and samples were taken by attaching
vacuum bottles. Analyses were performed on pooled samples to reduce the
effect of variation within the soil.
Chemical analyses
During the 12 weeks of the experiment, pH and total inorganic carbon (TIC)
concentration of infiltrating water and pore water were measured every 2
weeks (seven times in total). pH was measured with a standard Ag/AgCl electrode
(Orion, Thermo Fisher Scientific, Waltham, MA, USA) combined with a pH meter (Tim840
titration manager; Radiometer Analytical, Lyon, France). TIC was measured by
injecting 0.2 mL of sample into a compartment with 1 mL phosphoric acid (0.4 M)
in an infrared gas analyser (IRGA; ABB Analytical, Frankfurt, Germany),
after which concentrations of HCO3- and CO2 were calculated
based on the pH equilibrium. Concentrations of PO43-,
NO3- and NH4+ were measured colourimetrically on an
AutoAnalyser 3 System (Bran&Lubbe, Norderstedt, Germany) using ammonium
molybdate (Henriksen, 1965), hydrazine sulfate (Kamphake
et al., 1967) and salicylate (Grasshof and Johannse, 1972),
respectively. Concentrations of Ca, Fe, K, Mg, total P and SO4 were
analysed by inductively coupled plasma spectrometry (ICP-OES iCAP 6000;
Thermo Fisher Scientific).
Plant data
To preserve bare control soils and monocultures of the Sphagnum species, all
aboveground biomass of non-Sphagnum species was carefully removed every 2 weeks.
This vegetation consisted mainly of Typha latifolia and Juncus effusus seedlings. Every 2 weeks, growth
and expansion of the mosses were recorded. Mosses were allowed to grow
outside of their designated quarters to include the effects of competition
between species. After 8 weeks of moss growth, pH was measured within the
Sphagnum vegetation at 0.5–1 cm above soil level, using a pH meter (HQ 40d, Hach,
Loveland, CO, USA) and an Ag/AgCl pH electrode (Orion 9156BNPW, Thermo Fisher
Scientific, Waltham, MA, USA). After 12 weeks, all moss biomass was
harvested and the number of capitula (top 8–10 mm of the photosynthetically
active tissue of the mosses) was counted for each plot. Length of the moss
fragments was measured before living plant parts and dead parts were
separated and weighed. Biomass was dried for 48 h at 70 ∘C to
determine dry weight (DW). C and N contents (%) of dried moss material
were determined using an elemental analyser (Carlo Erba NA1500, Thermo
Fisher Scientific, Waltham, MA, USA).
Carbon fluxes
C fluxes were determined after 6 weeks of experimental treatments. Since the
cover of S. magellanicum had declined severely by this time, the remaining patch was too
small to cover with a closed chamber and the species was excluded from these
measurements. C fluxes from soils covered with S. squarrosum, S. palustre
or S. fallax and from bare control soils were measured in transparent and closed chambers (length, width and height of 10, 10 and 12 cm) for light and dark conditions, respectively. Dark measurements started at the end of the
8 h dark period and lights remained off during measurements, so that mosses
remained dark-adapted. Samples were taken from the headspace immediately
after placing the chambers on the aquaria, and subsequently after 2 and 4 h
using 1 mL syringes, which were first flushed with headspace. They
were analysed for CO2 using an IRGA (ABB Analytical, Frankfurt,
Germany) and for CH4 using a gas chromatograph (5890 GC, Hewlett
Packard, Wilmington, DE, USA). The slopes of the linear increases in both
gasses were used to determine areal net C fluxes for each Sphagnum species and for
bare peat. Measurements on CO2 and CH4 fluxes carried out under
light and dark conditions were combined to calculate daily C fluxes. Under
natural conditions, Sphagnum spp. tend to grow vertically, whereas in our experiment
– due to the absence of supporting neighbouring mosses – elongated moss
fragments fell over, causing an apparent horizontal growth. Our areal C
fluxes measured with the closed chambers covering only part of the elongated
fragments are therefore underestimates. We corrected for this by multiplying
the areal C fluxes with the ratio of the area covered by elongated
Sphagnum fragments and the area of the chamber.
