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Lability of dissolved organic carbon from boreal peatlands: interactions between permafrost thaw, wildfire, and season

Publication: Canadian Journal of Soil Science
25 February 2020

Abstract

Boreal peatlands are major sources of dissolved organic carbon (DOC) to downstream aquatic ecosystems, where it influences carbon cycling and food web structure. Wildfire and permafrost thaw alter peatland vegetation and hydrology and may affect the quantity and chemical composition of exported DOC. We studied the influence of wildfire and thaw on microbial and photochemical lability of near-surface porewater DOC, assessed through 7 d incubations. We carried out these incubations in spring, summer, and fall but only found differences in spring when DOC biodegradability (% loss during dark incubations) increased with lower DOC aromaticity and C/N ratios. During spring, the most labile DOC was found in recently formed thermokarst bogs along collapsing peat plateau edges (25% loss), which was greater than in mature sections of thermokarst bogs (3%), and peat plateaus with intact permafrost (9%). Increased DOC lability following thaw was likely linked to high DOC production and turnover associated with productive hydrophilic Sphagnum mosses and sedges, rather than thawed permafrost peat. A wildfire (3 yr prior) reduced DOC biodegradability in both peat plateaus (4%) and rapidly collapsing peat plateau edges (14%). Biodegradability of DOC in summer and fall was low across all sites; 2% and 4%, respectively. Photodegradation was shown to potentially contribute significantly to downstream DOC degradation but did not vary across peatland sites. We show that disturbances such as permafrost thaw and wildfire have the potential to affect downstream carbon cycling, particularly as the largest influences were found in spring when peatlands are well connected to downstream aquatic ecosystems.

Résumé

Les tourbières boréales déversent énormément de carbone organique dissous (COD) dans les lacs, ce qui a une influence sur le cycle de cet élément dans l’eau, en aval, ainsi que sur la structure de la chaîne alimentaire. Les feux de forêt et la fonte du pergélisol modifient la végétation et l’hydrologie des tourbières, avec les conséquences qu’on imagine sur la quantité et la composition chimique du COD exporté. Les auteurs ont examiné l’effet des feux de forêt et du dégel sur la labilité microbienne et photochimique du COD dans l’eau des pores, près de la surface, en procédant à des essais d’incubation de 7 jours. Ces essais se sont déroulés au printemps, en été et à l’automne. Les seuls écarts ont été observés au printemps, quand la biodégradabilité du COD (perte en pour cent durant une incubation dans le noir) augmente et s’accompagne d’une aromaticité plus faible ainsi que d’une diminution des ratios C/N. Au printemps, le COD le plus labile a été découvert dans les tourbières oligotrophes de formation récente, dans les thermokarsts le long des bords effondrés des plateaux de mousse de sphaigne (perte de 25 %). Une plus forte quantité de COD labile a été relevée dans les parties matures des tourbières de ces thermokarsts (3 %) et sur les plateaux de mousse au pergélisol intact (9 %). La plus grande labilité du COD après la fonte résulte sans doute de la forte production et du renouvellement du COD par la mousse de sphaigne et les carex hydrophiles vivants plutôt que de la fonte de la mousse de sphaigne gelée. Un feu de forêt (3 ans plus tôt) a diminué la biodégradabilité du COD à la fois sur les plateaux de mousse de sphaigne (4 %) et à la bordure des plateaux qui s’effondraient rapidement (14 %). Le COD s’était peu biodégradé en été et en automne à tous les sites, soit de 2 % et de 4 %, respectivement. La photodégradation pourrait concourir de façon appréciable à la dégradation du COD en aval, mais elle ne varie pas d’un lieu à l’autre. Les auteurs montrent que les perturbations comme la fonte du pergélisol et les feux de forêt sont susceptibles d’affecter le cycle du carbone en aval, surtout sachant que les plus grandes influences surviennent au printemps, quand les tourbières sont bien connectées aux écosystèmes aquatiques en aval. [Traduit par la Rédaction]

