Aquatic export of young dissolved and gaseous carbon from a pristine boreal fen: Implications for peat carbon stock stability

The stability of northern peatland's carbon (C) store under changing climate is of major concern for the global C cycle. The aquatic export of C from boreal peatlands is recognized as both a critical pathway for the remobilization of peat C stocks as well as a major component of the net ecosystem C balance (NECB). Here, we present a full year characterization of radiocarbon content (14C) of dissolved organic carbon (DOC), carbon dioxide (CO2), and methane (CH4) exported from a boreal peatland catchment coupled with 14C characterization of the catchment's peat profile of the same C species. The age of aquatic C in runoff varied little throughout the year and appeared to be sustained by recently fixed C from the atmosphere (<60 years), despite stream DOC, CO2, and CH4 primarily being sourced from deep peat horizons (2–4 m) near the mire's outlet. In fact, the 14C content of DOC, CO2, and CH4 across the entire peat profile was considerably enriched with postbomb C compared with the solid peat material. Overall, our results demonstrate little to no mobilization of ancient C stocks from this boreal peatland and a relatively large resilience of the source of aquatic C export to forecasted hydroclimatic changes.

The distinction between these two sources is relevant to the stability of a peatland's C stock, as well as providing information about the links between different flux components in the NECB. Changes in climate and hydrology are known to exert a strong control on the magnitude of each component in a peatland's NECB, including the net ecosystem exchange (NEE) (Nijp et al., 2015;Peichl et al., 2014), CH 4 emissions (Bellisario, Bubier, Moore, & Chanton, 1999;Wilson et al., 2016), C accumulation (Frolking et al., 2001), and aquatic C export (Leach et al., 2016). Whether these drivers influence the source dynamics of a peatland's aquatic C export is to a large extent unknown (Wilson et al., 2016). Identifying the source of aquatic C export will help predict NECB response to environmental changes.
The radiocarbon content ( 14 C) and stable isotope composition (d 13 C) of C can help elucidate the sources of C. The reported 14 C content of aquatic C export from peatlands is notably variable across spatial and temporal scales, as well as between C species (Butman, Wilson, Barnes, Xenopoulos, & Raymond, 2014;Evans et al., 2014;, thus drawing further attention to the complex mixture of sources contributing to aquatic C export in the NECB (Billett et al., 2015). The currently available data suggest that the age of stream DOC, CO 2 , or CH 4 exported from peat-dominated catchments can range from modern to >1,000 years (Billett et al., 2015;Evans et al., 2014;Garnett et al., 2012). Only a few studies have repeated 14 C characterization in streams over time Schiff et al., 1997;Tipping, Billett, Bryant, Buckingham, & Thacker, 2010). These studies suggest that stream C age can change significantly over a complete annual cycle, with changes in 14 C content corresponding to as much as 500 years when expressed in conventional 14 C years. Major discrepancies in 14 C content between DOC, CO 2 , and CH 4 within the same stream have also been observed, with differences of up to 1,000 years between the different C species (Billett, Garnett, & Harvey, 2007;Garnett, Hardie, Murray, & Billett, 2013;Leith, Garnett, Dinsmore, Billett, & Heal, 2014). Together, these results emphasize that aquatic C export from peatlands can arise from a variety of C sources and transport mechanisms. Hence, individual catchment studies are still needed to develop a more generalized understanding of the role of peatlands in the global C cycle.