Statistical analyses
All data were checked for normality of residuals and homogeneity of variance
using the Shapiro–Wilk test for normality and the Levene test of equality of
error variances, respectively. Differences in the chemical composition of
surface water and pore water of soils with and without moss cover were
analysed over time using linear mixed models. Differences between growth
parameters of Sphagnum mosses (Figs. 1 and 2, Table 3) and C fluxes were tested
using one-way ANOVAs with Tukey post hoc tests. In all tables and figures,
averages are presented with standard error of the mean (SEM). All
statistical analyses were carried out using SPSS for Mac (V21, IBM
Statistics).
Discussion
For Sphagnum species growing on top of floating peat monoliths, the influence of groundwater and surface water infiltration, buffered by Ca2+ -
HCO3-, into the peat was shown to be reduced, and even moderately
sensitive species were capable of growing under these conditions. It was
remarkable, however, to discover that while some of these species strongly
increased their biomass, Sphagnum-covered patches simultaneously showed a net C
efflux.
Tolerance of Sphagnum species to buffered conditions
Transitional mires are Sphagnum-rich systems characterised by the influence of
calcareous and alkaline surface water or groundwater in the subsoil and are
thus partly buffered systems. These environmental conditions are, however,
not limited to transitional mires and occur more widely, since local spots
with higher influence of groundwater or edges in contact with calcareous
surface water occur in many peatlands. The acid neutralising capacity (ANC)
of peatlands is mainly based on the presence of HCO3- and
Ca2+. When protons are released into a system, they are initially
buffered by the HCO3-–CO2 buffering system (Sherlock
et al., 1995; Lamers et al., 2015). Once most of the HCO3- has
been consumed, protons (H+) can be buffered by the cation-exchange
capacity (CEC) of the peat, where base cations bound to soil particles are
exchanged for H+ (Lamers et al., 2015).
Ca2+ is usually the main component of the CEC, since it is the dominant
divalent cation in many peatlands (Bache, 1984; Rippy and Nelson, 2007).
Several studies have indicated that Sphagnum can be sensitive to calcareous
groundwater and surface water due to Ca and HCO3- toxicity (Clymo,
1973; Andrus, 1986; Lamers et al., 1999; Hajek et al., 2006). This
sensitivity to one or both of the major buffering components of groundwater
and surface water is species-specific and it strongly affected the
performance of the Sphagnum species in our study. S. magellanicum appeared to be the most sensitive to
the tested conditions as this species decreased both in number of capitula
and biomass. S. fallax and S. palustre, on the other hand, increased in biomass, even though they
were obviously outcompeted by the better-adapted S. squarrosum.
S. squarrosum was able to increase
both horizontally, in number of capitula, and vertically, by stem
elongation.
S. squarrosum is one of the few Sphagnum species that is still vital in systems with a higher
influence of calcareous and therefore HCO3--rich water and is
even able to tolerate (temporary) immersion in these systems (Clymo,
1973; Vitt and Chee, 1990). Other species, including S. magellanicum, are known to be highly
sensitive to increased pH and buffered conditions in their habitat
(Clymo, 1973; Granath et al., 2010), which explains why S. magellanicum showed a strong
decrease in our study. The typical habitats of S. fallax, S. palustre and S. magellanicum are all characterised
by low pH (4.5–4.8) and low Ca2+ content (35–40 µmol L-1)
(Vitt and Chee, 1990; Hajek et al., 2006), although S. fallax and S. palustre can tolerate a
wider range of environmental conditions in terms of acidity and trophic
level than S. magellanicum (Daniels and Eddy, 1990). S. squarrosum, on the other hand, often occurs
in rich to moderately rich fens (Vitt and Chee, 1990; Hajek et al.,
2006), which are characterised by pH values of 5.1 to 6.7 and Ca2+
concentrations of 270–500 µmol L-1 (Vitt and Chee, 1990;
Kooijman and Bakker, 1994).
Succession of Sphagnum species
The transition of mineral-rich fens to acidic “poor fens” to oligotrophic
bogs is believed to be initiated by the acidification of pioneer Sphagnum species
(Wilcox and Andrus, 1987; Rydin and Jeglum, 2006; Granath et al., 2010).
These pioneer species are expected to tolerate mineral-rich conditions, have
a high growth rate and a high acidification capacity under more buffered
conditions, which will allow them to change a mineral-rich fen into an acidic
poor fen within a few decades (Granath et al., 2010). S. squarrosum may act as such a
pioneer species and is often responsible for rapid succession in fens
(Giller and Wheeler, 1988; Haraguchi et al., 2003), especially under
nutrient-rich conditions (Kooijman and Bakker, 1995).
