Pools are common features of peatlands and can represent from 5 % to 50 % of
the peatland ecosystem's surface area. Pools play an important role in the
peatland carbon cycle by releasing carbon dioxide and methane to the
atmosphere. However, the origin of this carbon is not well constrained. A
hypothesis is that the majority of the carbon emitted from pools
predominantly originates from mineralized allochthonous (i.e.,
plant-derived) dissolved organic matter (DOM) from peat rather than in situ
primary production. To test this hypothesis, this study examined the origin,
composition, and degradability of DOM in peat porewater and pools of an
ombrotrophic boreal peatland in northeastern Quebec (Canada) for 2 years
over the growing season. The temporal evolution of dissolved organic carbon
(DOC) concentration, the optical properties, molecular composition
(THM-GC-MS), stable isotopic signature (δ13C-DOC), and
degradability of DOM were determined. This study demonstrates that DOM, in
both peat porewater and pools, presents a diverse composition and
constitutes highly dynamic components of peatland ecosystems. The molecular
and isotopic analyses showed that DOM in pools was derived from plants.
However, DOM compositions in the two environments were markedly different.
Peat porewater DOM was more aromatic, with a higher molecular weight and
DOC : DON (dissolved organic nitrogen) ratio compared to pools. The temporal dynamics of DOC concentration
and DOM composition also differed. In peat porewater, the DOC concentration
followed a strong seasonal increase, starting from 9 mg L-1 and reaching a plateau above 20 mg L-1 in summer and autumn. This was
explained by seasonal peatland vegetation productivity, which is greater
than microbial DOM degradation. In pools, DOC concentration also increased
but remained 2 times lower than in the peat porewaters at the end of the
growing season (∼ 10 mg L-1). Those differences might be
explained by a combination of physical, chemical, and biological factors.
The limited hydraulic conductivity in deeper peat horizons and associated
DOM residence time might have favored both DOM microbial transformation
within the peat and the interaction of DOM aromatic compounds with the peat
matrix, explaining part of the shift of DOM compositions between peat
porewater and pools. This study did not report any photolability of DOM and
only limited microbial degradability. Thus, it is likely that the DOM might
have been microbially transformed at the interface between peat and pools.
The combination of DOM quantitative and qualitative analyses presented in
this study demonstrates that most of the carbon present within and released
from the pools originates from peat vegetation. These results demonstrate
that pools represent a key component of the peatland ecosystem ecological
and biogeochemical functioning.
Introduction
Northern peatlands constitute one of the most important terrestrial
reservoirs of organic carbon (C) containing about 530 ± 160 Pg C while
only covering ∼ 3 % of the global terrestrial land surface (Yu, 2012). The ecosystem carbon accumulation rates
of peatlands are typically greater than the losses to the atmosphere through
peat degradation and lateral transfer (Billett
et al., 2006, 2012; Blodau et al., 2007; Tunaley et al., 2017). Peatlands
are characterized by a mosaic of surface microforms, such as hummocks,
lawns, hollows, and pools (Charman, 2002). Considering peatlands as
a patchwork of microforms rather than a homogeneous ecosystem is critical to
accurately quantify their carbon balance and the role they play in the
modern global carbon cycle. Carbon dynamics between microforms are closely
related to the vegetation type and water table depth, which influence the
carbon dioxide (CO2) and methane (CH4) exchange with the
atmosphere (Nungesser, 2003; Chaudhary et al.,
2018). Among the different microforms, pools can constitute an important
proportion of peatland ecosystem surface areas, ranging from 5 % to 50 % (White, 2011; Pelletier et al.,
2014, 2015), and represent a net carbon source to the atmosphere (Pelletier et al., 2014). While most studies of peatland carbon dynamics have focused on terrestrial microforms (Nungesser,
2003; Pelletier et al., 2011; Shi et al., 2015; Chaudhary et al., 2018;
Graham et al., 2020), the composition and processes of production and
degradation of organic carbon in pools remain poorly documented.
The composition of dissolved organic matter (DOM) has previously been
documented in peatland porewater. A complex mixture of compounds with a
diversity of composition, functional groups, and ages seem to coexist (Tipping
et al., 2010; Kaplan and Cory, 2016; Raymond and Spencer, 2015; Tiwari et
al., 2018; Dean et al., 2019; Tfaily et al., 2018). The production of DOM in
peat porewater is controlled by vegetation productivity, peat temperature (Rydin et al., 2013; Kane et al., 2014), and
microbial activity (Worrall et al., 2008).
It has also been shown that pools can represent active compartments of
peatland ecosystems for DOM (Laurion
and Mladenov, 2013; Deshpande et al., 2016; Payandi-Rolland et al., 2020;
Folhas et al., 2020; Laurion et al., 2021) – a topic that has been less
studied. The DOM of pools may derive from surrounding terrestrial peat
(i.e., allochthonous) or be the result of their internal primary production
through phytoplankton and microbial production (i.e., autochthonous). In
both peat porewater and pools, DOM is affected by biodegradation processes
and by photodegradation in pools (Lapierre
and del Giorgio, 2014; Vonk et al., 2015). Changes in dissolved organic carbon (DOC) concentrations and
DOM composition are commonly observed and associated with a wide range of
degradation rates (Frey
et al., 2016; Payandi-Rolland et al., 2020; Moody and Worrall, 2021). The
composition and reactivity of DOM transferred from the terrestrial to
aquatic compartments of peatlands highly depend on the hydrology and
hydroclimatic context, as well as the biological and chemical processes occurring
during their transfer (Jaffé et al., 2012; Kaplan
and Cory, 2016). The DOM transfers between peatlands and aquatic ecosystems
are well documented for streams (Elder
et al., 2000; Billett et al., 2006, 2012; Austnes et al., 2010; Knorr, 2013;
Frey et al., 2016; Buzek et al., 2019; Dean et al., 2019; Rosset et al.,
2019) but more rarely for pools (Banaœ, 2013; Arsenault et al., 2019; Payandi-Rolland et al., 2020).
Differences in DOM composition and concentration between peat porewater and
pools have been observed, but the processes involved remain unclear (Schindler et
al., 1997; Payandi-Rolland et al., 2020). Studies conducted in temperate
peatlands have highlighted that the morphology (e.g., size, shape, depth, and
slope of banks) and surrounding vegetation influence the carbon content and
hydrochemistry in the pools (Banaœ, 2013; Arsenault et al., 2018, 2019). Others have explained the changes in
DOM composition in pools as the result of photodegradation and
biodegradation (Laurion
and Mladenov, 2013; Arsenault et al., 2019; Laurion et al., 2021). Studies
investigating the changes in DOM composition in peatland porewater and pools
have mostly been focused on temperate (Banaœ, 2013;
Arsenault et al., 2019), subarctic, and Arctic regions (Laurion
and Mladenov, 2013; Deshpande et al., 2016; Burd et al., 2020;
Payandi-Rolland et al., 2020; Laurion et al., 2021; Moody and Worrall,
2021), but there is no insight about changes in DOM compositions in boreal
peatlands not affected by permafrost.