Despite the highly variable 14 C content of C export in streams, a few reoccurring patterns can be discerned in the existing literature (Marwick et al., 2015). On many occasions, stream DIC is found to be older than DOC. Such differences are typically attributed to a partial sourcing of the DIC from carbonate dissolution (Billett et al., 2007;Mayorga et al., 2005;Vihermaa, Waldron, Garnett, & Newton, 2014). But, aged DIC has also been reported in peatland streams where such minerals do not seem to contribute to DIC, suggesting instead a partial sourcing from mineralization of older peat  or preferential mineralization of 14 C-depleted fractions of the OC pool (Mccallister & Del Giorgio, 2012). The occurrence of 14 C-depleted DOC, on the other hand, has been linked to both anthropogenic disturbance and increasing sediment load to inland waters (Butman et al., 2014;Marwick et al., 2015;Moore et al., 2013). Older CH 4 compared with CO 2 has also been reported in several peatland streams and attributed to ebulitive transport of CH 4 from deep peat horizons or geological sources Leith et al., 2014). Several studies have also reported an increase in postbomb 14 C content with increasing stream discharge, suggesting that high-flow events tend to mobilize younger material, typically associated with more superficial hydrological flowpaths Dyson et al., 2011;Garnett et al., 2012). In northern high latitudes, stream discharge can vary by several orders of magnitude over the course of a full year cycle, with a majority of the annual runoff generated during the spring freshet, contributing to more than half of the annual aquatic C export ( Agren, Berggren, Laudon, & Jansson, 2008;Dyson et al., 2011;Leach et al., 2016).
In this study, we characterized the 14 C content of aquatic DOC, CO 2 , and CH 4 exported from a boreal peatland over a complete hydrological year with the aim of determining their sources and major controls. Our case study was the Deger€ o Stormyr, an extensively studied pristine nutrient-poor fen in northern Sweden, where organic C has accumulated for >8,000 years (Larsson, Segerstr€ om, Laudon, & Nilsson, 2016;Leach et al., 2016;Nilsson et al., 2008;Peichl et al., 2014;Figure 1). A previous study on aquatic C export at Deger€ o suggested that a significant proportion of the runoff could be generated from deep peat horizons (Leach et al., 2016). Based on these combined findings, we hypothesized that aquatic C export will be comprised of a major 14 C-depleted component due to partial sourcing from breakdown of ancient peat C located in deep soil horizons. To test this hypothesis, we coupled our analysis of aquatic 14 C export with a full peat depth profile characterization of the 14 C content of the same three dissolved C species and previously published 14 C characterization of the solid peat material Nilsson, Klarqvist, Bohlin, & Possnert, 2001). This allowed us to identify the sources and main drivers of aquatic C export, information that is critical to determining the stability of the peatland C stock and its resilience to changes in climate and hydrology.

| Study site and instrumentation
The study was conducted within the Deger€ o Stormyr mire complex which is located ca 60 km north-west of Ume a, northern Sweden.
The mire complex is located on a topographic high point (~270 m.a.s.l; 64°11 0 N, 19°33 0 E) and has a total surface area of 6.5 km 2 , with a 55 m elevation gradient (Leach et al., 2016). Deger€ o Stormyr consists of a mosaic of interconnected mires divided by islets and ridges of glacial till and is classified as an oligotrophic fen. The underlying bedrock is composed predominantly of base-poor Svecofennian metasediments/metagraywacke and includes no known carbonate containing minerals. The water table position is typically within 20 cm of the peat surface during the growing season. The climate at the site is cold temperate humid with a persistent snow cover during November to April. The maximum thickness of the snow pack is usually around 60 cm. The depth of the winter soil frost typically ranges from 10 to 30 cm (Granberg, Ottosson-L€ ofvenius, Grip, Sundh, & Nilsson, 2001). The 30 year (1981-2010) mean annual precipitation is 614 mm, of which 35% usually falls as snow, based on observations from the nearby (10 km) Svartberget climate station (Swedish University of Agricultural Sciences). The mean annual temperature is +1.8°C, ranging from a maximum average of +14.7°C in July and minimum average À9.5°C in January . Annual peak stream discharge at the mire outlet, as measured at the C-18 station, typically occurs during spring due to snow melt runoff. Large rain events during summer and autumn can also generate peak flows in some years.
Winter is dominated by low flow conditions.