Introduction

The delivery of dissolved organic carbon (DOC) from terrestrial soils to freshwater ecosystem represents an important component of the global carbon cycle, with a large proportion of this DOC mineralized and released as carbon dioxide (CO2) to the atmosphere before reaching the ocean (Drake et al. 2018). High-latitude terrestrial ecosystems are estimated to export 34 Tg DOC yr−1 to the Arctic Ocean (Holmes et al. 2012), much of it derived from boreal peatlands (Kicklighter et al. 2013). Boreal peatlands have globally significant soil carbon stores (Hugelius et al. 2014) and are often hydrologically well connected to downstream aquatic ecosystems. Peatlands thus act as important landscape DOC sources, and they strongly influence the concentration and chemical composition of DOC in downstream aquatic ecosystems (Ågren et al. 2008). Both microbial and photochemical processes contribute to the mineralization of DOC into CO2 in streams and lakes, and the rates of both processes are influenced by the chemical composition of the DOC (Sobek et al. 2003; Cory et al. 2013, 2014). High-latitude regions are experiencing the greatest rates of climate warming (Meredith et al. 2019), which is further associated with an increased occurrence of peatland disturbances such as permafrost thaw and wildfire (Helbig et al. 2016). Disturbances in peatlands may potentially alter the soil DOC composition and its susceptibility to mineralization in downstream aquatic ecosystems. Thus, it is possible that disturbances such as wildfire and permafrost thaw in peatlands have cascading effects on the carbon balance of aquatic ecosystems that are relevant to understand the overall landscape carbon balance and its response to climate change.
Wildfire and permafrost thaw are common and widespread disturbances in the discontinuous permafrost zone of western Canada (Flannigan et al. 2009; Tarnocai et al. 2009; Grosse et al. 2011). Peatlands cover ∼40% of this region and are a mosaic of peat plateaus with permafrost, and channel fens and thermokarst bogs that are permafrost free (Quinton et al. 2009; Baltzer et al. 2014). The peat plateaus are forested with black spruce and thus carry fire well, whereas the thermokarst bogs and channel fens are wetter, open ecosystems that are not susceptible to burn. Wildfire in peatlands has in other regions been found to increase the relative availability of aromatic over aliphatic DOC, due to both the combustion of the surficial fresh soil material and the creation of condensed aromatics during combustion (Neff et al. 2005; Olefeldt et al. 2013a). Approximately 25% of peat plateaus in the discontinuous permafrost zone of western Canada have burned in the last 30 yr (Gibson et al. 2018). Over the same period, approximately 15% of peat plateaus have undergone complete permafrost loss (Gibson et al. 2018). Complete permafrost loss from peat plateaus causes land surface collapse and the development and expansion of thermokarst bogs. The development of thermokarst bogs involves a vegetation change to wet-adapted Sphagnum species and sedges at the actively thawing edge of peat plateaus during the initial thaw stage (recent thaw), and succession to a more dry-adapted Sphagnum species and ericaceous shrubs as the system matures and accumulates peat above the water table (Camill 1999; Beilman 2001; O’Donnell et al. 2012). Rates of thermokarst bog expansion have accelerated in the last few decades, and wildfire significantly accelerates permafrost thaw (Zoltai 1993; Myers-Smith et al. 2007; Gibson et al. 2018). These disturbances may, thus, affect the downstream delivery of peatland DOC, due to impacts on vegetation composition, leaching from char, soil moisture conditions, or the altered hydrological connectivity that accompanies permafrost thaw (Quinton et al. 2009; Kokelj and Jorgenson 2013).
The chemical composition of soil DOC varies across ecosystems and influences the DOC composition in Arctic rivers depending on the dominant land cover types (Amon et al. 2012; Mann et al. 2016). Characterization of the DOC chemical composition is complex, and often bulk characteristics are assessed from proxies. Aromaticity of DOC is for example often assessed from the specific UV absorbance at 254 nm, whereas the degree of decomposition can be assessed from the C/N ratio (Kuhry and Vitt 1996; Weishaar et al. 2003). Streams draining peatland-rich catchments generally have DOC with higher concentrations and higher aromaticity than streams draining peatland-poor catchments (Ågren et al. 2008). However, DOC characteristics have been shown to vary substantially even within peatlands, where leachates from humified peat layers have relatively low DOC concentrations and high aromaticity, compared with leachates from live Sphagnum vegetation and root exudates which have relatively higher DOC concentrations and lower aromaticity (Kalbitz et al. 2003; Wickland et al. 2007; Olefeldt et al. 2013a; Pinsonneault et al. 2016). These differences in DOC chemical composition are likely to influence how this carbon is transformed in downstream aquatic ecosystems, as both microbial and photochemical processes are known to selectively act on specific moieties of the DOC pool.
Microbial mineralization preferentially degrades aliphatic DOC components associated with fresh litter and vegetation inputs, with lower biomineralization rates observed with increasing aromaticity associated with DOC from more humified soils (Fellman et al. 2008; Olefeldt et al. 2013b; Pinsonneault et al. 2016). Photochemical mineralization can either degrade aromatic DOC to CO2 (Corin et al. 1996; Bertilsson and Tranvik 2000) or convert it to bioavailable compounds thus stimulating further biomineralization (Obernosterer and Benner 2004). Both microbial and photochemical mineralization processes alter the composition of the remaining DOC pool, where microbial processes generally increase DOC aromaticity though selective removal of non-aromatic DOC, whereas photochemical processes reduce DOC aromaticity through selective removal of aromatic DOC (Wickland et al. 2007; Ward and Cory 2016). Elevated levels of photo- and biodegradation of permafrost-derived DOC have been reported in thermokarst features such as thaw slumps and erosional gullies in tundra in Alaska (Cory et al. 2013; Abbott et al. 2014), yet these processes remain unknown in permafrost peatlands disturbed by wildfire and permafrost thaw.
The amount and composition of DOC in freshwater systems is in addition largely influenced by runoff (Clark et al. 2007; Ågren et al. 2008; Broder et al. 2017), which in northern latitudes is characterized by high discharge during the spring freshet (Quinton et al. 2009; Amon et al. 2012; Holmes et al. 2012). This short-lived event represents a large portion of the annual DOC export in sub/arctic catchments (e.g., Burd et al. 2018), and its composition is dominated by fresh vegetation inputs and aliphatic components (Amon et al. 2012; Holmes et al. 2012). The spring season is of particular importance in the discontinuous permafrost zone of western Canada, as it hydrologically connects different land cover types such as peat plateaus, channel fens, thermokarst bogs, and shallow open water wetlands (Quinton et al. 2009). After spring, this hydrological connectivity decreases, as some of these peatland types become disconnected. The implications of interactions between hydrological connectivity and seasonally changing DOC composition for different peatland types have so far not been well studied.
The objective of this study was to examine the effects of wildfire, permafrost thaw, and seasonality on DOC characteristics and its susceptibility to photochemical and microbial degradation. We conducted incubation experiments at three different times during the growing season, using porewater collected from a partially burned peatland complex in the discontinuous permafrost zone of boreal western Canada. We predicted that permafrost thaw would enhance DOC biodegradability due to the shift in ground cover from shrubs and lichens on peat plateaus to productive Sphagnum mosses in thermokarst bogs, and that wildfire would have the opposite effect due to increased DOC aromaticity. Our study continues a tradition of peatland soil science conducted at the University of Alberta, which has included studies of peat microbiology (Christensen and Cook 1970), controls on peat decomposition (Lieffers 1988), development of Sphagnum-dominated peatlands (Kuhry et al. 1993), stability of permafrost peatlands (Vitt et al. 1994), post-thaw peatland carbon cycling (Turetsky et al. 2002), and peatland hydrology (Devito et al. 2012). As such, our study build on previous research and provides insights on the composition and reactivity of DOC from peatlands affected by wildfire and permafrost thaw, and it contributes to a better understanding of potential impacts for downstream carbon cycling in peatland-rich regions with discontinuous permafrost.

