Biodegradability of dissolved organic carbon in permafrost soils and waterways

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Introduction
Boreal and Arctic ecosystems contain more than half of global terrestrial organic carbon (Tarnocai et al., 2009;Hugelius et al., 2014), much of which will be vulnerable to microbial processing and release to the atmosphere by the end of the century (Slater et al., 2013;Schaefer et al., 2014;IPCC 2013).At high latitudes, ecosystem carbon balance depends largely on aquatic processes (Kling et al., 1992;Striegl et al., 2012;Vonk and Gustafsson, 2013) with lakes, wetlands, rivers, and streams covering more than half of the land surface in many regions (McGuire et al., 2009;Loveland et al., 2000;Lammers et al., 2001;Aufdenkampe et al., 2011;Avis et al., 2011).However, little is known about mechanistic controls on persistence or processing of organic carbon currently flowing through Arctic watersheds (Mann et al., 2012;Wickland et al., 2012), and even less is known about the behavior of permafrost-derived organic carbon that is delivered to arctic freshwater and marine ecosystems (Cory et al., 2013;Vonk and Gustafsson, 2013).
Arctic watersheds transport an average of 34 Tg C yr −1 of dissolved organic carbon (DOC) and 6 Tg C yr −1 of particulate organic carbon (POC) to the Arctic Ocean (Holmes et al., 2012;McGuire et al., 2009), not including fluxes from coastal erosion.Though no model projections of future circum-arctic hydrologic carbon flux exist, a few recent studies predict that organic carbon loading to the circum-arctic watershed may increase in the future (Abbott et al., 2015;Kicklighter et al., 2013).However, observed patterns of changes in hydrological carbon loading in permafrost regions are inconsistent, with Figures increases in DOC export from areas with extensive peat deposits (Frey and McClelland, 2009), but decreases in discharge-normalized DOC export in other regions, due to increasing flow paths, and increased mineralization in soils (McClelland et al., 2007;Petrone et al., 2006;Striegl et al., 2005;Tank et al., 2012).Furthermore, conflicting patterns of DOC biodegradability exist with respect to seasonality and permafrost extent (Kawahigashi et al., 2004;Striegl et al., 2005;Holmes et al., 2008;Balcarczyk et al., 2009;Frey and McClelland, 2009;Vonk et al., 2013b;Abbott et al., 2014).The scarcity of long-term data as well as a lack of conceptualization of the processes controlling DOC transport and processing represent an important source of uncertainty in the permafrost-regional carbon balance.
In both terrestrial and aquatic ecosystems, carbon mineralization is often limited to the dissolved form, since DOC is composed of lower molecular weight compounds that can be directly transported across microbial cell membranes (Battin et al., 2008), though particulate matter provides surface area for bacterial attachment in aquatic ecosystems (del Giorgio and Pace, 2008).Biodegradability of DOC (BDOC), therefore, is a key regulator of ecosystem metabolism in general and the rate of permafrost carbon release to the atmosphere specifically (Holmes et al., 2008;Mann et al., 2012;Wickland et al., 2012;Abbott et al., 2014).While promising proxies of BDOC have been identified, including optical signatures, molecular characteristics and nutrient concentrations (Balcarczyk et al., 2009;Wickland et al., 2012;Abbott et al., 2014), BDOC is typically assessed through incubation experiments, representing a simple metric of microbial uptake and mineralization.Most studies measure BDOC through: (i) production of dissolved inorganic carbon (DIC), (ii) consumption of DOC, or (iii) consumption of O 2 (McDowell et al., 2006).While these methods can give comparable results, differences in experimental factors can directly influence BDOC, including duration of incubation, temperature, light exposure, type of filtration, and the addition of bacterial inoculum.While this methodological diversity complicates direct comparison of BDOC measurements from across the Arctic permafrost-region, it also represents an opportunity to identify fundamental controls on DOC processing.Introduction

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Full We synthesized results from 14 BDOC studies within the Arctic Ocean watershed representing a total of 551 individual incubations to identify controls and patterns of DOC biodegradability across spatial and temporal scales (Sect.2.1).Based on findings from these studies we developed a standard incubation method, which we tested on water from soils, streams, and rivers from throughout the permafrost region and across seasons (Sect.2.2).We examined the role of seasonality, permafrost extent, and incubation design on metrics of BDOC and recommend a protocol for future BDOC incubations.A meta-analysis of the combined results of our standardized circum-arctic incubations and literature synthesis allowed us to identify temporal and landscapescale patterns in BDOC across Arctic regions.This study represents the first to include both soils (soil leachates) and aquatic systems (streams, lakes, rivers) to explore geographical and seasonal patterns of BDOC in the Arctic.