The contemporary net ecosystem carbon budget (NECB) at Deger€ o is estimated to be À24 g C m À2 yr À1 (Nilsson et al., 2008), while the estimated Holocene long-term average rate of peat C accumulation is 13 g C m À2 yr À1 , pointing to a similar or higher NECB in current times compared with the Holocene average Figure 1b). The contemporary net ecosystem exchange (NEE, 2001(NEE, -2012 is À58 (AE21) g C m À2 yr À1 (Peichl et al., 2014) and the combined C loss as CH 4 emission and total aquatic C export corresponds to about 50% of the NEE (Leach et al., 2016;Nilsson et al., 2008;Figure 1b). The vegetation covering the fen is dominated by lawns and carpet plant communities dominated by Eriophorum vaginatum L., Trichophorum cespitosum (L.) Hartm., Vaccinium oxycoccos L., Andromeda polifolia L., and Rubus chamaemorus L., with both Carex limosa L. and Schezeria palustris L. as well as sparse occurrence of Carex rostrate.
The studied stream is the outlet of a sub-catchment within the Deger€ o mire complex, draining a total area of 2.7 km 2 , representing 40% of the total mire complex (Figure 1a). The fen covers 70% of the stream's catchment area, while the remaining 30% consists of coniferous forest underlain by soils derived from glacial till, located in the periphery of the fen, as seen from LIDAR scan of the catchment, available in Leach et al. (2016). Water balance assessment indicates a low potential for hydrological sources coming from outside the delineated catchment area (Peichl et al., 2013). Runoff water generated within the nearest forested portion of the catchment must cross at least 950 m (horizontal distance) through the fen in order to reach the headwater source. During this transport, much of the original chemical character of this water from the podzolized mineral soils underlying the forest is likely lost, particularly its original C load, which is heavily  (Figure 1). Stream discharge was determined by applying a stage height-discharge rating curve to hourly water level measurements. A theoretical rating curve for the flume was calibrated using manual discharge measurements made over a range of flow conditions.
To make continuous measurements of aquatic CO 2 concentrations, both stream stations were instrumented with Vaisala CARBO-CAP GMP221 nondispersive infrared (NDIR) CO 2 sensors (range 0%-5%), which were enclosed with a water-tight, gas-permeable  (Nilsson et al., 2008) representing the net ecosystem exchange (NEE), estimated based on a 12 year record of gross ecosystem production (GEP) and ecosystem respiration (ER) during 2001-2012 (Peichl et al., 2014). The aquatic C export was also estimated from a 12 year record of TOC, DIC, and CH 4 export from the stream at the C17-Headwater source and C18-Flume stations (2003-2014) (Leach et al., 2016). The long-term C accumulation was based on peat core dating from Larsson et al. (2016). The CH 4 emissions from the peat surface and contemporary peat accumulation were estimated based on a 2 year record presented in Nilsson et al. (2008) [Color figure can be viewed at wileyonlinelibrary.com] CAMPEAU ET AL.
Both stations were also instrumented with pressure transducers (MJK 1400, 0-1 m, MJK Automation AB) for water table height measurements and temperature sensors (TO3R, TOJO Skogsteknik) that recorded hourly. All continuously measured data were stored on external data loggers (CR1000, Campbell Sci.). The CO 2 and CH 4 concentrations were measured from the same sample, using the acidified headspace method ( Aberg & Wallin, 2014;Wallin, Buffam, € Oquist, Laudon, & Bishop, 2010). The stream water DIC, CO 2 , and CH 4 concentrations were calculated from GC determined headspace pCO 2 and pCH 4 corrected for in situ stream pH and water temperature. The precision of sampling and analysis were estimated to an average of 10% (standard deviation (SD)), based on replicate sampling (Nilsson et al., 2008;€ Oquist, Wallin, Seibert, Bishop, & Laudon, 2009). The sensor-derived and manual spot measurements of stream water CO 2 concentration showed a close correspondence, with an average difference of À7% (t-test p = .61). The detection limit for CH 4 concentration corresponded to 2.0 lg C L À1 at a stream water temperature of 2°C and with the water-air volume ratio used (1 ppm CH 4 from the instrument). This limit was exceeded in all stream and soil water samples.