Because DOM may derive from different sources and be subjected to various
processes of transformation and degradation, apprehending the complexity of
the origin, composition, and properties of the molecules that compose the
DOM is challenging. The use of complementary analytical methods is a good
approach to characterize DOM and attempt to understand its origin and
composition (Folhas
et al., 2020; Tfaily et al., 2013). The DOC : DON (dissolved organic nitrogen) elementary ratio is used to
estimate the extent of microbial processing of DOM (McKnight et al.,
1994; Autio et al., 2016). The absorbance and fluorescence are recognized
tools to estimate the average DOM molecular weight and aromaticity (Haan
and Boer, 1987; Weishaar et al., 2003; Helms et al., 2008), discriminate the
origin of DOM between microbial and plant sources (McKnight et
al., 2001; Cory et al., 2010), and highlight microbial degradation (Parlanti, 2000; Wilson and Xenopoulos, 2009). The
stable carbon isotopic signature of DOC (δ13C-DOC) can be used
to discriminate between terrestrial and aquatic plant-derived DOM and also
the extent of microbial processing of DOM (Elder
et al., 2000; Billett et al., 2012; Buzek et al., 2019). Finally, the
analysis of target molecules using THM-GC-MS allows the definition of
indicators of DOM sources and processing degradation stage (Jeanneau
et al., 2014, 2015; Kaal et al., 2017, 2020).
The aim of this study is to clarify the role that pools play in the
production, transfer, and transformation of organic carbon within peatland
ecosystems. We identified three possible scenarios. First, pools are
mineralization hotspots that can decompose the laterally exported fresh
organic matter from adjacent peat porewater. Second, pools represent passive
pipes that only collect the remaining refractory DOM laterally exported from
peat porewater. Third, pools represent a sub-ecosystem within the peatland
where both primary productivity and heterotrophic respiration exist. To
determine which scenario is the most accurate, we studied the spatiotemporal
variability of DOM over two growing seasons in a boreal ombrotrophic
peatland. The objectives of this study were to (i) identify the differences
in the origin, quantity, composition, and degradability of DOM between peat
porewater and pools; (ii) understand which factors induce those differences;
and (iii) clarify the contribution of DOM in pools to the peatland carbon
cycle.
Study site
The study site is located in northeastern Quebec, Canada, within the Romaine
River watershed (14 500 km2), adjacent to the Labrador border. It is located in the eastern spruce–moss bioclimatic domain of the closed boreal
forest (Payette, 2001) at the limit of the coastal plain and the
Highlands of the Laurentian Plateau of the Precambrian Shield (Dubois, 1980). The Bouleau peatland (unofficial name;
50∘31′ N, 63∘12′ W; altitude 108 ± 5 m) is an
ombrotrophic, slightly dome-shaped bog with a total surface area of 1.18 km2 (Taillardat et al.,
2022). Peat accumulation was initiated at ca. 9260 cal BP, and the maximum peat depth reaches 440 cm (Primeau and Garneau, 2021). The mean annual temperature is 1.5 ∘C, and the mean
annual precipitation is 1011 mm, of which 590 mm falls as snow. The average
monthly positive temperatures occur from May to October with 1915 growing
degree days above 0 ∘C (Havre-Saint-Pierre meteorological station,
mean 1990–2019; Meteorological Service of Canada and Environment and Climate Change Canada, 2019).
The surface microforms of the Bouleau peatland show a clear patterned surface of alternating dry hummocks, lawns, hollows, and pools. The surface vegetation varies according to the microtopography with Sphagnum fuscum, S. capillifolium, and Cladonia rangiferina on
hummocks; S. magellanicum, S. rubellum, S. cuspidatum, and Trichophorum cespitosum on lawns; and S. majus and S. pulchrum on hollows (Primeau and Garneau, 2021). Pools cover
9 % of the peatland surface area and are characterized by their elliptical morphology, steep banks, and slightly concave bottoms. Because of the steep
banks, no aquatic vegetation is observed along the edges of the pools.
Material and methodsWater sampling
Sampling was performed five times during the 2018 growing season (June,
July, August, September, and October) and four times in 2019 (June, August,
September, and October).
The peat porewater was sampled from six wells (P1 to P6, Fig. 1) located
along a topographic transect from the dome to the southern edge of the
peatland. Two meters long PVC wells were perforated and covered with a nylon
sock to avoid infilling by peat. They were inserted in peat to collect water
in the first 2 m of the peat column. This method allows collecting
the fluctuating water table which moves through the peat. In 2018, six pools
were chosen along a north–south axis giving a distance gradient to the stream
outlet (M01 to M06, Fig. 1). In 2019, the pools were divided into five class
sizes, and one pool was chosen into each class (M11 to M15, Fig. 1). The
pool sizes varied from 30 to 2065 m2 with a mean depth between 70 and
120 cm. Pools were sampled from their banks and from the surface of the
water column.
Aerial view of the Bouleau peatland with the location of the sampling sites (green dots: wells for peat porewater; yellow triangles: pools; blue diamond: stream outlet). The aerial photo was provided by Hydro-Québec.
The physicochemical parameters (temperature, pH, specific conductivity, and
dissolved oxygen saturation) of peat porewater and pool water were measured
using a multi-parameter portable meter (MultiLine Multi 3620 IDS, WTW,
Germany) at each sampling site and calibrated before each field visit. All
water samples were collected in clean polypropylene (PP) bottles and
filtered on pre-combusted (4h at 450 ∘C) GF/F filters (Whatman).
Samples collected during each field campaign and analyses performed are
synthesized in Table S1 in the Supplement.
Water-level and temperature monitoringInstrumentation
The six wells were equipped with a water-level data logger U20-001-04 in
2018 and replaced with a U20l-04 in 2019 (HOBO, Onset, USA) for continuous
measurements of the water table depth (WTD) and temperature from June 2018
to October 2020. Water temperature was recorded hourly in pools M11 to M15
using HOBO TMC50 probes coupled with a HOBO U12-008 data logger (Onset, USA)
from June 2019 to August 2020. A water-level data logger was installed in
pool M11 (Fig. 1), and water-level variations were measured from 20 May to 28 August 2020. Height variations (in cm) between the
peatland surface at wells P5 and P6 and adjacent pool M11 were measured using
a ZIPLEVEL Pro-2000 (Technidea, USA). Those measurements allowed the water
levels in the pool to be compared with those in the two wells (Fig. S2 in the Supplement).
An EXO2 multiparameter probe (YSI, USA) was installed at the outlet of the
peatland stream to record water temperature hourly, from June 2018 to August
2020.
Season definition
Samples from the 2 studied years were pooled according to seasons. In this
study, seasons were defined based on air and water temperatures measured at
the site (Fig. S3). Spring was defined by the end of the seasonal thaw
that occurred in May to the end of June. Summer included the months of July
and August when air and water temperatures were at their warmest. Finally,
the autumn season corresponded to the months of September and October when
air and water temperature decreased to zero.
Quantitative analyses
The filtered water samples (through GF/F filters) were prepared for DOC and
total nitrogen (TN) analyses by acidification to pH 2 with 1 M HCl and stored
in 40 mL glass vials. The DOC and TN concentrations were analyzed using the
catalytic oxidation method followed by the non-dispersive infrared (NDIR)
detection of the CO2 produced (TOC analyzer TOC-L, Shimadzu, Japan)
with limits of quantification of 0.1 mg C L-1 and 0.2 mg N L-1.