Materials and Methods

Study site

This study was conducted at a peatland complex consisting of peat plateaus and thermokarst bogs in the boreal discontinuous permafrost zone of the Northwest Territories, Canada (61.19N, 120.08W) (Fig. 1). The climate of the region is characterized by short summers and long winters with average annual temperatures of −3.2 °C and total annual precipitation of 369 mm, of which 46% falls as snow (Quinton et al. 2009). The peatland was partially affected by wildfire in 2013, which burned some peat plateaus but did not burn the bogs due to their non-forested, wetter conditions. We established two transects at the site in 2016; one in a section affected by wildfire and one outside the fire boundary (Fig. 1). Each transect (<100 m in length) starts on a peat plateau, and traverses a thawing edge of the peat plateau into a thermokarst bog. The unburned transect included a peat plateau site (“unburned plateau”), a recently thawed site at the edge of the thermokarst bog (“unburned edge”), and a thermokarst bog site away from the thawing edge (“mature bog”). The burned transect consisted of a burned peat plateau site (“burned plateau”), a recent fire-induced thaw site at the peat plateau burned edge (“burned edge”), and a mature thermokarst bog (“mature bog”). The mature bog sites in each transect had similar vegetation, DOC characteristics, and similar results from the incubation experiments, and as such we will henceforth combine these into a single mature bog site. The unburned plateau was elevated by 1–2 m above the surrounding thermokarst bogs and had an open canopy of ∼5 m tall black spruce (Picea mariana) and an understory dominated by labrador tea (Rhododendron groenlandicum) and lichens (Cladonia spp.). All black spruce were dead at the burned plateau, and the ground was extensively charred with sparse labrador tea. The active layer (i.e., the seasonally thawed peat layer above the permafrost) was ∼60 cm at the unburned plateaus and ∼100 cm in the burned plateau. The water table at both plateau sites was generally found at the base of the seasonally thawing front, although some internal depressions have a water table ∼10–20 cm below the peat surface. The unburned edge was dominated by Sphagnum riparium and rannoch-rush (Scheuchzeria palustris), with a water table between 5 cm above the peat and 10 cm below the peat surface. Permafrost thaw at the unburned edge is likely to have occurred in the last few decades, as indicated by chronologies of cores taken at a nearby area (Pelletier et al. 2017). The burned edge site is likely to have undergone permafrost thaw following the wildfire, i.e., within 3 yr prior to the study, since the ground cover was identical to the burned plateau but the peat surface had collapsed, and the water table was at or above the peat surface. At both mature bogs, vegetation was similar and dominated by Sphagnum fuscum and leather-leaf shrubs (Chamaedaphne calyculata), indicating that permafrost has been absent for >100 yr (Camill 1999; Pelletier et al. 2017; Estop-Aragonés et al. 2018a). There was no evidence of wildfire affecting either of the mature bog sites. Peat depth at the mature bogs was between 250 and 300 cm, which we assume is representative of the overall site.
Fig. 1.
Fig. 1. Transects and sampling sites in a permafrost peatland complex partially affected by a recent wildfire, the Northwest Territories, Canada (61.19N, 120.08W). Panel (a) shows drone image (provided courtesy of W.L. Quinton) of the peatland complex which was affected by a wildfire in 2013, 3 yr prior to the study. Non-permafrost thermokarst bogs do not carry fire and were not affected by the wildfire. The wildfire had, however, caused substantial permafrost thaw and thermokarst bog expansion along edges of burned peat plateaus. Panels (b)–(f) show ground views of porewater sampling sites where peat porewater was collected: an unburned peat plateau, an unburned thermokarst bog along the edge of a thawing peat plateau, a mature thermokarst bog, a burned thermokarst bog which has developed along a peat plateau edge due to the recent wildfire, and a burned peat plateau, respectively. Panel (g) shows a scheme of water flow, water table position, and permafrost conditions in each site. Water flows off the plateaus and bogs into the subsided thermokarst where it can move downstream. For more detailed sampling site information, refer to Table 1. [Colour online.]