Literature synthesis
We gathered and analyzed data from permafrost-region BDOC studies that met the following criteria: (1) located in the Arctic Ocean watershed (including the Yukon River watershed); (2) used CO 2 evasion (DIC production) or DOC loss over time to assess biodegradability (we excluded studies based on O 2 loss due to complicating factors such as respiratory coefficients); and (3) incubation was performed in the dark to avoid autotrophic effects or photodegradation.
When an incubation was performed at "room temperature" we assumed 20 • C. For watersheds crossing permafrost boundaries we chose the spatially-dominant permafrost type.Additionally, we sorted the data into soil leachate and aquatic incubations, with subcategories for the aquatic data: lakes, streams (< 250 km 2 ), large streams (250 km 2 to 25 000 km 2 ), rivers (25 000 km 2 to 500 000 km 2 ) and large rivers (> 500 000 km 2 ).

Circumarctic standardized incubation experiment
In June to September of 2013 we performed BDOC experiments with leachates from three soil cores (from near Toolik Field Station, Alaska), water from two streams (Richardson Creek,Alaska;Y3,Siberia), and water from three major Arctic rivers (Yukon, Mackenzie and Kolyma Rivers) (Fig. 1).Soil leachates were performed by adding 500 mL DI water to soil volumes of ca. 2 L, letting this stand for 24 h, and extracting using a pore water sampler measuring total leachate volume extracted.Water samples were collected from the surface in pre-cleaned, pre-rinsed containers and transported (dark and cool) to filtration facilities within 12 h.We developed an incubation methodology adapted for implementation at remote field sites to assure applicability to future work.We measured DOC loss over time rather than O 2 loss or CO 2 production as it did not require specialized supplies or instrumentation in the field.All samples were filtered through pre-combusted Whatman GF/F filters (nominal pore size 0.7 µm), which are commonly used throughout the literature and can be pre-cleaned through combustion (450 • C > 4 h).We set up triplicate incubations with three different treatments to test the The incubated samples were re-filtered through 0.7 µm filters to remove flocculation after 0, 2, 7, 14 and 28 days (using separate vials, in triplicate, for each time step).
Re-filtration removes the majority of the microbial biomass, resulting in a measured DOC loss including both DOC mineralization and assimilation.Samples were immediately acidified with 30 µL of concentrated HCl (high quality grade; to pH ≤ 2).Acidified sample vials were capped and stored refrigerated in the dark until analysis within three months.At the time of analysis, acidified samples were sparged with CO 2 free air for 8 min at 75 mL min −1 and run as non-purgable organic carbon (NPOC) on either a Shimadzu TOC-V or TOC-L analyzer.DOC was calculated as the mean of between three and seven injections and the coefficient of variance was always < 2 %.BDOC was calculated as the change in DOC concentration during the 28 day incubation, relative to the initial DOC concentration, and is reported in percent loss.

Statistical analyses
We combined the literature meta-analysis of 14 papers (n = 551) with data from our circum-arctic incubation experiment (n = 192).Each of the studies identified used different methods for assessing BDOC, complicating and limiting the analyses available.
To examine trends across the total dataset (n = 743) we therefore performed categorical principle component analysis (CATPCA) via optimal scaling.This approach allowed us to compare the effect of multiple variables with mixed measurement levels (scalar, nominal, ordinal).After, we performed a standard principle component analysis (PCA) using the optimally-scaled results to aid in data interpretation.Data normality was as-Introduction