Samples for pH analysis were collected in 50 ml high-density polyethylene bottles, which were slowly filled and closed under water in order to avoid pockets of air in the bottle. The pH was measured in the laboratory using a Mettler Toledo MP 220 pH Meter with an accuracy of AE0.1 units. The DOC concentration was analyzed from 10 ml of stream or peat pore water, filtered at 0.45 lm in the field and stored in high-density polyethylene bottles.
Previous analysis showed no statistically significant differences between the filtered and unfiltered samples, indicating that DOC is a reasonable proxy for TOC (Laudon et al., 2011;Nilsson et al., 2008).
Prior to analysis, samples were acidified and sparged to remove inorganic carbon. The samples were analyzed using a Shimadzu Total Organic Carbon Analyzer TOC-V CPH , following storage in 4°C refrigerators for 2-3 days periods (Leach et al., 2016).
Peat pore water samples and a subset of stream water samples were analyzed for d 13 C-DIC and d 13 C-CH 4 . Samples for d 13 C-CH 4 analysis were collected in a 100 ml glass vial, filled completely with stream or peat pore water samples and closed airtight with a rubber septum. One ml of 50% w/v ZnCl 2 solution was injected in each glass vial directly after sample collection for preservation. Samples for the d 13 C-DIC analysis were directly injected into 12 ml septum-sealed glass vials (Labco Limited) prefilled with N 2 gas, and preinjected with phosphoric acid in order to convert all DIC species to CO 2 (g). The samples for d 13 C composition were analyzed using a Gasbench II and a Thermo Fisher Delta V mass spectrometer. The Manual spot measurements of 14 C-CO 2 were carried out with the super headspace method whereby CO 2 samples were collected onto MSCs [see Garnett, Billett, Gulliver, and Dean (2016) for further details]. The method for manual spot measurement of 14 C-CH 4 followed a protocol similar to that for 14 C-CO 2 , with the exception that the recovered gas volume was injected into 10 L foil bags (SKC Ltd, UK), rather than MSCs [see   Alternatively, an even mixture now consisting of 93 %modern and 97 %modern, can result in the same total 14 C content and age, yet incorporate no postbomb 14 C. In light of these issues, we purposefully avoided reporting the measured 14 C contents in terms of ages from conventional 14 C dating. Instead, we focused our interpretation of 14 C results using the relative differences in 14 C contents between C forms or changes over time in order to draw conclusions on the sources and controls of dissolved C in stream and peat pore water.
The 14 C content of the calibrated solid peat stratigraphy, obtained from Larsson et al. (2016), was also reported here for comparison with the 14 C-content of the dissolved and gaseous C species.
It is important to note that these 14 C-contents were determined on plant fragments from mosses or aboveground vascular plant tissues to ensure a representative dating of the specific depth Nilsson et al., 2001; Figure 3a and Section S2). Both the hydrolyzable and fine fraction (<0.045 mm) of the peat core samples were removed from this 14 C analysis. These fractions are known to be generally be more 14 C-enriched than the age of corresponding depth . For simplicity, we refer to these 14 C-contents as the solid peat material.

| Stream C export and isotopic composition
The stream water DOC concentration at the headwater source, aver- The total aquatic C export at the headwater source station was estimated at 10.8 g C m À2 yr À1 , of which DOC export contributed 76% (8.2 g C m À2 yr À1 ), with CO 2 and CH 4 export representing 2.4 and 0.2 g C m À2 yr À1 (22% and 2%), respectively during the study year. The stream discharge ranged across nearly three orders of magnitude, from the lowest flow conditions at 0.5 L/s during the winter and summer, to peak flow conditions of 382 L/s during the spring freshet in April and May (Figure 2b).
The average flow over the study period was 21.9 (AE40.1) L/s.
There was a significant negative relationship between the stream water concentration of all three C species and discharge over the studied period (Fig. S3). Spring freshet contributed 71% of the annual runoff, as well as 60% of the annual DOC export, 53% of the CO 2 export, but only 26% of the annual CH 4 export. Two major rain events occurred during the open water season, the first CAMPEAU ET AL.
| 5527 in late July (peak flow = 40.8 L/s) and the second in late September (peak flow = 70.9 L/s). Together, they contributed ca 10% of the annual runoff as well as 13%, 16%, and 23% of DOC, CO 2 , and CH 4 annual export, respectively.