Porewater sampling

Porewater was collected at each site in spring (21 May), summer (18 July), and fall (12 Sept.) to determine seasonal changes in porewater chemistry, DOC composition, and lability. The spring sampling occurred during the receding limb of snowmelt freshet, suggesting that surface water in the peatland complex was hydrologically connected to the catchment stream network at this time, considering the shallow water table at the sampling sites (Fig. 2). Heavy storms during summer and fall can contribute significantly to the annual runoff generation in peatlands (Clark et al. 2007). In 2016, there were no major storms, however, and peatland hydrological connectivity to the catchment stream network after the spring sampling period was likely limited. Electrical conductivity ranged from 38 to 62 μS cm−1 and pH from 3.8 to 4.2 (Table 1) suggest that all sites are largely ombrotrophic with minor groundwater connectivity.
Fig. 2.
Fig. 2. Timing of peatland porewater collection, and the hydrological runoff regime in 2016 of a nearby peatland-rich Scotty Creek catchment. Discharge from Scotty Creek outlet (61.42N, 121.45W) located ∼60 km west of the studied peatland is used as an example to indicate the hydrological connectivity of the peatland to downstream ecosystems at different times of the year. Discharge data obtained from Environment Canada (https://wateroffice.ec.gc.ca/).
Table 1.
Table 1. Soil and water characteristics at the studied peatland sites.
a
Variability within site throughout the sampling period between May and September, with wetter conditions in spring and drier in fall. Positive values indicate position above the peat surface.
b
Values are the mean ± standard deviation of electrical conductivity (EC) and pH from three locations in each site.
c
Sylvic peat aggraded under permafrost conditions typically identified by the presence of well-decomposed woody remnants and lichen (Zoltai 1995; Robinson 2000).
Peat porewater was collected using MacroRhizon samplers with a 0.15 μm pore size (Rhizosphere Research Products, Wageningen, the Netherlands), thus removing most microbial biomass. Samplers were inserted into the soil at or just below the water table position to collect DOC that is likely to be exported during runoff generation because hydrological connectivity decreases with peat depth (Quinton et al. 2008). In the drier peat plateau sites, samplers were inserted close to the base of the seasonally thawed peat layer in spring and summer (10–40 cm depth) and at 40 cm depth in the fall sampling. At each site, porewater was obtained from three locations ∼10 m apart to account for site spatial variability, and pooled into a single sample until sufficient volume (∼2 L) was collected for analysis and incubation preparations. Samples were kept dark overnight in 4 L polyethylene bottles in a coolbox with ice packs (∼4 °C).

Porewater analysis

An aliquot of 60 mL from the collected porewater was transferred to an amber glass bottle and acidified (0.6 mL 2 mol L−1 HCl) for analysis of DOC and total dissolved nitrogen (TDN) concentrations. DOC and TDN concentrations were determined within 8 d on a TOC-L combustion analyzer (Shimadzu, Kyoto, Japan), and all measurements are based on four injections, where the average standard deviation for injections of the same sample was 0.07 mg C L−1 and 0.008 mg N L−1. The DOC/TDN analysis used an eight-point calibration, with calibration points between 0.5 and 50 mg C L−1 and 0.05 and 5 mg N L−1, respectively; ranges which encompassed all analyzed porewater samples.
Another aliquot of 60 mL was stored in amber glass bottles for analysis of total dissolved phosphorous (TDP), and major anion and cation (Ca2+, K+, Na+, Mg2+, Cl, SO42−, and Fe2+/3+) concentrations. Concentrations of TDP were determined by standard acid hydrolysis methods for flow-injection analysis, with concentrations determined based on the stannous chloride method. Major ion concentrations were determined by ion chromatography and ICP-OES at the Natural Resources Analytical Laboratory at University of Alberta within 15 d of sampling.
UV–Vis absorbance was determined the day after sample collection in the field by transferring aliquots of 3 mL from the porewater sample into a quartz cuvette and measuring absorbance between 230 and 600 nm using a UV–Vis portable spectrometer with deionized water as the blank (Flame-DA-CUV-UV-VIS, Ocean Optics, Dunedin, FL, USA).

Incubation experiment

An incubation experiment was initiated the day after porewater collection to determine the interactive effects of site and season on porewater DOC lability as assessed by dark (i.e., microbial DOC mineralization) and light (i.e., photochemical and microbial DOC mineralization) treatments. We used the previously measured absorbance at 254 nm (A254) to standardize physicochemical conditions for the incubation experiment by diluting the collected porewater with a 0.001 mol L−1 NaHCO3 solution to a target A254 of 0.35. This dilution was done to standardize pH and DOC concentrations among samples, to reduce the likelihood of anoxia during the incubation time, and to standardize the amount of sunlight absorbed among light-exposed samples. Diluted samples were further inoculated at a 1% volume ratio with a solution consisting of 1:1 mixed stream and peatland porewater filtered using 1.6 μm pore size glass microfibre filters (Grade GF/A, Whatman). The stream water was collected on the day of incubation initiation from the stream which drains the studied peatland, at a downstream culvert where it crosses the road (Notawohka Creek, 61.16N, 119.92W). The coarser filtration of the inoculation sources does not exclude the microbial communities, and as such this inoculation was done to standardize the microbial community among samples, while excluding microbial grazers. Once the dilution was prepared, UV-light transparent 1 L tedlar bags (Keika Ventures, Chapel Hill, NC, USA) were filled with 400 mL of the prepared sample. Five replicates each for both the dark (tedlar bags covered with opaque tape) and the light treatments were prepared for each site, except for the pooled sample of the mature thermokarst bog sites, where eight replicates were prepared for both the dark and light treatments.
Incubations were carried out in a 0.1 ha shallow peatland pond (59°29′N, 117°10′W) for 7 d to mimic the potential in situ conditions for DOC processing during water export from peatlands. Bags were randomly placed in submerged floating mesh baskets attached to a PVC tube secured with cord to limit movement in the pond. We assessed DOC mineralization by quantifying changes in concentration and UV–Vis spectral properties of DOC between days 0 and 7. Water samples were drawn from each bag prior to deployment and after 7 d in spring, summer, and fall campaigns for absorbance, DOC, and TDN concentration analysis. Samples for DOC and TDN concentration analysis were acidified to inhibit further microbial DOC mineralization, and stored in cool, dark amber bottles until being analyzed within 8 d of collection, as described above. Sample for absorbance spectra was filtered with 0.7 μm glass microfiber filters (Grade GF/F, Whatman) and analyzed immediately in the portable UV–Vis instrument. In spring, tedlar bags were submerged 10 cm below the pond water surface, which yielded very low rates of DOC loss attributed to photodegradation (see results below). Thus, to assess the effect of photodegradation, we instead placed the tedlar bags at 2 cm depth during the summer and fall incubations to enhance incoming radiation. Incoming photosynthetically active radiation (PAR), which we assume to be proportional to incoming UV light, was monitored in a floating station above the water surface in the pond (Photosynthetic Light Smart Sensor, Onset, MA, USA). Due to longer days in summer, incoming PAR was doubled during summer relative to the fall incubation, averaging 422 and 214 μmol m−2 s−1, respectively. Loggers attached to the floating baskets were used to monitor pond water temperature (HOBO pendant temperature logger, Onset, MA, USA) during the incubation time, which averaged 14, 21, and 11 °C for the spring, summer, and fall incubations, respectively.