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Full sessed using the Shapiro-Wilk test (p > 0.05).Separate CATPCA and PCA analyses were performed on the aquatic and soil leachate datasets, as well as for methodological and environmental parameters (Table 1).Validity of each PCA was tested using the Barlett tests of sphericity (p < 0.001) and Kaiser-Meyer-Olkin measures of sampling adequacy.Direct oblimin rotation was applied and rotated scores used throughout, allowing for correlation between scores (Manisera et al., 2010).CATPCA runs assigned measures from scalar data (initial DOC, % BDOC, latitude, longitude, Julian day, bottle size, incubation time, and incubation temperature), nominal data (method of C loss, shaking, nutrient addition, inoculum, oxygen availability, location in fluvial network) and ordinal data (filter pore size, and permafrost extent).We considered final rotated PCA correlations of > 0.7 as strong, between 0.5 and 0.7 as moderate, and < 0.5 as weak or absent (Quinn and Keough, 2002).Although this approach was not ideal, in our opinion it proved the best available approach given the diverse dataset which included repeated measures (i.e.multiple time points) of a given parameter (i.e.BDOC; Bradlow et al., 2002).Additionally, we combined data from all studies carried out from 15-25 with incubation durations of 28-34 days, which represented the most common temperature and length used, to test for environmental trends (Figs.3-5).Here we tested for differences among means using analysis of variance (ANOVA).All ANOVA, CATPCA and PCA analyses were conducted in SPSS 22.

Literature synthesis
The 14 literature studies comprised a total of 551 data points of which 418 were aquatic.Most studies were located in North America (242 in Alaska, USA and 227 in Canada; Fig. 1), 234 were from regions without permafrost, and 230 were from regions with continuous permafrost (Fig. 2e).The most common incubation temperatures were 17.5 or 20 • C (41 and 36 % of the data, respectively).The majority of studies (60 % Introduction

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Full of data) used a 0.7 µm glass fiber filters cut-off to determine DOC.Half of the BDOC assays were incubated for between 14 and 40 days (Fig. 2).Furthermore, most incubations in our synthesis were started after addition of an inoculum (80 % of aquatic incubations, 97 % of soil leachate incubations).

Methodological factors affecting BDOC
To examine the effects of inoculum addition and inoculum concentration on BDOC, we compared mean BDOC across our circum-arctic standardized incubation experiment (no inoculum, 1 and 10 % inoculum; n = 40 per treatment).Amount of inoculum (1 or 10 %) had no effect on the proportion of BDOC (ANOVA, p > 0.9).As the degree of inoculation had no clear systematic effect on BDOC loss we grouped all inoculated data (independent of concentration), and all non-inoculated data during further analyses.
In the sections below we examine the patterns present in the combined analysis of aquatic and soil literature results, including our circum-arctic incubation experiments.

Environmental factors affecting BDOC
Similar to Sect.3.2, here we present the statistical results of the combined literature synthesis and circum-arctic incubation experiments concentrating on how environmental variables co-vary with BDOC losses.

Methodological factors influencing BDOC
Aquatic BDOC losses were most strongly influenced by incubation time, with higher total BDOC observed in longer experiments.Despite total DOC loss increasing with longer incubation time, the fraction of BDOC removed over time likely decreases.These are two perspectives to look at the same process, but we want to briefly re-stress our usage of the term BDOC as a proxy for the potential of DOC to degrade in a specific environment.Soil leachate BDOC was not clearly affected by incubation time across experiments.We suggest that the effects of incubation time may have been masked by multiple additional methodological factors significantly influencing particularly soil BDOC experiments.For example, the presence of O 2 within incubations or regular bottle shaking appeared to play a crucial role in soil BDOC losses.As soil extractions typically have higher initial DOC concentrations (despite some degree of dilution applied in the experiment), they may be more susceptible to oxygen drawdown, increasing the importance of regular bottle shaking.Also, the method of assessing carbon loss appeared to play a critical role in the amount of BDOC measured during soil incubations, but not so Introduction

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Full  2006) that found largely comparable results between available methods.We, however, were forced to compare different methods conducted on different samples, which could explain our contrasting findings.