| DISCUSSION
4.1 | Intra-annual variability in 14 C content of aquatic C export Contrary to our original hypothesis, the stream water 14 C-DOC, 14 C-CO 2 , and 14 C-CH 4 was highly influenced by postbomb C, with relatively stable 14 C content in both stream and soil waters over the study year (Figures 2b and 3a). Other studies reported similar total 14 C-DOC content and intra-annual variability in streams from peatdominated catchments, indicating a regular occurrence of young material in aquatic DOC export (Palmer, Hope, Billett, Dawson, & Bryant, 2001;Schiff et al., 1997;Tipping et al., 2010). Few studies have repeatedly characterized 14 C-CO 2 in stream water over time, but a study from a Scottish peatland reported a similar intra-annual variability in the 14 C content, but with a much lower 14 C content, resulting in both older and wider span in 14 C content (from 91.6 to 86.0 %modern; Garnett et al., 2012)). Together, our results indicate that the sources of the different C species sustaining aquatic C export are relatively similar throughout the year and largely contain recently fixed C from the atmosphere (postbomb, i.e. 1964). This apparently constant 14 C source occurs despite considerable fluctuations in hydroclimatic conditions and C concentration in the stream (Figure 2a,b), which are known to affect both metabolic pathways and hydrological flowpaths delivering C to streams, both at this (Leach et al., 2016) and other mire sites (Dinsmore, Billett, & Dyson, 2013;Wilson et al., 2016;Winterdahl, Laudon, Lyon, Pers, & Bishop, 2016).
While the stream water 14 C-CO 2 and 14 C-CH 4 were similar across measurements, there was a persistent gap of~10 %modern between the 14 C-DOC and that of the gaseous C forms (Figure 2b).
Such disparity in 14 C content clearly indicates major differences in source material and formation processes, and/or transport mechanisms delivering the different C forms to the stream. Other studies have reported a higher content of postbomb C in stream DOC relative to aquatic CO 2 , which could be explained by the influence of carbonate minerals dissolution generating DIC (Billett et al., 2007;Leith et al., 2014;Tipping et al., 2010). To our knowledge, such DIC sources are absent in the Deger€ o Stormyr catchment, according to the bedrock composition and the molar Ca:Na in streams in the area (averaging 0.58 AE 0.12; Laudon et al., 2013). Therefore, our results indicate that large differences in 14 C content between DOC and gaseous C forms in streams can persist even in catchments with strict biogenic C sources. The 14 C characterization of DOC, CO 2 , and CH 4 down the peat profile also allowed to determine that a major flowpath for the aquatic C export occurs through the bottom 2-4 meters of the peat profile, where the 14 C content of the three different C forms matches most closely that of the same C species in the stream (Figure 3a).
The intra-annual variability in stream water 14 C-CO 2 is best explained by a constant supply of C from the deep peat layers (2-4 m; averaging 70% of the annual export), supplemented with superficial flowpaths during high-flow events (0-25 cm; Fig. S7). The peat bulk density profile also demonstrates a clear decrease in peat F I G U R E 6 Scatterplots showing the relationship between the antecedent stream discharge (9 days earlier), expressed in L/s and using a logarithmic scale and (a) the stream water 14 C-DOC (gray squares), and (b) the 14 C-CO 2 (black circles), expressed in %modern, with the full lines representing the least-square linear regression model (Equations 4 and 5). In panel (b), the light gray triangles represent the 14 C-CH 4 , the large black circles represent manual spot measurements of 14 C-CO 2 , and the small black circles represent the time-integrated measurements of 14 C-CO 2 , with bars representing the standard deviation of the discharge over the measurement period. The black dot with a white cross represents the time-integrated 14 C-CO 2 measurement taken from January to March, which was considered an outlier. (c) scatterplot showing the R 2 of individual least-square linear regression models between 14 C-DOC (gray squares) or 14 C-CO 2 (black circles) and stream discharge under different lag periods starting from t = 0, up to 2 weeks earlier, with the dotted line highlighting the best fit, found at À9 days' lag period CAMPEAU ET AL.