Data reporting and statistical analysis

We attribute DOC loss during the incubation to microbial activity (biodegradation) in our dark treatment and to a combined effect of both photodegradation and biodegradation in our light treatment. Although we did not exclusively assess photochemical mineralization (light treatment includes biodegradation), we consider our light treatment to be more representative of natural DOC processing in aquatic networks as both processes co-occur in situ. We express biodegradation as the percent DOC loss during the incubation relative to the initial concentration (BDOC). We express photodegradation as absolute DOC loss in mg C L−1 and calculate it by subtracting the site averages of DOC loss in the dark treatment from site average of DOC loss in the light treatment (light–dark DOC losses). Specific UV absorbance at 254 nm (SUVA) is a measure of DOC aromaticity and was calculated by dividing the decadic absorption coefficient at 254 nm by the DOC concentration of each sample (Weishaar et al. 2003). Iron(III) concentrations may interfere with the absorbance in this spectral range, but this effect was ruled out due to the low total iron concentrations relative to DOC concentrations and low pH in our samples associated with reduced iron forms (Poulin et al. 2014). The porewater C/N ratio, a potential indicator for the degree of decomposition of DOC, was calculated using the molar ratio of DOC and TDN.
Statistical analyses were carried out using R Studio (version 1.0.44). Data were checked for normality using the Shapiro–Wilk’s test and for homogeneity of variances using a Levene’s test. A two-way analysis of variance (ANOVA) was used to test the effects of site and season, and their interaction, on BDOC and on total DOC loss. Post hoc analyses were conducted using Tukey’s honest significant difference test. Differences in DOC losses from photodegradation between summer and fall were compared using a paired t test. Linear regressions were used to assess relationships between BDOC and several water quality measures (C/N ratio, SUVA, and ion concentrations).

Results

Porewater DOC composition and water chemistry

Seasonal differences in porewater composition between sites are shown in Fig. 3. DOC concentrations, SUVA values, and C/N ratios increased from spring to fall in most sites, with the greatest change generally occurring between spring and summer. The unburned edge had the highest DOC concentrations, particularly during spring when it was almost twice as high as any other site. The unburned edge also had the most distinct DOC composition across all sites and seasons during the spring sampling, with the lowest observed SUVA and C/N ratios, and the highest TDP concentration. When comparing the unburned and burned plateau sites, we found that the burned site had higher SUVA and C/N in spring and summer, and higher TDP during all three sampling occasions. The burned edge was also found to have high concentrations of TDP, particularly in spring. There was a positive linear relationship between C/N and SUVA during spring (R2 = 0.72, p < 0.05), but this relationship was not observed in summer or fall when the range of SUVA values were much narrower across sites.
Fig. 3.
Fig. 3. Seasonal variability of (a) dissolved organic carbon (DOC) concentration, (b) specific ultraviolet absorbance (SUVA), (c) carbon to nitrogen ratio, and (d) total dissolved phosphorus (TDP) concentration in the peatland porewater at the study sites. Within-site variability was not assessed, as preparations for the incubation experiment required porewater from three locations within each site to be pooled in the field.
Ion concentrations (Ca2+, Mg2+, Na+, Fe2/3+, K+, Cl, SO42−, and Al3+) generally increased from spring to fall, with the exception of the unburned edge where the highest concentrations were found in spring. In general, the unburned edge also generally had the highest ion concentrations (Fig. 4). When comparing the unburned and burned plateau, we found that the burned plateau had elevated concentrations of Fe2/3+, K+, SO42−, and Al3+.
Fig. 4.
Fig. 4. Porewater ion concentrations across peatland sites. Each bar represents an average of three samples, one each from spring, summer, and fall porewater collections. Error bars show one standard error of the mean.