Permafrost extent and longitude
Aquatic and soil BDOC losses were significantly lower in regions without permafrost than in discontinuous or continuous permafrost regions (Fig. 3).This could be explained by the unique dissolved organic matter (DOM) composition of permafrost (Spencer et al., 2015) allowing for more rapid degradation.Furthermore, permafrost DOM is relatively well-preserved under a lack of processing of OC in soils due to long-term frozen conditions (Khvorostyanov et al., 2008;Schuur et al., 2008), though permafrost-derived DOC still shows some signs of processing prior to freezing (Wickland et al., 2012;Abbott et al., 2014).Continuous permafrost regions thus seem to receive relatively well-preserved, unique DOC into soil leachates and aquatic systems leading to higher losses, whereas discontinuous permafrost regions and regions without permafrost receive DOC that has already been subject to some degree of degradation.The presence of permafrost also impacts hydrological flowpaths and transport times, which may result in more efficient delivery of relatively less-processed terrestrial DOC to aquatic systems (Striegl et al., 2005;Walvoord et al., 2012).Alternatively, preferential sorption of specific compounds, freeze-thaw effects, or sub-zero metabolism in permafrost could increase DOC biodegradability (Abbott et al., 2014 and references therein).The difference in BDOC with permafrost extent is stronger in soils than in aquatic systems (Table 3, Fig. 3), likely attributable with a less altered permafrost DOC signature in soils, that is subject to more rapid degradation (Spencer et al., 2015).
Aquatic BDOC was negatively correlated to longitude.Judging from the prevailing geographical regions in the dataset (Fig. 1) this suggests that aquatic BDOC in Alaska Introduction

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Full and Canada was on average higher than in Eastern Siberia.This could be related to a combination of the spatial spread in our dataset with the distribution of yedoma.Yedoma is Pleistocene-aged permafrost (Zimov et al., 2006) predominantly present in northeast Siberia that releases extremely biolabile DOC upon thaw (BDOC between 40-65 % after 30-40 days of incubation, Vonk et al., 2013b;Abbott et al., 2014).In our meta-analysis, most of the aquatic BDOC incubations with yedoma-derived DOC are located in Alaska, which could explain the longitudinal pattern.

Patterns within the fluvial network
In continuous permafrost regions, aquatic BDOC decreases within the fluvial network (Fig. 4).Here, large rivers (defined as watersheds larger than 500 000 km 2 ) showed significantly lower BDOC than streams, large streams, rivers and large rivers.We should note here that streams (< 250 km 2 , n = 149) and large rivers (> 500 000 km 2 , n = 60) are overrepresented in the continuous permafrost dataset, when compared to large streams (250-25 000 km 2 , n = 46) and rivers (25 000-500 000 km 2 , n = 18).Nevertheless, this suggests that continuous permafrost regions release DOC that degrades more rapidly in the fluvial network and that these sources may be absent in regions with discontinuous or no permafrost.Pleistocene yedoma could be such a source, as its strong degradation potential (Vonk et al., 2013a, b;Abbott et al., 2014) leads to preferential utilization in headwater streams (Mann et al., 2015;Spencer et al., 2015).

Seasonality
BDOC decreased with Julian day for rivers and large streams in both continuous and discontinuous permafrost regions, whereas streams (Fig. 5b) and soil leachates (Fig. 5c) showed no seasonal pattern.This pattern may be associated to shifts in carbon source (winter and spring DOC in large Arctic rivers is more biolabile than summer DOC; Wickland et al., 2012;Mann et al., 2012;Holmes et al., 2008) but it is likely more related to a changing hydrologic residence time.In boreal and Arctic systems Introduction

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Full soil thaw-depth increases throughout the summer, resulting in longer water residence times in soils and headwater streams (Harms and Jones, 2012;Jones and Rinehart, 2010;Koch et al., 2013).This allows more time for biodegradable carbon compounds to be mineralized before reaching the river late in the season, effectively reducing measured BDOC in higher-order streams and rivers later in the season.Increasing water temperature through the season could magnify this effect with little mineralization early in the year when soils and streams are cold but accelerating biolabile carbon removal in summer.Hydrologic connectivity between soils and surface waters is generally weaker later in summer (Striegl et al., 2005;Spencer et al., 2008;Koch et al., 2013), which could explain the decoupling in seasonal trends.Furthermore, soil core leachates from a near-surface core that developed fresh plant growth during the growing season showed higher BDOC than cores without fresh plant growth (Fig. 6).These local plant growth-induced spikes in BDOC, likely induced by root exudates (Marscher and Kalbitz, 2003) could also mask seasonal trends in soil leachate BDOC and instead highlight spatial variability.