| 5531 density at 2 m depth (Fig. S4), suggesting the presence of macropores or preferential flowpaths with increased hydrological conductivity at this specific depth. In addition, the relationship between stream discharge and DOC, CO 2 and CH 4 concentrations demonstrated that stream C concentrations agree best with the bottom 2-4 m of the peat profile during base flow conditions (Fig. S3). All of these inferences were further supported by the patterns in water stable isotopes (d 18 O), pH, and water temperatures (Figures 5, S5 and S6). This interpretation of the sources of aquatic C export agrees well with the proposed hydrological model for the same catchment from Leach et al. (2016). Several studies have suggested that superficial flowpaths dominate the runoff generation in peatlands, with deep soil horizons often mostly hydrologically inactive (Ingram, 1982;Ronkanen & Kløve, 2007;Tipping et al., 2010), a conception that is clearly challenged by our results and those of others (Glaser et al., 2016;Holden & Burt, 2003) including a nearby peatland catchment (Laudon et al., 2007;Peralta-Tapia et al., 2015).

| Sources of DOC, CO 2 , and CH 4 across the peat profile
The 14 C-DOC, 14 C-CO 2 , and 14 C-CH 4 across the peat profile deviated considerably from the 14 C content of the solid peat material (Figure 3a). Both the 14 C-CO 2 and 14 C-CH 4 decreased with depth, by ca. À3 %modern per meter, which is much lower than the drop in 14 C content observed in the bulk solid peat (~20 %modern per meter; Figure 3a). The 14 C-CO 2 and 14 C-CH 4 near the surface of the peat profile (À25 cm; averaging 107.1 and 108.8 %modern, respectively), also deviated significantly from the atmospheric 14 C content during the studied period (102.5 % modern; Figure 3a). This suggests that direct root respiration is unlikely to be the sole source sustaining gaseous C concentrations near the peat surface. Instead, the 14 C content of CO 2 , CH 4 was most similar to the 14 C-DOC at that depth (Figure 3a), suggesting that a substantial fraction of the near surface gases is derived from DOC mineralization.
The close similarity in 14 C content between CO 2 and CH 4 in both peat pore water and stream water is a clear indication of the shared sources and transport processes controling both gases ( Figure 4a, Equation 1; Chanton et al., 2008;Clymo & Bryant, 2008;Garnett, Hardie, & Murray, 2011). Other studies have reported considerably larger differences in 14 C content between CO 2 and CH 4 in peat pore water or associated streams, which has been linked to disparity in sources or geological influences on either CO 2 or CH 4 (Chasar, Chanton, Glaser, Siegel, & Rivers, 2000;Garnett et al., 2013;Leith et al., 2014). The aCO 2 -CH 4 value in all peat pore waters (ranging from 1.047 to 1.10), suggested a persistent dominance of hydrogenotrophic methanogenesis, in contrast to acetoclastic methanogenesis, as the main metabolic pathway within the entire peat profile (Whiticar, Faber, & Schoell, 1986; Figure 3c). The dominance of hydrogenotrophic methanogenesis may explain the significant positive relationship between the 14 C content of both gases (Figure 4a, Equation 1).