Biodegradation of DOC during dark incubations

Concentrations of DOC at the start of the incubations, after dilution to a standardized absorbance (0.35 cm−1) and addition of the inoculate, varied between 9.2 and 18.5 mg C L−1 in spring, 7.0 and 12.2 mg C L−1 in the summer, and 7.6 and 13.3 mg C L−1 in fall. All incubations had detectable losses of DOC, ranging between 0.05 and 4.0 mg C L−1. As such, it is unlikely that rates of microbial activity were restricted by low oxygen levels (∼9.0 mg O L−1 at the start of the incubation).
Biodegradability of the DOC was assessed for the dark incubation from the change in DOC concentration over the 7 d incubation relative to initial DOC concentration (expressed as a %). There were statistically significant differences in BDOC between sites and seasons, with the main and interactive effects of these factors interpreted through post hoc analyses (Fig. 5, Table 2). The BDOC was greatest in spring (12.5%), followed by fall (4.4%), and least during summer (2.5%). During summer and fall, there were no differences in BDOC between sites, whereas during spring, there were significant differences between sites. The greatest BDOC during spring occurred at the young bog (25.6%), followed by the burned edge (14.0%) and the unburned plateau (9.5%), which all were greater than the burned plateau (4.8%) and the mature bog (2.8%) (Fig. 5).
Fig. 5.
Fig. 5. Biomineralization of porewater dissolved organic carbon (BDOC) across peatland sites and seasons. BDOC was assessed from 7 d dark incubations, and expressed as the incubation DOC loss compared with the initial DOC concentration. Incubations were done in a peatland pond, and as such the incubation temperatures varied between seasons (14, 21, and 11 °C for the spring, summer, and fall incubations, respectively). Bars represent mean BDOC (five replicated incubations) and error bars show one standard error of the mean. Lowercase letters indicate statistically significant differences (p < 0.05) between sites and seasons, see Table 2 for details.
Table 2.
Table 2. Two-way analysis of variance assessing the effects of site, season, and their interaction on biodegradability of the dissolved organic carbon.
In spring, BDOC was significantly negatively related to C/N (R2 = 0.80, p < 0.05) and SUVA (R2 = 0.76, p < 0.05) (Fig. 6). In spring, BDOC was also significantly positively related to TDN (R2 = 0.95, p < 0.01) and TDP (R2 = 0.81, p < 0.05) (not shown). None of these relationships were apparent during summer or fall, when BDOC overall was much lower and had no differences between sites. BDOC was not significantly related to any ion concentration during any season (all p > 0.1, data not shown).
Fig. 6.
Fig. 6. Relationships between the biodegradability of dissolved organic carbon (BDOC) from various peatland stages during a 7 d dark incubation, repeated three times during the 2016 season, and (a) the C/N ratio and (b) the specific ultraviolet absorbance (SUVA) of the DOC. Lines indicate significant (p < 0.05) linear regressions for the spring incubations. These relationships were not significant during the summer and fall incubations. Symbols show mean values of BDOC and of initial (day 0) C/N and SUVA of the incubated DOC (five replicates for each incubation). Error bars show one standard error of the mean and are occasionally smaller than the symbols.

Losses of DOC during light incubations

No results from the light incubation in spring are shown, as DOC losses were not different from DOC losses during the dark incubations — resulting from submerging the samples too deep in the pond, thus restricting the amount of light which was absorbed by the DOC. During the summer and fall incubations, we found consistently greater DOC loss in the light incubation than the dark incubations (Fig. 7). Mean DOC loss in the light treatments ranged between 0.59 and 0.91 mg C L−1 across all sites and both seasons. There were, however, no differences between sites (p = 0.88, F = 0.45, df = 4) or seasons (p = 0.77, F  = 0.045, df = 1), and no interactive effect (p = 0.45, F  = 0.94, df = 4) with regards to DOC losses during the light incubations (two-way ANOVA).
Fig. 7.
Fig. 7. Loss of dissolved organic carbon (DOC) during 7 d dark and light incubations, and their differences (light-dark incubation losses). Each bar represents the average DOC loss across all five peatland sites (unburned plateau, unburned edge, mature bog, burned edge, and burned plateau), and the error bars indicate one standard error.
The additional DOC losses enabled by the light treatment compared with the dark incubation, either from direct photomineralization or photo-enhanced biodegradation, were calculated from the difference between mean site DOC loss during the light and dark incubation. We found that this DOC loss due to photodegradation was greater in summer than in fall, with the range across sites being 0.35–0.78 mg C L−1 in summer, whereas being 0.21–0.42 mg C L−1 in fall (Fig. 7). This seasonal difference in photodegradation was not statistically significant (paired t test, p = 0.131, n = 5; comparing mean site DOC loss in summer and fall); however, it is likely related to the amount of light that was absorbed by the DOC samples during the 7 d incubations. Incoming average PAR during the summer incubation was nearly double that during the fall incubation (422 vs. 214 μmol m−2 s−1). As such, the photodegradation losses were proportional to incoming light. We found no relationships across sites or season between biodegradation DOC losses and DOC composition (C/N or SUVA) or with ion concentrations.

Discussion

This study shows that disturbances like wildfire and permafrost thaw have important impacts on the composition and lability of porewater DOC in boreal peatlands, with potentially significant implications for downstream aquatic ecosystems. However, our study also shows that these differences in DOC lability were dependent on the season, with differences between different peatland sites found only in spring when we sampled porewater at the end of snowmelt. It is, however, this time of the year which may be of the greatest importance from a landscape perspective, as this is when peatlands are most likely to be hydrologically connected and deliver the majority of their DOC export to downstream receiving ecosystems. During this spring period, we found that recently thawed peatland sites had significantly elevated microbial lability of their porewater DOC, whereas peatland sites affected by wildfire had reduced microbial DOC lability. Our light treatment found no difference in DOC photodegradation between sites or seasons, but we note that we were not able to complete a successful light treatment during the important spring period. Below, we discuss potential reasons for the observed patterns, linking peatland disturbances with characteristics of porewater DOC and its lability.