Our data confirm that S. squarrosum potentially acts as a foundation species for other
Sphagnum spp. This species simultaneously increased its biomass considerably and
acidified its environment most effectively, lowering pH to values around 4.5
despite continuous infiltration of surface water with an alkalinity of 3 meq L-1,
while the other three species could not lower pH below 5.2.
Sphagnum species show differences in acidification rate, based on differences in
their cation-exchange capacity (Rippy and Nelson, 2007).
Additionally, however, Sphagnum acidification rates depend on their species-specific
performance under certain environmental conditions. High growth rates
combined with low decomposition rates (5–35 % mass loss yr-1;
Clymo, 1965; Coulson and Butterfield, 1978; Verhoeven and Toth, 1995;
Limpens and Berendse, 2003) result in a fast build-up of the peat layer and
succession in species composition, which, in floating transitional mires,
will slowly reduce the influence of the underlying calcareous water.
Carbon dynamics
Increase of the thickness of the peat layer due to Sphagnum growth shows that these
species can sequester a significant amount of C. Sphagnum biomass can increase by
approximately 70 to 600 g DW m-2 yr-1 (Gerdol, 1995; Graf and
Rochefort, 2009; Hajek, 2009; Samaritani et al., 2011), which corresponds to
a CO2 fixation rate of approximately 28 to 240 g C m-2 yr-1.
If we extrapolate the daily CO2 fixation rates of the
three growing species in our experiment, S. squarrosum, S. fallax and S. palustre, to calculate yearly
production rates, based on a growing season of 8 months, we find high
CO2 fixation rates of approximately 100–450 g C m-2 yr-1.
These values, however, overestimate actual field growth of these mosses,
since the experiment was carried out indoors under summer conditions only.
Still, even with these high CO2 fixation rates, we found net C
emissions from both bare peat and from peat covered with growing Sphagnum mosses.
Bare peat showed C emission rates of around 0.3 g C m-2 d-1
(Fig. 4), which consisted of 98 % CO2 and 2 % CH4. Both bare
peat and vegetated plots were a small source of CH4, with average
emission rates of 2 to 20 mg C m-2 d-1, which fall within the
range of 4 to 500 mg C m-2 d-1 usually reported for saturated
peatlands (e.g. Bartlett and Harris, 1993; Byrne et al., 2004; Saarnio et al., 2007; Salm et al., 2009). Still,
the contribution of CH4 to the greenhouse gas emission is much higher
in terms of CO2 equivalents, since the global warming potential of
CH4 is 34 times that of CO2 (IPCC, 2013). The
higher greenhouse gas emissions (as CO2 equivalents) from the plots
vegetated by S. squarrosum were, however, not due to differences in CH4 emissions,
but resulted from the much higher emissions of CO2 from these plots.
Origin and rates of C fluxes (in g C m-2 d-1) of
peat covered with different species of Sphagnum. Net C fluxes, gross C fixation
rates and gross C emissions are based on closed chamber measurements carried
out under light and dark conditions. Other fluxes are calculated using Eq. (1), with the fraction of autotrophic respiration based on the maximum value
found for Sphagnum respiration in the literature (31 %, Laine
et al., 2011).
Net C flux
Gross C fixation
Gross C emission
Bare peat
Autotrophic respiration
Additional HCO3-
(B+C-F)
(F)
(B+R+C)
(B)
(R)
-derived CO2 (C)
S. squarrosum
1.1 ± 0.2
1.9 ± 05
3.0 ± 0.7
0.3 ± 0.1
0.6
2.1 ± 0.7
S. fallax
0.5 ± 0.1
0.9 ± 0.2
1.5 ± 0.2
0.3 ± 0.1
0.3
0.9 ± 0.2
S. palustre
0.2 ± 0.1
0.4 ± 0.1
0.6 ± 0.3
0.3 ± 0.1
0.1
0.2 ± 0.3
Schematic overview of a transitional floating mire
influenced by HCO3--rich groundwater or surface water,
illustrated by dashed arrows in the figure above. Due to differences in the
thickness of the floating peat and the origin and composition of the
HCO3--rich water, there is high heterogeneity within these
systems. Part of the floating raft is shown in more detail below. Here, peat
soils are covered with different Sphagnum species. Rates of C fixation in peat
(downward arrow) and C emission to the atmosphere (upward arrows) are both
derived from C flux measurements and presented in g C m-2 d-1. As
the mosses showed differences in final biomass, higher or lower amounts of
biomass are depicted in the figure. Furthermore, the mosses differ in
acidification rate, with significantly higher amounts of acids produced by
Sphagnum squarrosum (left) than the other species. Since Sphagnum magellanicum declined severely in biomass due to
its sensitivity to the calcareous water, its C fluxes could not be measured
and the species was excluded from this figure.