The slope of this relationship (0.9) is similar to that reported from other peatlands (Chanton et al., 2008). Both methanogenic pathways are known to occur in peatlands, sometimes co-existing and shifting with seasons (Hornibrook, Longstaffe, & Fyfe, 2000;Throckmorton et al., 2015). However, hydrogenic methanogenesis seems to be the dominant pathway in acidic and nutrient-poor ecosystems, which characterizes well this poor fen (Bellisario et al., 1999;Holmes, Chanton, Tfaily, & Ogram, 2015;Kotsyurbenko et al., 2004). Despite the similarities between the 14 C-CO 2 and 14 C-CH 4 content, there was also a small but persistent gap in 14 C content of the two gases, with CH 4 being slightly 14 C-enriched (ca. 2 %modern) relative to CO 2 , yielding an intercept of +6 for this relationship. This intercept is noticeably more positive than reported from other peatlands (from À9 to À23) (Chanton et al., 2008; Figure 4a), which likely reveal differences in the source material contributing to the both C gases across catchments. In our case, this systematic gap in 14 C content between CO 2 and CH 4 could be explained by a comparatively larger input of atmospheric CO 2 through direct root respiration or by degradation of root exudates in the CO 2 pool. While it is known that dissolved and gaseous C forms can be subject to different transport processes across the peat profile (e.g. mass flow, advection, and diffusion) (Chanton et al., 2008;Clymo & Bryant, 2008;Flury, Glud, Premke, & Mcginnis, 2015), the exact processes leading to the reported 14 C patterns across peat depths remain to be fully resolved. More in depth hydrological studies are needed to determine more clearly the origin of the C found across this peat depth profile. The peat profile characterized here is found at a hydrological confluence of multiple flowpaths. The large C con- Overall, the mineralization of solid peat material from deep horizons appears to play a minor role in the production of dissolved C in the peat, particularly near the surface and bottom of the peat profile ( Figure 3a, Section S2). Our results clearly suggest that organic substrates, derived from recently fixed C from the atmosphere (<60 years), contribute significantly to sustaining DOC, CO 2 , and CH 4 concentrations across the peat profile and in the stream (Figures 2b and 3a). Other studies have also reported large isotopic disconnects between the dissolved C species and the surrounding solid peat (Aravena et al., 1993;Chanton et al., 1995Chanton et al., , 2008Charman, Aravena, Bryant, & Harkness, 1999;Chasar et al., 2000;Clymo & Bryant, 2008;Corbett et al., 2013;Glaser et al., 2016;Schiff et al., 1998), but those differences are typically smaller than the ones reported in this study. Several of these studies have compared results from bogs and fens, leaving some indications that fens could contain comparatively younger dissolved C likely reflecting their differences in hydrological controls (Bellisario et al., 1999;Chanton et al., 1995Chanton et al., , 2008Corbett et al., 2013;Glaser et al., 2016). The underlying causes of these contrasting patterns in dissolved 14 C content across peatlands have yet to be explained, but the degree of anthropogenic pressure is likely a key factor controling the mobilization of ancient C stocks (Butman et al., 2014;Evans et al., 2014;Moore et al., 2013). Peatland systems are also well recognized for their diversity in terms of genesis and biogeochemistry, so further studies at other poor fens may reveal similar patterns to the ones reported here.
4.3 | Controls on aquatic 14 C export and implications for peat C stock stability The intra-annual variability in stream water 14 C-DOC and 14 C-CO 2 was best explained by stream discharge, albeit with a significant time delay (Figure 6, Equations 4 and 5). This relationship likely reflected shifts in contributing flowpaths, delivering C to the stream with a higher modern fraction during high-flow periods associated with superficial flowpaths (0-25 cm) and with more 14 C-depleted C exported during base flow conditions where contributions from deeper flowpaths dominate (2-4 m) (Fig. S7). The clear improvement of the explanatory power of discharge on 14 C-CO 2 and 14 C-DOC with a lag time of up to 9 days possibly suggests differences in hydrological conductivity between the source areas feeding aquatic C export ( Figure 6c). These differences may cause the increase in stream discharge to occur before the resulting increase in 14 C content, suggesting that the deeper and more 14 C-depleted source area (2-4 m) responds faster to increasing runoff than the surficial and more 14 Cenriched source area. A similar discharge-dependent relationship was also observed for a Scottish peatland stream, where the modern fraction of C export also increased during high-flow events, but without a noticeable time delay . Our characterization of stream 14 C-CO 2 and 14 C-DOC also extended over the icecovered season, a period of the year so far uncharacterized in northern peatlands. One stream water 14 C-CO 2 sample, collected during the lowest winter flow conditions (Jan-March), was identified as an outlier in the relationship with discharge, since it had a higher postbomb C content than predicted (Figure 6b). It is likely that the sources of CO 2 within the peat profile and hydrological flowpaths delivering C to the stream during winter conditions differ compared with other seasons, but the exact processes leading to such shifts remain to be determined. Nonetheless, our results suggested no major changes in the source of aquatic C export between the icefree and ice-covered season.