Differences in peatland porewater characteristics across seasons

We found large differences in porewater characteristics between different peatland sites, both with regards to DOC characteristics and general water chemistry. Our study design required us to pool sampled porewater from several locations within each site to prepare for the incubation experiment, and thus it did not yield replicate analyses of porewater chemistry for each site/season, which precludes statistical assessment of differences between sites. However, many of the key differences between sites with regards to DOC characteristics were in agreement with previous findings at other sites. This includes the lower aromaticity and higher lability of DOC during spring freshet, along with the increased aromaticity and decreased lability of DOC following wildfire. The recently thawed unburned edge site emerged as an important hotspot for biogeochemical cycling during the spring period, which might be linked to both its hydrological position within the peatland complex (where they often receive and convey runoff from peat plateaus to adjacent fens), the release of DOC from previously frozen peat layers, and the shifts in vegetation composition.
The lower aromaticity (SUVA) observed in spring at most sites could be explained by a relative dominance of labile DOC after the winter due to the effects of low but prolonged microbial activity at low temperatures. Freezing of the soils during winter may result in damage of belowground plant tissues and microbial biomass, leading to a release of simple sugars and amino acids, a feature described for riparian soils and related to decreased SUVA values after frost (Haei et al. 2012). A build-up of microbial biomass has also been shown to occur in wet meadow soils during winter (Edwards and Jefferies 2013). This labile DOC pool may thus dominate at the beginning of the growing season, during and following freshet, and result in reduced porewater DOC aromaticity. Additionally, as the water table is closest to the surface in the spring, shallow flow paths bypassing deeper, more aromatic DOC in subsurface soils likely rendered lower SUVA values (Pinsonneault et al. 2016; Broder et al. 2017). The C/N ratios of the DOC were also lowest in spring for all sites and generally increased during the growing season. Microbial biomass has lower C/N than plants and peat (Kuhry and Vitt 1996; Preston et al. 2012), and a greater proportion of this component accumulating in soil during winter could explain the consistently lower spring C/N values.
Wildfire on permafrost peat plateaus was found to lead to higher DOC aromaticity, increased C/N ratio, and reduced DOC lability in porewater. This effect was apparent both in the comparison between the burned and unburned peat plateaus, as well as the comparisons between the burned and unburned recently thawed and collapsed edges of the thermokarst bogs. Similar findings with higher SUVA of DOC following fire have previously been found in Alaskan streams (Larouche et al. 2015) and boreal peatlands (Olefeldt et al. 2013a, 2013b). This increase in DOC aromaticity is likely related both to the combustion of fresh plant material at the peat surface, which otherwise is a source of aliphatic compounds when leached (Pinsonneault et al. 2016), and to the formation of char, produced due to the impartial combustion of organic matter, which leaches hydrophobic, and highly aromatic DOC (Preston and Schmidt 2006). The effect on downstream water chemistry is, however, not limited to DOC characteristics, as wildfire was also found to increase the concentrations of TDP as well as several ions (Fe2/3+, Al3+, K+, and SO42−). These shifts may be due to both the release of these constituents from peat following fire (Smith et al. 2001), as well as a reduction in plant uptake.
The unburned recently thawed edge had the most distinct porewater chemistry, especially in spring. These thermokarst bog edges are locations where peat plateaus are likely to have thawed in the last few decades (Pelletier et al. 2017). The collapse of peat plateaus lead to inundation of old peat, and colonization of hydrophilic Sphagnum species and sedges. The flooded, anoxic, conditions inhibit rapid degradation of the deeper, older peat (Estop-Aragonés et al. 2018b), whereas rapid Sphagnum and sedge growth at the surface is likely to lead to high rates of DOC production. Leachates of live Sphagnum have been shown to have low aromaticity and high biodegradability in western Canada, south of the permafrost boundary (Olefeldt et al. 2013a). Rapid growth of Sphagnum species has also been shown to yield high rates of DOC and TDN production and support higher concentrations of microbial biomass (Lindholm and Vasander 1990). Highly microbially labile algal DOC may also have contributed to the DOC pool in both the unburned and burned edges, as these were the only partially inundated sites in the study (see Figs. 1c and 1e) (Wyatt et al. 2012). Overall, we consider it likely that the distinct DOC characteristics at the unburned edge were due to high rates of DOC production associated with Sphagnum, sedge species, and algae, rather than the production of DOC from recently thawed deeper peat.
Despite the limited areal extent of recently thawed thermokarst bogs within peatland complexes, they are likely to contribute disproportionally to overall peatland DOC export to downstream aquatic ecosystems. Recent thermokarst bogs have been estimated to cover approximately 10% of peatland complexes in the study region (Gibson et al. 2018). However, they are located at points within the peatlands which receive water from their surrounding peat plateaus, and then convey it further downstream to streams or channel fens. As thermokarst bogs expand, many previously isolated thermokarst bogs also become hydrologically connected to streams (Quinton et al. 2009). As such, accelerated thermokarst bog expansion due to permafrost thaw may increase the peatland export of both nutrients (Abbott et al. 2014) and DOC, particularly the delivery of highly labile DOC during freshet.