The samples were prepared for cation and anion analyses and stored in
high-density polyethylene (HDPE) vials without acidification. Those ions
(chloride, ammonium, nitrites, and nitrates) were analyzed by
high-performance liquid chromatography (HPLC) coupled with a Dionex
ICS-5000+ analyzer for anions (Thermo Fisher Scientific) and a Dionex
DX-120 analyzer for cations (Thermo Fisher Scientific).
The reference materials included ION-915 and ION 96.4 (Environment and
Climate Change Canada, Canada). The analyses were performed at EcoLab (UMR
5245 CNRS – UT3 – INPT, France).
Dissolved organic nitrogen (DON) corresponds to the difference between the
concentration of TN and the sum of concentration of inorganic nitrogen
(ammonium, nitrites, and nitrates).
Qualitative analysesStable isotopic analyses
Analyses of δ13C-DOC were realized on 41 samples selected from
peat porewater (n=20) and pools (n=21; Table S1) at the Ján Veizer
stable isotope laboratory (University of Ottawa, Canada) following the
method developed by Lalonde
et al. (2014). The samples were acidified to pH 2 with 1 M HCl and stored in
40 mL quality certified ultra-clean borosilicate glass vials. The first step
involved the catalytic oxidation of DOC followed by a solid-state
non-dispersive infrared (SS-NDIR) detection of the CO2 produced (OI
Aurora 1030C, Xylem Analytics, USA). The produced CO2 was passed
through a chemical trap and a Nafion trap prior to 13C isotopic
analyses using isotope-ratio mass spectroscopy (IRMS, Thermo Finnigan
DeltaPlus XP, Thermo Electron Corporation, USA). The results were
standardized with organic standards (KHP and sucrose), and the
13C /12C ratios were expressed as per mill deviations from the
international standard VPDB.
Optical analyses
The samples for ultraviolet–visible spectroscopy analyses were stored at 4 ∘C in glass vials following filtration on GF/F filters. The absorbance was
measured from 180 to 900 nm with a 5 nm resolution. The absorbance analyses
were performed on an Ultrospec 3100 instrument (Biochrom, United Kingdom) for 2018 samples
and on a Duetta instrument (Horiba, Japan) for 2019 samples, over a wavelength range from
190 to 900 nm at 2 nm intervals. All analyses were performed at the GRIL
laboratory (GRIL, UQAM, Canada). For comparison, 10 samples from the 2019
campaign were randomly selected and analyzed on both instruments, Duetta and
Ultrospec 3100. A pairwise t test revealed slight but significant
differences between absorbance at 254 nm from the two series (t=-3.9013,
df = 9, p value = 0.0036). As no significant effect was observed between
years on absorbance indices, no correction was performed on absorbance
spectra.
The absorbance indices were calculated to provide information about DOM
composition. Those indices were SUVA254 (L mg-1 m-1), which is
a proxy of the DOM's aromatic content (Weishaar et al., 2003); E2:E3 ratio; and spectral slope ratio (SR), which are proxies of the average
DOM molecular weight (Haan
and Boer, 1987; Helms et al., 2008).
In 2019, spectrofluorometric analyses were also conducted on Duetta (Horiba,
Japan) at the GRIL laboratory. Samples were excited at a range from 230 to
450 nm (at 2 nm resolution), and fluorescence was measured at a range from
240 to 600 nm (at a 5 nm resolution). Prior to the analyses, the samples
were diluted when necessary to maintain an absorbance intensity at 254 nm
below 0.6. A blank sample with Milli-Q water (Merck Millipore, Germany) was
measured prior to the sample analyses. The sample spectra were obtained by
subtracting the blank spectra to eliminate the Raman scatter peak. The
operation was conducted automatically by the analytical equipment.
Two indices were calculated to provide qualitative information on the
fluorescent fraction of the DOM: the fluorescence index (FI), lower values
(FI ≈ 1.4) of which indicate a plant origin while higher values (FI ≈ 1.9) indicate a microbial origin of DOM (Cory et al.,
2010; McKnight et al., 2001); and the β:α index, which is known
as a proxy of biological activity, and an increase in the ratio of which
corresponds to an increasing proportion of the recently produced DOM derived
from microbial activity (Parlanti, 2000; Wilson and
Xenopoulos, 2009).
Molecular analyses
Thermally assisted hydrolysis methylation–gas chromatography–mass
spectrometry (THM-GC-MS) was performed on 37 samples from peat porewater (n=18) and pools (n=19; Table S1). Those samples were selected to
include summer and autumn 2018 and spring, summer, and autumn 2019. The
THM-GC-MS analyses were conducted on freeze-dried samples from 100 mL of
water previously filtered on GF/F filters (Whatman) and followed the
procedure described by Jeanneau et al. (2015). One milligram of the sample was introduced into an 80 µL stainless-steel reactor with an excess of tetramethylammonium hydroxide (6 mg). The THM reaction was performed at 400 ∘C using a vertical
microfurnace pyrolyzer PZ-2020D (Frontier Laboratories, Japan). The reaction
products were injected into a gas chromatograph GC-2010 (Shimadzu, Japan)
equipped with an SLB-5ms capillary column in split mode (60 m × 0.25 mm i.d., 0.25 µm film thickness). The compounds were detected with a
mass spectrometer QP2010+ (Shimadzu, Japan) operating in full scan mode.
Analyses were realized at the Géosciences Rennes laboratory (UMR 6118 –
Univ. Rennes – CNRS, France).
For each chromatogram, the compounds were identified based on known m/z ratios
(Table S1) through comparison with the NIST library. The area of each
compound was integrated for each m/z and corrected by a mass spectra factor
(MSF). The MSF corresponds to the reciprocal of the integrated fragment
proportion and the entire related fragmentogram in the NIST library. The
relative proportion of each compound was calculated by dividing the compound
area (for all cumulated peaks) by the sum of total integrated compound areas
and expressed as a percentage.
All compounds were classified into five groups, and their relative
proportions were calculated: %CAR of carbohydrates compounds (derived
from both plant and microbial metabolism), %LMW_FA for low-molecular-weight fatty acids (derived from microbial metabolism),
%HMW_FA for high-molecular-weight fatty acids, %SOA for
small organic acids, and %Phenols for phenol markers (derived from plant
metabolism). The indices were calculated for each sample, derived from
molecular analyses, and presented as in Jeanneau et al. (2015). The C/V ratio
corresponds to the sum of coumaric and ferulic acids divided by the sum of
vanillic acid, vanillaldehyde, and acetovanillone. The deoxyC6 : C5 ratio is a
mixing model based on the proportion of deoxyC6 carbohydrates (derived
mainly from microorganisms) and the proportion of C5 carbohydrates (derived
mainly from plants). Values close to 0.5 suggested a dominant contribution
of plant-derived DOM, while values close to 2 corresponded to the
contribution of microbial-derived DOM (Rumpel and
Dignac, 2006). The last index corresponds to the proportion of plant-derived
markers, fVEG, which is the difference between the total markers and the
microbial-derived markers, fMIC. The fMIC corresponds to the proportion of
microbial carbohydrates multiplied by the total proportion of carbohydrates,
summed up by the proportion of microbial fatty acids, and multiplied by the
total proportion of fatty acids. The MIC : VEG index corresponds to the ratio
of microbial-derived markers divided by the proportion of plant-derived
markers.
Incubation of dissolved organic matterExperimental design
The objective of DOM incubation experiments was to test the sensitivity of
DOM to biodegradation and photodegradation and how it could affect its
composition. The incubation experiments were designed to test the effects of
temperature (in situ versus controlled) and total organic carbon versus
dissolved organic carbon on DOM degradation rates.