Other factors affecting BDOC
There are multiple factors that affect BDOC that neither we nor the investigated literature studies have considered.One of these factors is the effect of light.Photochemical processes can lead to rapid DOC losses (up to 30 % in 14 days; Mann et al., 2012) and may alter the DOC composition so that it is more susceptible to microbial degradation (Cory et al., 2013).Furthermore, the presence of POC also serves as an important catalyst in DOC biolability (Battin et al., 2008).In this study we do not investigate any potential co-metabolizing effects of POC degradation, or for the biodegradability of POC itself, which could be substantial (Sánchez-García et al., 2011;Richardson et al., 2013).
Something we could not directly address in our synthesis was the effect of DOM composition which can be related to the depth of the active layer and the associated retention of (certain fractions of) the DOC pool.For example, sugars and microbially-

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Full derived organic matter appear more biolabile than plant-derived organic matter (Balcarczyk et al., 2009;Mann et al., 2012).Also, permafrost DOM appears to be enriched in hydrogen-rich, aliphatic compounds that are preferentially degraded in incubation experiments (Spencer et al., 2015).The preferential degradation of biolabile components of the bulk DOC results in an enrichment of more recalcitrant components in soil pore waters (Wickland et al., 2007) and in larger rivers downstream (Spencer et al., 2015).
Another factor that could affect BDOC is leaching of nitrogen stocks in permafrost (Harden et al., 2012) with increased thaw depth (Keuper et al., 2012).Higher concentrations of inorganic nitrogen have been shown in some studies to increase BDOC in the receiving ecosystems (Holmes et al., 2008;Wickland et al., 2012), even though we do not find a strong correlation in our meta-analysis and other studies show little response of BDOC to inorganic nutrient additions (Mann et al., 2015).

Geographical and seasonal patterns in BDOC
We identified distinct large-scale patterns in the biodegradability of DOC, which we illustrate in a conceptual diagram (Fig. 7).The percentage BDOC in both soil and aquatic systems increased from regions without permafrost to regions with continuous permafrost.We attribute this increase to better preservation of DOC in permafrost regions where frozen storage has limited processing of the soil organic matter.Furthermore, within aquatic networks, BDOC was lower in large river systems compared with small streams, and this pattern was most obvious in continuous permafrost regions.This suggests that continuous permafrost regions release DOC sources that degrade rapidly in the fluvial network such as Pleistocene yedoma (Vonk et al., 2013b;Abbott et al., 2014;Mann et al., 2015;Spencer et al., 2015).
Aquatic BDOC in large streams and rivers decreased as the Arctic summer progressed.This pattern was absent for soils and small streams.This could be related Introduction

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Full to a variety of factors such as seasonal shifts in carbon sources, changing DOC residence time related to increasing thaw-depth, increasing water temperatures later in the summer, as well as decreasing hydrologic connectivity between soils and surface waters when the season progresses.Alternatively, the integrating character of rivers and larger streams could mask local-scale heterogeneity that is more apparent in small streams and soil leachates.

Circumarctic fluxes of BDOC
Evaluating aquatic DOC export fluxes through sampling at river mouth locations near the Arctic Ocean underestimates the importance of the fluvial network for processing DOM.Literature estimates of watershed-scale aquatic C gas fluxes vary roughly between 0.5 and 10 g C m −2 yr −1 (all normalized to catchment area; Striegl et al., 2012;Lundin et al., 2013;Denfeld et al., 2013;Crawford et al., 2013).When extrapolated to the Arctic Ocean watershed (20.5 × 10 6 km 2 ; Holmes et al., 2013) this could result in a total gaseous C emission between 10 and 200 Tg C yr −1 .These estimates seem reasonable compared to an annual Arctic Ocean watershed DOC flux of 34 Tg (Holmes et al., 2012), where 34 Tg is based on river mouth monitoring and ignores processing within the watershed prior to arriving at the river mouth.Also, a significant fraction of the emitted flux originates from weathering and soil respiration sources (Striegl et al., 2005;Humborg et al., 2009).Gaseous losses of C during aquatic processing in the watershed remain hard to de- values for each permafrost zone (20, 15 and 8 % BDOC for continuous, discontinuous and no permafrost regions, respectively; mean values from Fig. 3b) we can calculate the permafrost-normalized average soil BDOC, tobe16 %.Inclusion of DOC processing within soils is likely to significantly raise the 2.3 Tg C yr −1 estimate for aquatic networks alone (Wickland et al., 2012).However, questions about the linkages between soil and stream BDOC with deepening active layer depths remain.Changes in hydrological flow paths associated with deepening active layers could reduce the inputs of DOC due to mineral sorption and additional processing during transport (MacLean et al., 1999;Striegl et al., 2005;O'Donnell et al., 2010) but the net effects of permafrost thaw on BDOC inputs to streams are not yet well characterized.