When plots were vegetated by growing Sphagnum spp., CO2 emissions increased,
despite the accumulation of biomass by all three species (Fig. 4), which
indicates that the source of this CO2 could not solely be the
decomposition of Sphagnum litter. The only likely explanation for this remaining net
CO2 efflux is therefore the chemical reaction R (1) that occurs when
HCO3--rich water comes into contact with the acidifying mosses
(Fig. 4). The transition of HCO3- to CO2 is the first step in
the ANC of aquatic systems and will occur much faster than other buffering
mechanisms, such as cation exchange of Ca2+
(Lamers et al., 2015). Active acidification was
mainly observed in S. squarrosum, while S. fallax and S. palustre did not significantly lower pH more than the
dying S. magellanicum.
HCO3-+H+→H2O+CO2
To further disentangle the different CO2 sources responsible for the
net CO2 emission from plots vegetated with different species, we used a
mass approach (Eq. 1; Table 4). Net CO2 fixation was estimated based on
the difference between light and dark CO2 fluxes, whereas CO2
emission was estimated based on dark fluxes. This CO2 emission can be
further divided into separate contributors, as is shown in Eq. (1).
Net C flux to atmosphere=B+R+C-F
Here, B represents the CO2 flux from bare peat to the atmosphere, R is
the dark plant respiration, C represents the flux of chemically produced
CO2 according to Reaction (R1) and F is the gross CO2 fixation, calculated
as the light CO2 flux minus the dark CO2 flux. Bare peat
respiration was derived from dark fluxes of non-vegetated plots. For the
Sphagnum respiration factor R, we have used the maximum value (31 %) from the
range reported in the literature (12 to 31 % of photosynthetic C fixation,
Haraguchi et al., 2003; Laine et al., 2011; Kangas et al., 2014). As a
result, we obtain a conservative estimate of the C flux emitted through the
chemical Reaction (R1), driven by the acid production of the Sphagnum mosses (C).
Although the likely higher respiration rates during the light period and the
use of estimated Sphagnum respiration impede an exact quantification of factor C, the fact that we find CO2 emissions during the light period in
growing Sphagnum patches clearly points out that there is a considerable chemical
CO2 source.
Table 4 shows the different sources of the C fluxes as presented in Eq. (1).
Furthermore, the implications that these values have on the landscape scale
are depicted in a schematic overview of a floating transitional mire that is
being fed by HCO3--rich water (Fig. 4). Here, we show
simultaneous C fixation and C emission of the three growing Sphagnum species from our
experiment, with their different growth and acidification rates.
The production of HCO3--derived CO2 will occur in any
situation where HCO3--rich water comes into contact with an acidic
environment, such as in the highly acidic lower layers of floating bog
systems influenced by HCO3--rich water (Lamers et al., 1999;
Smolders et al., 2003). Therefore, CO2 effluxes measured from the
slightly acidic bare peat in our experiment are likely at least partially
derived from acid-driven CO2 production from HCO3-, as is
illustrated in Fig. 4. Our finding that the most strongly acidifying and
fastest growing mosses such as Sphagnum squarrosum show the highest C effluxes strongly
suggests that active acidification enhances the production of
HCO3--derived CO2.
This leads to the apparent contradiction that while growth of Sphagnum will lead to
accumulation of organic matter and thus contributes to the build-up of a
peat layer, it is accompanied by a large net efflux of CO2 ranging from
0.2 to 1.1 g C m-2 d-1 (Table 4, Fig. 4). While we show this
phenomenon here in a controlled laboratory setting, net CO2 effluxes
have indeed been reported for transitional mires, with rates ranging from
-0.34 to +0.16 g C m-2 d-1 (Moore and Knowles, 1987; Koch et
al., 2008; Salm et al., 2009). As mentioned before, however, this phenomenon
may not be limited to transitional mires. For example, bogs typically show
an outflow of acidic water (H+ and organic acids) and therefore
Sphagnum-produced acids may also cause chemical CO2 production outside the
peatland system, thereby counteracting at least a part of the C
sequestration realised by peat growth.