DOM from peat porewater and pools was incubated during three sampling
periods in 2019, from 7 to 13 June, 31 July to 7 August, and 4 to 10 September. An incubation time of 6 d had to be adjusted to 7 d during the last campaign due to
logistical constraints. Pool M11 was used to monitor the water level using a
barometric pressure sensor and was also sampled for incubations. The peat
porewater samples consisted of a mix of equal water volumes between five
different wells. This strategy was used because the water quantity in
piezometers was limited and not sufficient to perform all incubation
conditions.
The incubation experiments were performed on 100 mL of water filtered on
GF/F filters (F) and in unfiltered (UF) conditions. Amber borosilicate glass
vials of 125 mL were used to test biodegradation (BIO) only, and transparent
borosilicate vials of 125 mL were used for biodegradation and photodegradation
(BIO+PHOTO). Each condition was incubated in triplicates with a headspace
of 25 mL, and bottles were tightly closed. Considering the absence of
standardized incubation media between porewater and pools (Vonk et al., 2015), measured
biodegradation rates could be dependent on the abundance and the activity of
microorganisms in the samples from each environment.
For in situ incubations (IS), the peat porewater samples were placed 1–2 cm below
the water surface at the outlet of the peatland (Fig. 1), where water
temperature was recorded hourly with the EXO2 probe. The pool samples were
placed 1–2 cm below the water surface of pool M11 (Fig. 1). For controlled conditions (CC), the vials were placed in a dark room in a laboratory space at
Havre-Saint-Pierre, where the temperature was maintained between 18 and
20 ∘C and controlled twice each day. Both in situ and controlled
conditions started the same day. There is no value available for F conditions in pools in August due to variability between the incubated water
volume, suggesting that vial caps were loose.
Post-incubation analysis
In the end, samples incubated under UF conditions (n=18) were filtered
on a GF/F filter to analyze only the dissolved fraction. All samples (n=36) were prepared for DOC, TN, and inorganic N quantification, as well as absorbance
analyses, before and after the incubation experiments. The apparent removal
rate of dissolved organic carbon (RDOC), expressed in milligrams per day (mg d-1),
corresponds to the amount of DOC removed during incubation, reported per
day, and calculated following Eq. (1).
RDOC(mgd-1)=DOCpre-incubation-DOCpost-incubation/incubation time[DOC]pre-incubation(mgL-1):DOC concentration at the beginning of incubation[DOC]post-incubation(mgL-1):DOC concentration at the end of incubation
The degradation rates correspond to the proportion of DOC lost per day of
incubation and are expressed in percent of carbon per day (%C d-1) according to Eq. (2).
Degradationrate%Cd-1=DOCpre-incubation-DOCpost-incubationDOCpre-incubation×100/incubationtime
Changes in the DOC : DON ratio and absorbance indices were determined in
proportion to the initial values per day for the variable i following Eq. (3).
Δi(d-1)=ipost-incubation-ipre-incubationipre-incubation/incubationtimeΔi is the change of the variable i during the incubation, ipre-incubation is the initial value of the variable i at the beginning of
incubation, and ipost-incubation is the value of the variable i at the end of incubation.
Statistical analyses
All statistical tests were performed on R (CRAN project) through the RStudio
interface (RStudio inc., USA), and all figures were realized with the package
ggplot2 (Wickham, 2016).
Comparisons of variance tests were performed, and in the following sections,
the mention of significant differences refers to statistical tests using the
following method. First, normal distribution was tested using the Shapiro
and Wilk test, and a normal distribution was considered true when the p value
was >0.05. If the distribution was not normal, a Kruskal and
Wallis test was performed to compare the averages, and significant
differences were considered true when the p value was <0.05. Dunn
tests were performed as post hoc pairwise comparison tests to determine which group
was significantly different (when the p value <0.05). Second, the
homogeneity of variance was tested using the Levene test and was considered
true when the p value was >0.05. If the homogeneity of variance
was not true, a Welch ANOVA was performed, and significant differences were
admitted when the p value was <0.05. Estimated marginal means tests
were performed as post hoc tests to determine significantly different groups
(p value <0.05). In cases where the normal distribution and
homogeneity of variances were true, an ANOVA was performed, and significant
differences were true when the p value was <0.05. When there were
significant differences, the Tukey tests were performed as post hoc tests to
determine which groups were significantly different (when the p value
<0.05). The results of the statistical tests are summarized in
Table S2.
Principal component analyses (PCAs) were used to explore relationships
between DOM qualitative variables in peat porewater and pools. The selected
variables were quantitative variables as DOC concentrations and qualitative
variables as the DOC : DON ratio, optical indices (SUVA254, E2:E3 ratio,
and SR), and molecular indices (deoxyC6 : C5, fVEG, fMIC, MIC : VEG ratio,
C/V ratio, and Ac : Al(V) ratio), as well as molecular compound proportions
(%SOA, %CAR, and %CAR_MIC, %LMW_FA, %HMW_FA, and %Phenols). Environmental and seasonal variables were used as supplementary qualitative variables. Prior to PCA, a
correlation matrix was performed to identify strong correlations between the
variables (Fig. S1). One of the correlated variables was excluded from PCA
when the correlation was >0.90 or <-0.90, with p values
<0.05. Therefore, the DOC : DON ratio (DOC : DON ∼ DOC,
r= 0.90, p value <0.0001), %CAR_MIC
(%CAR_MIC ∼ deoxyC6 : C5 ratio, r= 0.99, p value
<0.0001), and fMIC (fMIC ∼ MIC : VEG ratio, r= 0.98,
p value <0.0001) were excluded from the PCA data set. The PCA was
performed with the package FactoMineR (Lê
et al., 2008). The ellipses in the representation of the first two axes of
the PCA correspond to the function addEllipses from the R package FactoMineR used to
add concentration ellipses to the plot.
ResultsHydrodynamics and physicochemical characteristics
Pool and peat water levels followed the same seasonal trend, although the
water level in the pools was always lower than in peat. Thus, the
preferential water flow goes from peat porewater to pools. The response of
the water level to precipitation was slower and buffered in pools compared
to peat (Fig. S2), and an average time lag of 13 h was measured between
the WTD peak of peat and pools.
Peat porewater temperatures were constantly lower compared to pools, with
13.4 ± 4.4 ∘C in peat porewater against 17.1 ± 5.5 ∘C in pools when averaged over the two growing seasons. In
both environments, the pH was acidic with an average of 4.9 ± 0.7 in
peat porewater and 4.4 ± 0.3 in pools. Specific conductivity was on
average almost 2 times higher in peat porewater than in pools, with 33.0 ± 19.3 and 14.0 ± 6.1 µS cm-1 in
peat porewaters and pools, respectively. Pool waters were characterized by
their constant saturation in dissolved oxygen, with 99.9 ± 5.2 % saturation
on average, while dissolved oxygen saturation was 50.04 ± 17.1 % saturation
in peat porewater (Table 1).