Method considerations and recommendations
In order to compare BDOC losses across Arctic, and alternate systems, it is crucial to standardize the methods with which biodegradability is assessed.Our meta-analysis highlighted the significant variability in incubation design across the currently available literature making robust comparisons of BDOC across studies challenging.We suggest the following DOC incubation method, which is intentionally kept simple to be feasible at more remote field sites (a more detailed protocol is available in the Supplement).
Additionally, we suggest a few optional protocol steps that could be used to assess further environmental controls on BDOC.

Standardized incubation protocol
-As soon as possible after collection, filter water samples through pre-combusted (450 • C > 4 h) 0.7 µm glass fiber filters and chill (ca. 4 • C) until ready to incubate.
-Rapid incubation setup is strongly recommended since many biolabile DOC compounds have turnover times of hours.We advocate against freezing samples due to DOC flocculation, compositional and structural changes in the DOC, and bacterial viability (Fellman et al., 2008).Introduction

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Full -Decant filtrate into triplicate sets of 40 mL pre-combusted (550 • C > 4 h) glass vials, and fill each vial with 30 mL filtrate.Use a triplicate glass vial set for each time point in your incubation.We recommend five time points at which one triplicate set will be consecutively removed from incubation: T = 0, T = 2, T = 7, T = 14 and T = 28 days.Use caps with silicone or teflon septa (avoid rubber which can leach DOC).
-Our reason for recommending 40 mL glass vials are multiple; they are commonly available, can be cleaned through pre-ashing, the required total volume per incubation is relatively small but sufficient for analysis, and our analyses suggest that variation in bottle size may affect BDOC results.
-Inoculation of samples is not needed as filtration through 0.7 µm allows for a sufficient amount of bacteria to pass the filter.
-Incubate the vials in the dark (to avoid autotrophic respiration and photodegradation), with loose caps and shake regularly to avoid oxygen-depletion.
-We recommend performing sample incubation at room temperature (20 • C), as this is most common and relatively easy to maintain.Document the temperature throughout the experiment precisely.
-If possible, the incubations should be carried out at a stable temperature for example by using an oven or incubator.
-Re-filter the incubated samples through pre-combusted (450 -For logistical reasons, we recommend assessment of BDOC through DOC loss.
-For details regarding DOC analysis, see the Supplement.

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Full -Ambient incubation temperature: incubate at the ambient temperature of the water or soil from where the sample was collected to allow for application of results to ambient conditions.Run control incubations at 20 • C.
-Nutrient amendment: because the effect of nutrients on DOC processing is unclear, we recommend running experiments both with and without added nutrients.Amount of added nutrients should be adapted in relation to initial nutrient concentration according to the Redfield ratio, but in general an amendment of NO -DIC production: if field and laboratory settings allow we recommend also assessing C loss through DIC production, to provide BDOC estimates through two independent methods and allowing for the calculation of bacterial growth efficiencies.We suggest to measure the CO 2 concentration in the headspace of the incubation flask and calculate the change in DIC (headspace CO 2 plus dissolved CO 2 , carbonate, and bicarbonate in the aqueous phase).This method is detailed in Kalbitz et al. (2003).Keep all other parameters (such as filter pore size, incubation temperature approximate sample volume) similar to the control incubation that measures DOC loss.
-Light incubation: dark incubations eliminate effects of autotrophic respiration and photodegradation, however to simulate realistic DOC drawdown, light is a critical factor (Mann et al., 2012;Cory et al., 2013).
-DOC "quality" (composition) measurements: if possible, we recommend to provide DOM compositional information on, at least, initial water or soil leachates and, if possible, also on incubated waters and soil leachates.These measures may Figures include optical properties (specific ultraviolet absorbance, fluorescence excitation emission matrices), and compound-specific analyses (carbohydrates, amino acids, lignin phenols, Fourier transform ion cyclotron resonance mass spectrometry, etc.).