The flow conditions that occurred during our study captured 99.5% of the long-term flow variability observed at Deger€ o Stormyr  (Figure 7a; Leach et al., 2016). However, the most extreme flow conditions were not characterized in our stream 14 C sampling (>75 L/s, 90% exceedance), thus representing a source of uncertainty ( Figure 7a). Nonetheless, the observed relationship between the stream 14 C-DOC and 14 C-CO 2 and discharge spanned nearly the full range of 14 C content characterized across the peat profile, indicating low potential for alternative C sources contributing to the C export under more extreme flow conditions (Figures 2b, 3a, and 6). We estimated the possible inter-annual variability in the 14 C export (see details in Section S1), based on the two empirical relationships (Equations 4 and 5) and the long-term (2003-2016) record of stream discharge from Leach et al. (2016) (Figure 7b). The total annual runoff across these different years varied from 240 to 834 mm and the total C export varied from 8 to 26 g C m À2 yr À1 (see Leach et al. (2016) for more information). This exercise allowed us to estimate that the 14 C content of the total annual C export may increase by ca 2 %modern with nearly a tripling of the annual runoff ( Figure 7b). Therefore, we conclude that the source of aquatic C export is relatively constant and resilient to changes in hydroclimatic conditions. Similar conclusions were reached by a recent study addressing potential influence of rising air temperatures (Wilson et al., 2016). The stability of the source of aquatic C export may persist given the forecasted hydroclimatic changes for the region (annual precipitation >17%, mean annual temperature >3.7°C, in the coming century) (Teutschbein, Grabs, Karlsen, Laudon, & Bishop, 2015).
The C cycling in northern peatlands is highly sensitive to a number of environmental changes, most of which are driven by climatic changes or anthropogenic disturbances. The flux components of the NECB are recognized to be influenced by changes in temperature (Dinsmore et al., 2013;Peichl et al., 2014;Wilson et al., 2016;Wu & Roulet, 2014), precipitation and cloud cover (Dinsmore et al., 2013;Nijp et al., 2015), timing and magnitude of the spring freshet ( Agren et al., 2008;Dyson et al., 2011), atmospheric CO 2 increase (Freeman et al., 2004) or changes in vascular plant communities (Lafleur, Roulet, Bubier, Frolking, & Moore, 2003;Lund et al., 2010), sulfur and nitrogen deposition (Eriksson, € Oquist, & Nilsson, 2010), and peatland management (Jauhiainen, Limin, Silvennoinen, & Vasander, 2008;Waddington & Price, 2000). Here, we demonstrated that the source of aquatic C export from this boreal poor fen remains relatively unchanged despite large annual hydroclimatic variation. In addition, the aquatic C export was only to a small degree sustained by degradation of ancient peat C stocks and was instead mainly supported by young C sources. It is worth noting that a growing body of evidence demonstrates the highly sensitive nature of peat C stock mobility to anthropogenic disturbances (Butman et al., 2014;Evans et al., 2014;Moore et al., 2013), an aspect that is not significant for this pristine boreal fen. Some of the 14 C patterns reported here have seldom been documented in previous studies and appear to differ noticeably from the existing literature. Further 14 C characterization of dissolved and gaseous C at other sites will be necessary to determine whether Deger€ o is unique in terms of C sources and transport dynamics, and to more completely assess the overall stability of northern peatland C stocks. It is however, noteworthy that a boreal peatland that has been accumulating organic C since~8,000 BP currently appears resilient, showing little or no evidence of ancient C release into the aquatic environment.