Peat porewater and pool seasonal average (± SD) of physicochemical variables (water temperature, pH, specific conductivity, and dissolved oxygen saturation), dissolved organic carbon (DOC) concentrations, and the ratio of DOC to dissolved organic nitrogen (DON) (DOC : DON), isotopic signature of DOC (δ13C-DOC), optical indices (SUVA254, E2:E3 ratio, and spectral slope ratio), fluorescence indices (FI and β:α index), and molecular indices (fVEG, fMIC deoxyC6 : C5, and C/V ratio, as well as %Phenols, %CAR, %CAR_MIC, %SOA, %LMW_FA, and %HMW_FA).
The DOC concentrations in peat porewater were significantly higher than in
pools (Fig. 2a). In both environments, the DOC concentrations showed the
same seasonal trends with a significant increase from spring to summer. The
DOC concentrations increased significantly in peat porewater from 9.2 ± 4.2 mg L-1 in spring, reaching a plateau above 20 mg L-1 during summer and autumn. In pools, the DOC concentrations also increased
significantly from 7.5 ± 3.2 mg L-1 in spring to a plateau above
10 mg L-1 in summer and autumn.
Box plots (a to b and d to f) and dot plots (c and g to i): (a) DOC concentrations, (b) DOC : DON ratio, (c)δ13C-DOC, (d) SUVA254, (e)E2:E3 ratio, (f) spectral slope ratio, (g) fluorescence index, (h)β:α index, (i)C/V ratio, (j) deoxyC6 : C5, (k)fVEG, and (l) MIC : VEG ratio. Each plot represents the evolution of variables during the growing season (SPR: spring; SUM: summer; AUT: autumn) in peat porewater and pools. Dot plots were used when n<5 for at least one season. Error bars represent standard deviations. Box plots were used when n>5 for each season. The dots represent each individual measurement, and boxes represent the lower (25th percentile) and the upper quartile (75th percentile); the median (50th percentile) is represented by the bold black horizontal bar in the boxes. Whiskers represent the interquartile range. Letters represent the significant differences between seasons. For each individual plot, conditions which share a letter do not present statistical differences.
Peat porewater presented a significantly higher DOC : DON ratio than pools. In
both environments, the DOC : DON ratio increased significantly from spring to
a plateau in summer and autumn (Fig. 2b). In peat porewater, the DOC : DON
ratio increased from 32.3 ± 12.4 in spring to 52.8 ± 22.5 and
56.6 ± 8.2 in summer and autumn, respectively (Fig. 2b). In pools,
the DOC : DON ratio increased from 26.2 ± 7.7 in spring to a plateau of
32.0 ± 7.5 in summer and 31.7 ± 6.6 in autumn.
Evolution of the isotopic compositions of DOM
Different trends for δ13C-DOC were identified between peat
porewater and pools (Fig. 2c). In peat porewater, δ13C-DOC
decreased significantly from spring, when the ratio was -26.0± 0.9 ‰, to autumn, when the ratio dropped to -27.5± 0.5 ‰. In pools, δ13C-DOC showed a
nonsignificant increase from -27.5± 0.3 ‰ in
spring to -26.8 ± 0.8 ‰ in autumn. In summer,
δ13C-DOC was significantly different between peat porewater and
pools.
Evolution of the optical and fluorescent properties of DOM
The DOM presented different optical properties between peat porewater and
pools. Among those, SUVA254 was significantly higher in porewater than
in pools during the whole growing season, indicating a higher aromaticity of
peat porewater DOM (Table 1). During the growing season, there were no major
changes of SUVA254 in peat porewater, but a slight increase was
observed in pools during the autumn (Fig. 2d).
The E2:E3 ratio was significantly higher in pools than in peat porewater,
indicative of a lower average molecular weight. Compared to SUVA254,
the E2:E3 ratio showed no significant trends in peat porewater, but it
slightly increased in pools from 4.02 ± 0.11 in spring to 4.41 ± 0.18 in autumn, suggesting a decrease in the average molecular weight during
the growing season (Fig. 2e).
The lower spectral slope ratio (SR) of peat porewater DOM also
suggested a higher molecular weight than in pool DOM. During the growing
season, the SR was steady in peat porewater with no significant changes
between seasons, suggesting a homogeneity of the molecular weight of DOM
(Fig. 2f). In pools, SR values increased from spring to summer and
decreased in autumn. Thus, according to the SR, the lowest average
molecular weight was reached during the summer in pools.
The fluorescence index (FI) was significantly higher in peat porewater than
in pools but varied within a narrow range, close to typical
terrestrial-derived organic matter (Fig. 2g). During the growing season,
the index remained steady in both environments with an average of 1.36 ± 0.10 in peat porewater against 1.27 ± 0.04 in pools.
The β:α index did not differ significantly between peat
porewater and pools, where it was on average 0.62 ± 0.07 and 0.64 ± 0.07, respectively (Fig. 2h). During the growing season, the index
remained steady in peat porewater. In pools, the β:α index
increased significantly from spring to summer: from 0.62 ± 0.03 to
reaching a peak at 0.69 ± 0.07. As the changes observed for the FI,
variations of the β:α index were limited to a small range.
Evolution of the molecular composition of DOM
Phenol markers dominated (54 %) the molecular markers of DOM for both peat
porewater and pools (Table 1). A similar proportion of small organic acids
(22 % on average) was measured in both environments. Carbohydrates
represented 6 % of the total markers in peat porewater and up to 8 % in
pools. The distribution of fatty acids differed between the two
environments. While low-molecular-weight fatty acids showed similar
proportions in peat porewater (5.8 %) and pools (6.7 %), high-molecular-weight fatty acids, which are associated with plant inputs, were almost
3 times higher in peat porewater (6.1 %) than in pools (2.3 %).
In addition, three modifications of the molecular composition of DOM between
the two environments must be highlighted. First, the C/V ratio (Fig. 2i), a
lignin compositional proxy, was significantly higher in peat porewater than
in pools (p value <0.01). While it remained almost stable in pools,
it decreased in peat porewater from 0.37 ± 0.12 during spring and
summer to 0.22 ± 0.07 during autumn. Secondly, the deoxyC6 : C5 ratio
(Fig. 2j), a carbohydrate ratio, was significantly higher in pools (0.97 ± 0.28) than in peat porewater (0.57 ± 0.20) (p value <0.0001). While it remained almost stable in peat porewater, it was maximal
in summer (1.10 ± 0.19) compared to spring (0.73 ± 0.10) and
autumn (0.91 ± 0.33). In pools, this evolution emphasized an increase
in the contribution of microbial exudates among the carbohydrate compounds
in pools. Finally, the fraction of plant-derived compounds among the
identified markers, fVEG (Fig. 2l), was always higher than 50 % in both
environments, highlighting the dominance of plant-derived DOM. However,
fVEG was significantly higher in peat porewater than in pools (p value = 0.02). Comparatively to the variations observed for the C/V ratio, fVEG
remained almost stable in pools, while it decreased in peat porewater in
autumn.
Global assessment of DOM quality in peat porewaters and pools
The PCA analyses of the peat porewater and pool samples indicate that the
first two components, represented by the two axes of Fig. 3, accounted for
56.3 % of the total variance. Individuals represented in the first two
dimensions showed a clear separation of both environments along the first
dimension (Fig. 3). The major contributors of the first axis were SR
(19.8 %), E2 : E3 ratio (14.4 %), deoxyC6 : C5 (12.8 %), DOC concentration
(12.7 %), and finally MIC : VEG ratio (11.8 %). For the second axis, the
major contributors were the proportion of phenols (%PHENOLS; 20.8 %),
C/V ratio (18.5 %), and high-molecular-weight fatty acids (%HMW_FA;
17.3 %). Other variables contributed less than 10 % to the first two
axes.