Conclusions
Half of the global belowground soil OC pool is stored in circum-arctic permafrost but little is known about the processes controlling transport and degradation of DOC, the key regulator of the rate of permafrost carbon release from the Arctic watershed to the atmosphere.We synthesized results from 14 BDOC studies from the permafrost region and complemented this with novel BDOC data from across the Arctic.We observed a large variability in soil and aquatic BDOC, even under standardized conditions.Nonetheless, we found that both soil and aquatic DOC is more labile in regions with continuous permafrost compared to regions without permafrost.Within continuous permafrost regions, the degradability of DOC decreased from headwater streams to larger river systems, suggesting that permafrost DOC is preferentially utilized within the network.Furthermore, we discovered that aquatic BDOC in large streams and rivers decreased as the Arctic summer progressed, whereas this pattern was absent for soils and small streams.
Based on the findings of our synthesis of BDOC studies and additional measurements, we predict that slow future transformation of continuous permafrost into discontinuous permafrost regions could release a first, relatively short-term, pulse of biodegradable DOC but would on longer timescales possibly lead to the release of DOC that is more recalcitrant.The total gaseous watershed C flux could however increase as more DOC could be processed within soils prior to release into aquatic networks, due to deeper thaw depths and increasing residence time (Striegl et al., 2005).
The Arctic is changing, and so is the coupling between its carbon and hydrologic cycles.There still are large uncertainties related to processing and transport of DOC, Figures and little data are available from northern Canada and Russia, from discontinuous permafrost regions, and across all seasons.We strongly recommend that future studies of DOC degradability assess BDOC by means of our standardized BDOC protocol, to facilitate optimal use and integration of future datasets with existing knowledge.
The Supplement related to this article is available online at doi:10.5194/bgd-12-8353-2015-supplement.Introduction

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Full Based on watershed size: streams < 250 km 2 ; large streams 250-25 000 km 2 ; rivers 25 000-500 000 km 2 ; large rivers > 500 000 km • C) after 28 day incubation for soil leachates from three cores collected near Toolik, Alaska, as part of our circum-arctic incubation experiment (see Sect. 2.1).Soil leachates were collected and incubated both in spring (circles) and fall (diamonds).In core 1 we observed active plant growth during the spring and fall incubations.
Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 84), and correlated negatively with bottle size (r = −0.77)and presence of inoculum (r = −0.51).Component 2 also did not explain much variation in BDOC, but correlated with filter pore size (r = 0.90), nutrient addition (r = 0.90), and the use of inoculum (r = 0.64).Component 3, explained the greatest proportion of BDOC variance (r = −0.83).Component 3 also closely correlated with incubation time (r = −0.85)and displayed a negative correlation with bottle size (r = 0.54).Effect of oxygen availability was not examined in aquatic incubations, as all previously published experiments were conducted under oxic conditions.Discussion Paper | Discussion Paper | Discussion Paper | Fig. 3a), sample latitude (r = 0.93), and initial DOC (r = −0.70).The second component was strongly negatively correlated with BDOC (r = −0.71),and was explained by sample longitude (r = 0.78).The third component did not correlate to BDOC but showed a strong correlation with sampling period (Julian day; r = 0.95).Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | clearly in aquatic experiments.This finding contradicts with the finding of McDowell et al. ( Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | termine.Wickland et al. (2012) estimated that the combined BDOC exported by the six largest Arctic rivers to the Arctic Ocean is 2.3 Tg C yr −1 , based on empirical relations between BDOC and DOC : DIN (dissolved inorganic nitrogen) ratios.Importantly, these watershed-scale estimates exclude processing and retention of DOC in soils, prior to delivery to aquatic networks.As we have seen in this study, soil BDOC is on average higher than aquatic BDOC.By using the % permafrost extent in the Arctic Ocean watershed from Holmes et al. (2013), 45 % continuous, 31 % discontinuous (including sporadic and isolated) and 26 % without permafrost, and average soil % BDOC Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 7 µm filters (to avoid problems with flocculation and remove microbial biomass) for each time step.Store the filtered samples in pre-combusted (550 • C > 4 h) 40 mL glass vials, acidify to pH 2 with 30 µL concentrated HCl.Cap tightly and store dark and chilled until analysis.
µm;Holmes et al., 2008) is appropriate for aquatic and soil leachates.Run control incubations without nutrient amendment.

Figure 1 .Figure 6 .
Figure 1.Map of the hydrological network (blue) in the Arctic Ocean watershed (boundary in red) with points showing literature data (blue for aquatic, red for soil) and experimental data (green for aquatic, orange for soil).