Science of the Total Environment 511 (2015) 381–392

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Effect of inundation, oxygen and temperature on carbon mineralization in boreal ecosystems Youngil Kim a,b, Sami Ullah c, Nigel T. Roulet a, Tim R. Moore a,⁎ a b c

Department of Geography, McGill University, Montreal, QC H3A 0B9, Canada Department of Forest Ecosystems and Society, Oregon State University, Corvallis, OR 97331, USA School of Physical and Geographical Sciences, Keele University, Staffordshire ST5 5BG, UK

H I G H L I G H T S • • • •

We measured CO2 and CH4 production from boreal forest and peat litter and soils. Flooding led to a 24% decrease in CO2 production and a 32% increase in CH4 production. Higher temperatures resulted in more CO2 and CH4 production under both flooded and non-flooded conditions. C release was generally litters N soils, but varied with treatment conditions.

a r t i c l e

i n f o

Article history: Received 29 July 2014 Received in revised form 19 November 2014 Accepted 21 December 2014 Available online xxxx Editor: C.E.W. Steinberg Keywords: Carbon dioxide production Methane production Boreal forests Peatlands Flooding Temperature Oxygen concentration

a b s t r a c t The inundation of boreal forests and peatlands through the construction of hydroelectric reservoirs can increase carbon dioxide (CO2) and methane (CH4) emission. To establish controls on emission rates, we incubated samples of forest and peat soils, spruce litter, forest litter and peatland litter collected from boreal ecosystems in northern Quebec for 16 weeks and measured CO2 and CH4 production rates under flooded or non-flooded conditions and varying oxygen concentration and temperature. CO2 production under flooded conditions was less than under non-flooded conditions (5–71 vs. 5–85 mg C g−1 C), but CH4 production under flooded conditions was larger than under non-flooded conditions (1–8158 vs. 0–86 μg C g−1 C). The average CO2 and CH4 production rate factor for flooded:non-flooded conditions was 0.76 and 1.32, respectively. Under flooded conditions, high oxygen concentrations increased CO2 production in peat soils but decreased CH4 production in forest and peat soils and spruce litter. Warmer temperatures (from 4 to 22 °C) raised both CO2 production in peat soils and peatland litter, and CH4 production in peat soils and spruce litter. This study shows that the direction and/or strength of CO2 and CH4 fluxes change once boreal forests and peatlands are inundated. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The inundation of forests and wetlands during the construction of hydroelectric reservoirs alters the biogeochemical cycles and exchange rates of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) with the atmosphere (Rosenberg et al., 1997; St. Louis et al., 2000; Tremblay et al., 2005). About 0.2% of the global land surface (~340,000 km2) has been flooded by hydroelectric reservoir construction (Barros et al., 2011) and may change the landscape from a sink to a source for atmospheric CO2 and CH4. Undisturbed boreal forests and northern peatlands can have

⁎ Corresponding author. E-mail addresses: [email protected] (Y. Kim), [email protected] (S. Ullah), [email protected] (N.T. Roulet), [email protected] (T.R. Moore).

http://dx.doi.org/10.1016/j.scitotenv.2014.12.065 0048-9697/© 2014 Elsevier B.V. All rights reserved.

carbon (C) uptake rates of ~40–180 g C m−2 yr−1 (Luyssaert et al., 2007) and ~10–60 g C m−2 yr−1 (Loisel et al., 2014), respectively, and Rudd et al. (1993) reported that reservoir creation resulted in a net release of C stored in boreal soils and vegetation at an average rate of 60 g C m−2 yr−1, as CO2 and CH4. The inundation of a small wetland catchment in the Experimental Lakes Area (ELA), northwestern Ontario, Canada, led to large summer effluxes of CO2 (0.6–10.1 g C m−2 d−1: Kelly et al., 1997) and CH4 (6–483 mg C m−2 d−1: Scott et al., 1999) compared to pre-inundation values, and these effluxes may continue for years or decades after flooding (Kelly et al., 1997; Scott et al., 1999; St. Louis et al., 2003). The magnitude of CO2 and CH4 exchange in flooded ecosystems is dependent on the characteristics of the substrate, such as soil and litter, on the environment where decomposition occurs, such as water temperature and oxygen (O2) concentration, and on C uptake by terrestrial