Representation of the first two dimensions of principal component analysis (PCA) of (a) physicochemical, quantitative, and qualitative parameters as variables and (b) individuals.
In pools, the DOM was characterized by a lower average molecular weight and
aromaticity and a higher contribution of microbial-derived DOM compared to
peat porewater. Inversely, in peat porewater, the DOC concentrations were
higher, and DOM presented higher aromaticity and a higher contribution of
plant-derived DOM, characterized by a higher fVEG. There was no effect of the
sampling season on the variances.
Experimental degradability of peat porewater and pool DOM
Statistical tests revealed no significant differences in the average
degradation rate between in situ and controlled conditions of biodegradation (Sect. 3.5.1). In addition, no significant differences appeared between
the average degradation rate where biodegradation only was tested and those
where biodegradation and photodegradation were both tested. This suggests
that temperature and sunlight had a limited effect on the DOM degradation.
As a consequence, all experimental conditions (both in situ and controlled) were
pooled in the following section. The DOM degradation rates were
significantly higher for peat porewater than pools. The degradation rates
were significantly higher for the incubation conditions of unfiltered
samples (UF) compared to filtered sample (F) conditions (Fig. 4).
Seasonal degradation rates (in % C d-1) for DOC and TOC incubation conditions in peat porewater and pools.
On average, the DOC degradation rates were 1.6 times higher for incubation
under unfiltered conditions (2.5 ± 1.5 % C d-1) compared to filtered conditions (1.5 ± 0.8 % C d-1) in peat
porewater. In pools, degradation rates were twice as high for UF (1.1 ± 1.1 % C d-1) than for F conditions (0.5 ± 0.6 % C d-1).
In peat porewater, the DOC degradation rates for F and UF conditions
followed similar seasonal trends. The DOC degradation rates were low in June
(0.6 ± 0.4 % C d-1) and twice as high for UF conditions (1.3 ± 1.0 % C d-1). The degradation rates reached a peak in
August, with 2.2 ± 0.5 % C d-1 for F and 4.5 ± 0.8 % C d-1 for UF conditions. Then, the DOC degradation rates decreased in
autumn to 1.7 ± 0.6 % C d-1 and 2.2 ± 0.5 % C d-1 for F and UF incubation conditions, respectively.
After excluding the UF condition of August, there was no persistent
significant difference between F and UF conditions. In June, the DOC
degradation rates were similar between the F and UF conditions with rates of
1.0 ± 0.5 and 1.3 ± 0.2 % C d-1,
respectively. Then, an increase to 2.1 ± 1.3 % C d-1 was
observed in August for UF conditions which were 2 times lower than the
rate measured in peat porewater under the same conditions. Finally, the DOC
degradation rates diminished in September, with 0.8 ± 0.3 % C d-1 for F and 1.1 ± 0.3 % C d-1 for UF incubation
conditions. Those degradation rates were 2 times lower than those observed in
peat porewater in autumn.
As observed in Fig. 5, ΔSUVA254 was strongly and positively
correlated with degradation rates, with specific dependence in pools (n=29, r= 0.82; p value <0.0001) and peat porewaters (n=33,
r= 0.65; p value <0.0001).
Relations between changes in SUVA254 (ΔSUVA254)
during incubation experiments and linear regression in peat porewater (solid line) and pools (dashed line).
DiscussionDifferences in DOM concentrations and composition between peat porewaters
and pools despite a similar source
The DOM concentration and composition strongly differed between peat
porewater and pools, despite a clear common plant origin. Peat porewater DOM
was characterized by high DOC concentrations and a DOM composed by both
recently produced and biodegraded DOM. In pools, DOC concentrations were
2 times lower compared to the peat porewater (Fig. 2a). Pool DOM was
characterized by a dominant contribution of allochthonous DOM (i.e.,
plant-derived) but also presented characteristics of microbial degraded DOM.
At our site located in the boreal ecozone, the average DOC concentration in
peat porewater increased from 9.2 to 22.5 mg L-1 from spring to autumn.
During the growing season, DOC concentrations are in general below 20 mg L-1 in boreal and subarctic regions, which are lower than in temperate
regions (Table S4). This latitudinal trend suggests that the balance
between DOM production and processing in peat porewater is controlled by
climate and most likely by temperature (Kane et
al., 2014). At our site, both DOM production and consumption followed a
strong seasonal trend in peat porewater, with DOM production being more
intense, as DOC concentrations were multiplied by 2.5 during the growing
season (Table 1).
DOM production by plants within peat porewater followed a strong seasonal
trend. This is revealed by three observations. First, peat porewater showed
a greater proportion of plant-derived DOM towards the end of the growing
season as indicated by the lowest δ13C-DOC measured in the
autumn. Second, the high DOC : DON ratios measured in peat porewater at our
study site – up to 6 times higher than those measured in a temperate
peatland by Austnes et al. (2010), as well as the DOC : DON ratio's increase along the growing season (Fig. 2b) – indicated a high contribution of recently
produced DOM. Third, the slight decrease in δ13C-DOC (Fig. 2c)
and the contribution of high-molecular-weight fatty acids from spring
to summer confirm the high contribution of plant-derived DOM. However, peat
porewater DOM composition also suggested a contribution from microbial
processing. First, the molecular analysis revealed the presence of microbial
markers, as high as 6.4 ± 2.7 %, as expressed by fMIC (Table 1).
Second, the incubation experiments highlighted that the labile fraction of
DOM represented 2.0 ± 1.3 % of peat porewater DOC (Fig. 4). Third,
the high SUVA254 values (Fig. 2d) we observed might reflect the
importance of biodegradation processes of DOM in peat porewater as
SUVA254 values increase with biodegradation (Hulatt et al., 2014; Autio et al.,
2016, Fig. 5). The average SUVA254 of 5.5 L mg-1 m-1
measured in the Bouleau peatland porewater was, in general, higher than
that previously measured in peatlands from temperate regions, i.e., <3.6 L mg-1 m-1 (Arsenault
et al., 2019; Tfaily et al., 2015; Heinz and Zak, 2018), except for Austnes et al. (2010), who reported a similar aromaticity and
average DOM molecular weight in a Welsh ombrotrophic peatland. These
indicators of microbial degradation within the peat also showed a seasonal
trend, with higher DOM biodegradability measured in summer (Fig. 4), and the
lowering of the fVEG at the end of the growing season. Then, most of the DOM
present in peat porewater is derived from the active vegetation at the
surface of the peatland but has been partially decomposed through microbial
degradation.
In pools, the DOM composition also presented specific features, with lower
DOC concentrations, aromaticity, and average molecular weight compared to
peat porewater (Fig. 2). Our results highlighted a dominant contribution of
allochthonous DOM in pools despite the presence of microbial-derived DOM.