Controls on DOC biodegradability

Biodegradability correlated with the C/N and SUVA of the DOC pool but also with overall TDN and TDP concentrations. The DOC biodegradability was greatest in spring, when the largest differences in DOC properties occurred between sites. Similar findings in DOC derived from ombrotrophic peat vegetation have reported that C/N, SUVA, and TDN are strong predictors for BDOC with fresh organic materials relative to that in peat or litter having a greater biodegradability (Pinsonneault et al. 2016). This is in agreement to our observation of highest BDOC in the unburned edge site. Although there was no relationship observed between BDOC and C/N in the summer or fall, BDOC was always <10% when C/N exceeded ∼45, suggesting this may be a threshold value, beyond which DOC has very limited biodegradability. Increasing C/N typically correlates to higher aromatic C content (Fellman et al. 2008), and thus SUVA is also a strong predictor of BDOC as previously found (Fellman et al. 2008; Abbott et al. 2014; Larouche et al. 2015; Mann et al. 2015; Pinsonneault et al. 2016). Higher values for either of these indices represent microbially recalcitrant DOC, and subsequently reduced BDOC is expected. We observed little BDOC (∼5% or lower) when SUVA values exceeded 3.2 L mg C−1 m−1. However, short incubations (11 d) conducted on soil leachates from burned boreal peatlands observed higher biodegradability (10%–20%) even when SUVA exceeded 3 L mg C−1 m−1 (Olefeldt et al. 2013b), whereas longer incubations (40 d) on stream outflows from thawed permafrost in uplands observed no real impact of high SUVA on BDOC (Abbott et al. 2014; Larouche et al. 2015). Hence, the relationships between BDOC and simple DOC indices such as SUVA are not straightforward and universal, and are likely influenced by sources of the DOC and to what degree it has already been microbially and photochemically processed.

Controls on photochemical DOC degradation

We observed higher DOC loss in light treatments relative to the dark treatments indicating photochemical degradation may be an important pathway for DOC transformation and degradation, in agreement to a growing body of evidence in northern aquatic ecosystems (Laurion and Mladenov 2013; Cory et al. 2014; Ward and Cory 2016). Unlike other studies, we did not find a relationship between DOC losses attributed to photochemical processes and the chemical composition of the DOC. However, our study could not include a light incubation during the spring period, and we had limited variability in DOC composition during the summer and fall incubations. We did find indications of greater photochemical DOC degradation during the summer than fall incubation, likely related to the greater amount of absorbed UV light during the longer days of the summer incubations. As such, our findings suggest that longer ice-free periods of boreal lakes may increase the amount of degradation of terrestrial DOC. Our findings also suggest that photodegradation may play an important role for overall degradation of terrestrially derived DOC, particularly outside of the freshet period and in regions with shallow lakes with long residence times.

Conclusions

The net effect of permafrost thaw and wildfire on downstream C and nutrient cycling requires understanding the sources contributing to water export and their associated temporal variability. We characterized seasonal porewater chemistry in permafrost peatlands disturbed by wildfire and permafrost thaw in the discontinuous permafrost zone in Canada and studied DOC transformation processes using in situ incubations. We found that permafrost thaw can lead to increased microbial DOC lability, whereas wildfire decreased DOC lability. These effects were most pronounced in spring after the freshet and were linked to variability in DOC chemical composition. Differences between sites occurred only in spring, likely due to between-site differences in the accumulation of degradation by-products during winter. Notably, the spring season is responsible for the majority of annual DOC export to downstream aquatic ecosystems, as this time period is when the catchment is hydrologically connected. Our study further indicates that photodegradation is an important process for transformation and degradation of DOC derived from peatlands, yet further investigations are required to understand whether DOC photodegradation losses vary across peatland sites or seasons. Overall, the observed differences in DOC composition and lability between peatland sites strongly suggest that disturbances linked to climate change such as permafrost thaw and wildfire have the potential to have cascading effects on downstream aquatic carbon cycling.

Acknowledgements

This study was funded by support from the National Science and Engineering Research Council Discovery grant (RGPIN-2016-04688); the Campus Alberta Innovates Program; the University of Alberta Northern Research Awards; and the Polar Knowledge Canada (POLAR) Science and Technology program. The research in this article was included as a data-chapter in the MSc thesis by Katheryn Burd (Burd 2017). We thank Liam Heffernan, Carolyn Gibson, Michael Barbeau, and Jessi Steinke for assistance in the field.

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cover image Canadian Journal of Soil Science
Canadian Journal of Soil Science
Volume 100Number 4December 2020
Pages: 503 - 515
Editor: M. Anne Naeth

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Received: 1 December 2019
Accepted: 12 February 2020
Published online: 25 February 2020

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This paper is part of a Special Issue entitled “University of Alberta Contributions to Soil Science: Past, Present, and Future”.

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Key Words

  1. dissolved organic carbon
  2. permafrost
  3. incubation
  4. peatlands
  5. wildfire
  6. biodegradability

Mots-clés

  1. carbone organique dissous
  2. pergélisol
  3. incubation
  4. tourbières
  5. feu de forêt
  6. biodégradabilité

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This paper is part of the Climate Change Knowledge Cooperative, a Canadian Science Publishing partnership with Kudos Innovation.

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Katheryn Burd
Department of Renewable Resources, University of Alberta, Edmonton, AB T6G 2H1, Canada.
Cristian Estop-Aragonés
Department of Renewable Resources, University of Alberta, Edmonton, AB T6G 2H1, Canada.
Suzanne E. Tank
Department of Biological Sciences, University of Alberta, Edmonton, AB T6G 2R3, Canada.
David Olefeldt olefeldt@ualberta.ca
Department of Renewable Resources, University of Alberta, Edmonton, AB T6G 2H1, Canada.

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10. Modeling Holocene Peatland Carbon Accumulation in North America
11. Landscape matters: Predicting the biogeochemical effects of permafrost thaw on aquatic networks with a state factor approach

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