The DOC concentrations in pools (10.5 mg L-1 in average) were similar
to those previously reported in the literature (Table S4), which, unlike
peat porewater, do not follow any latitudinal trend. The DOC concentration
remained relatively steady during the growing season, only multiplied by 1.6
compared to an increase by a factor of 2.5 in peat porewater (Table 1). The
SUVA254 values measured in the Bouleau peatland pools (3.4 ± 0.9 L mg-1 m-1 on average; Fig. 2d) were similar to those reported
from Arctic regions, with values of about 4 L mg-1 m-1 (Laurion
and Mladenov, 2013; Peura et al., 2016; Gandois et al., 2019; Laurion et
al., 2021), suggesting a contribution of plant-derived DOM from peat at our
site and supported by the high DOC : DON ratio, ranging from 9.4 to 51.4 (Fig. 2b).
Yet, the slightly higher deoxyC6 : C5 and the higher %LMW_FA in pools
indicate the presence of microbial markers. Microbial processing follows a
seasonal trend in pools (the highest microbial activity occurred in summer),
which appear to be stronger compared to peat porewater. This is revealed by
the evolution of δ13C-DOC, increasing during the growing season
(Fig. 2c) and revealing an increasing proportion of processed DOM. The
increase in aromaticity, from 2.9 to 3.9 L cm-1, might also reflect
this microbial processing and its increasing contribution during the growing
season. This is supported by indices as the SR, deoxyC6 : C5, MIC : VEG
ratio, and β:α index following a pronounced seasonal trend
with a peak reached in summer (Fig. 2). This suggests that pool DOM is
mostly derived from active vegetation in the peat but undergoes more intense
microbial degradation.
The DOM compositional differences between peat porewater and pools are
explained by hydrological, chemical, and biological processes
The observed differences in DOM composition between peat porewater and pools
were persistent during the growing season and under different hydroclimatic
conditions. We propose that those differences were driven by a combination
of hydrological, chemical, and biological factors. Along a peatland-to-pool
transect, both DOM concentrations and compositions remained stable within
the peatland and changed sharply at the interface between the peatland and
an adjacent pool (Fig. S4).
Hydrological flow paths in the peatland and at the transitional zone between
peat and pools might play a role in the shift of DOC concentrations and DOM
composition between porewater and pools. The two environments appear to be
hydrologically connected, based on synchronous variations of the water
levels in adjacent environments with a strong buffering of water levels in
pools (Fig. S2). This buffering can be explained by the decrease in
hydraulic conductivity with depth in peat which limits water exchanges (Holden et al., 2018). This
suggests that the preferential flow path for lateral advection occurs at
shallower depths when WTD is high (Birkel et al., 2017).
Alternatively, it has been shown that deep flow paths (below 2 m depth)
could supply surface flow (Levy et al., 2014;
Peralta-Tapia et al., 2015), might transport deeper DOM to the surface
water (Campeau et al., 2017), and
could contribute to water supply in pools. The DOM composition in deep peat
porewater has been reported to be relatively similar to shallow layers with
high aromaticity and average molecular weight (Tfaily et al., 2018). If this
process could provide DOM to pools, it could not explain the shift in DOM
composition between environments.
At our studied peatland, a decrease in the water storage coefficient (Riahi
et al., submitted) and an increase in peat density with depth has been
documented (Primeau and Garneau, 2021), which
should inhibit water flow movements. In addition to slower water
circulation, peat pore structure stimulates interactions between DOM and
partially degraded peat, which can adsorb both hydrophilic and hydrophobic
compounds (Kalbitz et al.,
2000; Rezanezhad et al., 2016). Changes in composition between peat
porewater and pools might be induced by the selective interaction between
DOM aromatic compounds and peat during their slow transfer. As we observed
SUVA254 values 1.6 times higher in peat porewater compared to
pools (Fig. 2), those aromatic products might selectively interact with peat
or at least reduce its mobility and explain the lower DOC concentration
and DOM aromaticity measured in pools (Table 1), since aromatic compounds
are known to constitute the hydrophobic fraction of DOM (Dilling and Kaiser, 2002).
Then, DOM microbial processing, occurring at different rates within peat, at
the interface between peat and pools and within the pool, might greatly
contribute to the observed differences in DOM composition. Peat porewater
DOM composition reflects microbial degradation occurring within the peat
(fMIC, Table 1) and shows a greater degradation potential compared to pool
DOM (Fig. 4). The slow water circulation and long residence time of DOM
within peat might promote interactions with microorganisms, allowing
microbial degradation of DOM (Kalbitz et al., 2000; Catalán
et al., 2016). Yet, a significantly higher contribution of microbial-derived
DOM was observed in pools, as expressed by a higher deoxyC6 : C5 and higher
%LMW_FA and lower fVEG indices, as well as the decrease in the average DOM
molecular weight as shown by the higher SR and E2 : E3 ratio. The
significantly lower C/V ratio measured in pools also supports the higher DOM
microbial processing in pools. The coumaric and ferulic acids, composing the
C fraction, are preferentially biodegraded compared the vanillic acid,
vanillaldehyde, and acetovanillone, composing the V fraction (Goñi and Hedges, 1992), resulting in a decrease in
the C/V ratio, as observed from peat porewater to pools.
The signature of microbial-derived DOM in pools supports the hypothesis that
DOM degradation processes occur at the interface between peat porewater and
pools (Fig. S4) and within the pools. A shift gradual sharp changes in
physicochemical parameters between the two environments, such as the slight
increase in pH and temperature, and the rise of dissolved oxygen
concentrations may favor the microbial turnover of the fraction of labile
peat porewater DOM (higher than in pool, Fig. 4; Schindler
et al., 1997; Kalbitz et al., 2000; Worrall et al., 2008; Peura et al.,
2016). Additionally, our data did not evidence any photodegradation during
DOM incubation in peat porewater and pools, suggesting that the DOM
photodegradation was not sizable by our experimental design. This contrasts
with previous studies which observed DOM photodegradation and changes in DOM
composition in boreal and temperate aquatic ecosystems of eastern Canada (Lapierre and del
Giorgio, 2014; Ward and Cory, 2016) and the United Kingdom (Jones et al., 2016). The absence of sizable photodegradation suggests that this process did not drive the DOM
composition in pools, compared to biodegradation. The clear pattern of the
SUVA254 increase observed during the incubation experiments was
independent to the exposition of DOM with the solar radiation. This is
consistent with the biodegradation of non-aromatic molecules (Spencer
et al., 2008, 2015; Mann et al., 2015; Worrall et al., 2017), leading to an
increase in SUVA254 (Hulatt et
al., 2014; Autio et al., 2016), while photooxidation has been shown to induce
a decrease in DOM aromaticity (Laurion and
Mladenov, 2013; Ward and Cory, 2016). This supports the hypothesis that
the peat-derived DOM biodegradation is an important driver of DOM
composition in the pools.
Implications of the DOM exchange from peat to pools for the peatland carbon
cycle
Boreal peatland pools were previously identified as a continuous source of
carbon dioxide (CO2) to the atmosphere during ice-free seasons,
offsetting some of the carbon uptake by the vegetation (Pelletier et al., 2014, 2015).
This release of CO2 was assumed to be the product of DOM mineralization
through microbial productivity in pools (Billett
et al., 2004; Striegl et al., 2012; Payandi-Rolland et al., 2020). Since our
results showed low degradation rates in pools, we suggest that DOM could
have been partially biodegraded within the peat and at the interface zone
between peat porewater and pools, limiting further its degradation within
pools (Payandi-Rolland et
al., 2020). This reactive interface could be comparable with the hyporheic
zone (riparian-water-saturated zone between the peat and the stream), which
can be an active component of the carbon cycle through the active DOM
mineralization and CH4 oxidation at this interface (Rasilo et al., 2017).
The long-term apparent rate of carbon accumulation (LORCA) measured in the
Bouleau peatland has been estimated to be 35.5 g C m-2 yr-1 and the
recent apparent rate of carbon accumulation (RERCA) 85.1 g C m-2 yr-1 (Primeau and Garneau, 2021). Based on a total pool volume of approximately 136 350 m3, an average DOC
concentration of 10.1 mg L-1 (Table 1), and an average potential
degradation rate between 1.9 % d-1 in peat porewater and 0.9 % d-1 in pools (Fig. 4), the degradation of pool DOM could average
between 1.5 and 3.1 g C m-2 y-1 for our
site. This is equivalent to 5.4 % to 11.1 % of the LORCA and 2.2 % to
4.6 % of the RERCA. Those proportions suggest that the processing of DOM
in pools might have a substantial impact on the peatland carbon budget. The
integration of carbon exchange at the pool–atmosphere interface would tend
to ultimately minimize the carbon sink capacity of peatlands often reported
from studies focusing on vegetation-to-atmosphere exchange. It is also
important to note that DOM in pools is mainly derived from the recently
produced DOM in peat, is unlikely from deeper (and older) peat layers, and
might not affect old carbon stocks from deeper peat horizons. However, DOM
degradation is not the only source of carbon emissions from pools which can
also be supplied by the lateral transfer of CO2 and CH4 (Rasilo et al., 2017) and by
CH4 ebullition (Repo et
al., 2007). However, the importance of the pools as a potential carbon
source to the atmosphere needs to be moderate in comparison with the
CO2 and the CH4 exported and emitted in the headwater stream of
the peatland. This flux of 8.8 g C m-2 yr-1 (Taillardat et al., 2022)
accounted for 22.8 % of the LORCA, which is between 2 and 4 times higher than
the carbon potentially emitted by pools.
Results presented in this study are from a boreal peatland, without
permafrost or anthropogenic disturbances that could influence the carbon
production and transformation processes through the peat–pool complex. The
morphology of pool banks and vegetation surrounding the pools may play an
important role in the DOM dynamics and DOC concentrations of pools, as
suggested by Arsenault et al. (2018, 2019), who studied 156 pools with a range of surface and depth
comparable to our study site. A study conducted on 10 peatland pools showed
that the size of the contact surface between water and peat (influenced by
pool size, depth, and the slope of the banks) influenced the concentrations
and composition of DOM (Banaœ, 2013). However, the pools studied by Banaœ (2013) were up to 10
times larger and deeper than in our studied peatland pools. At our site,
there was no significant difference between the pools in their range of size
(from 30 to 2065 m2) and depth (from 70 to 120 cm) except for
SUVA254 (Table S5). Despite the slight effect observed on
SUVA254, the DOM dynamics do not seem related to the pool morphology
and depth. It supports the hypothesis that DOM transfer and biodegradation
from peat porewater to pools are the main driver of its dynamic and
implication to the peatland carbon cycle rather than pool morphological
features.
Our study suggests that peat-derived DOM degradation and release through
pools could play a substantial role on the net carbon budget of our studied
peatland (<10 % of the LORCA). Moreover, the influence of pools
in the peatland carbon cycle should be considered from the perspective of
climate change. DOM production and biodegradation rates seem to be
controlled by temperature (Figs. 2 and 4) during the growing season, and
longer ice-free seasons and higher temperatures might impact the importance
of pools in the peatland carbon cycle.
Conclusion
This study demonstrated that DOM is a highly dynamic component of the carbon
cycle in peatland, with important differences identified in its
concentration and composition in both peat porewater and pools. Those
differences are persistent throughout the growing season and different
hydroclimatic conditions.
The strong increase in DOC concentrations in peat porewater over the growing
season highlighted the intense production of DOM in this environment. DOC
concentrations increased by 2.5 during the growing season (against a DOC
concentration increase by a factor of 1.7 of in pools), despite microbial
processing of DOM occurring within the peat.
The molecular analysis of DOM in pools revealed the dominant contribution of
allochthonous DOM derived from the peatland vegetation, supported by the
dominance of plant makers (fVEG and %Phenols) and high DOC : DON ratio.
Despite this similar plant origin, peat porewater and pools DOM had very
different concentrations, composition, and dynamics over the growing season.
The DOM in pools was less aromatic and showed lower molecular weight
compared to peat porewater.
Based on our investigations, we suggest that a combination of hydrological,
chemical, and biological processes explain those differences. The low
hydraulic conductivity in peat might favor DOM microbial processing before
its transfer to the aquatic compartments. Low hydraulic conductivity could
also lead to the selective adsorption of aromatic compounds with degraded
peat supporting the decrease in concentration and the lower aromaticity of
DOM observed in pools. We observed abrupt changes in DOM concentration and
composition at the interface between peat and pools which were persistent
during the growing season. The rapid modification of physicochemical
conditions (e.g., temperature and oxygen availability) between those two
environments might influence the biodegradation of DOM at the interface
between the peat and the pools and within the pools. This is confirmed by
the higher proportion of microbial molecular markers identified in the pool.
Although DOM is microbially degraded both at the interface and within the
pool, the carbon emissions generated by those processes could be substantial
(between 5.5 % and 11 % of the LORCA). The importance of pools in
the carbon cycle still needs to be studied in the context of increasing temperatures, which could stimulate DOM production in peat porewater and its microbial processing in peat porewater, as well as in pools after its transfer.
Data availability
The data sets used in this study are available online on the PANGAEA data
repository (10.1594/PANGAEA.945391, Prijac et al., 2022).
The supplement related to this article is available online at: https://doi.org/10.5194/bg-19-4571-2022-supplement.
Author contributions
LG, MG, AP, and PT carried out the conceptualization. AP performed the data
curation with input from LJ and PT, as well as the data analyses with input from LJ. LG and AP performed the formal analyses. MG was responsible for the funding acquisition. LG, LJ, and AP developed the methodology. AP and PT performed the data collection with the help of LG and MG. AP wrote the original draft and developed the figures. All coauthors contributed to the review and editing.
Competing interests
The contact author has declared that none of the authors has any competing interests.
Disclaimer
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Acknowledgements
We thank Katherine Velghe and Alice Parks from GRIL for
their laboratory training and assistance in absorbance and fluorescence
analyses, as well as Paul Del Giorgio for access to his laboratory.
Marine Liotaud, from Géosciences Rennes, is acknowledged for performing
THM-GC-MS analyses, and Frederic Julien, Virginie Payre-Suc, and Didier
Lambrigot, from Laboratoire Ecologie Fonctionnelle et Environnement, are acknowledged for
performing DOC/TN and cations/anions analyses. We thank Roman
Teisserenc (Ensat, Toulouse) and Charles Bonneau,
Charles-Élie Dubé-Poirier, Camille Girard, Pénélope
Germain-Chartrand, Léonie Perrier, Guillaume Primeau, Khawla Riahi, and Karelle Trottier for their assistance in the field.
Financial support
This research has been supported by the Natural Sciences and Engineering Research Council of Canada and Hydro-Québec funding to Michelle Garneau (grant no. RDCPJ 51421817).
Review statement
This paper was edited by Gwenaël Abril and reviewed by Audrey Campeau and one anonymous referee.
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