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ecosystems which is eliminated when vegetation is inundated (Kelly et al., 1997; Bodaly et al., 2004; Matthews et al., 2005). Carbon mineralization rates under water-saturated conditions are correlated with the quality and quantity of substrate as well as temperature: substrates including more labile C pools (e.g. litter) and warmer conditions promote C mineralization (e.g. Moore and Dalva, 1993; Blodau and Moore, 2003a; Oelbermann and Schiff, 2008). Carbon dioxide is the main byproduct of microbial respiration under both oxic and anoxic conditions (Segers and Kengen, 1998), with anoxic CO2 production smaller (ca. 20%) than oxic production (Magnusson, 1993; Glatzel et al., 2004). Methane can be consumed by methanotrophs under oxic conditions, and under anoxic conditions, CH4 is produced as an end-product of microbial respiration that occurs by methanogens as microbes switch to alternative electron acceptors (Jones et al., 1985). Given the significance of oxic and anoxic conditions for heterotrophic microbial respiration and their end-products of CO2 and CH4, it is important to quantify and model the effects of inundation (flooding) of terrestrial ecosystems on C mineralization from soil and litter, at different temperatures and varying O2 level representing the range of inundation conditions. We hypothesized that: (1) soils and litter under flooded conditions would have larger CO2 fluxes but smaller CH4 fluxes than those under non-flooded conditions; (2) under flooded conditions, larger CO2 fluxes and smaller CH4 fluxes would be found under oxic conditions than under anoxic conditions; (3) warmer temperatures would promote both CO2 and CH4 fluxes; and (4) litter that contains more labile organic substrates would have larger CO2 and CH4 fluxes than soils. Our objectives were: (1) to compare CO2 and CH4 production rates from soils and litter of boreal forest and peatland between flooded and nonflooded treatments; (2) to investigate how post-flooding O2 levels and temperatures affect CO2 and CH4 production rates; and (3) to examine differences in C production rates by substrates type, soils or litter. We incubated boreal soils and litter under inundated (flooded) or fieldmoist (non-flooded) conditions. The O2 concentration of the incubators was adjusted to three levels, (a) anoxic ([O2] ≈ 0%), (b) suboxic (~50% saturation of ambient O2) and (c) oxic (~ 100% saturation of ambient O2). The incubation temperatures were 4, 12 and 22 °C in a factorial design to represent temperatures in deep, middle and shallow sections of a boreal reservoir.

Soil and litter samples were collected from a typical mature black spruce forest (lat. 52°6′N, long. 76°11′W) and a typical ombrotrophic peatland, locally known as the Lac Le Caron bog (lat. 52°17′N, long. 75°50′W). The forest site is dominated by black spruce and shrubs (e.g. Kalmia and Rhododendron spp.) with ground vegetation consisting of feather moss (Pleurozium spp.) and lichen (Cladonia spp.). Soils in these forests have an O-horizon (8 to 16-cm thick) overlying a sandy and well-drained A-horizon layer (Ullah et al., 2009). The peatland site has a dense cover of Sphagnum mosses, ericaceous shrubs (e.g. Chamaedaphne and Rhododendron spp.) and sedges (e.g. Carex and Eriophorum spp.); trees (e.g. P. mariana, Alnus incana and Larix laricina) are sparse. The peat depth ranges from 1 to 5 m and contains 110 kg C m−2 with peat accumulation beginning ~5700 years BP (van Bellen et al., 2011).

2. Materials and methods

Three types of soil (FS: forest soil; PS-1: peat soil 0–5 cm; and PS-2: peat soil 5–15 cm) and three litters (BL: black spruce litter; FL: forest litter; and PL: peatland litter) were collected (Table 1). The vegetation mixtures of FL and PL represent the distribution of understory biomass in the study forest and peatland, applying proportions of each ground vegetation type (e.g. moss, lichen and shrub) determined by Lemieux (2010) and Pelletier et al. (2011) for forest and peatland, respectively. During the incubation, the soil or litter samples were either completely submerged under water to create the ‘flooded’ treatment or were kept close to field-moist conditions in the ‘non-flooded’ treatment (Table 1). The experimental design was chosen to mimic the flooded or non-flooded conditions and varying O2 concentration and temperature in the study site. However, during sampling, transport and storage, as well as the incubation method employed, artifacts could be created in the results obtained. One-L Mason jars were used for the flooded incubation with three rubber tubes with stopcock valves fixed on the caps; two short tubes for air sampling and N2 purging and one long tube for water sampling and dissolved C and N analyses. To start the flooded incubation, the weighed soil or litter was placed in the jar, and 400 to 450 mL of distilled water was added to fill to 8 cm in depth, in a 17-cm tall jar. The water was flushed with O2-free N2 gas for 5 min to remove all O2 before it was added. The incubators were tightly capped to eliminate gas exchange between the enclosed headspace and the outside air. After caps were secured, O2 concentration was adjusted for anoxic and suboxic treatments by flushing with O2-free N2 gas for 30 min. For the suboxic treatment, half of the volume of the headspace (250 mL) was

2.1. Study sites The study sites are located near the Eastmain-1 (EM-1) hydroelectric reservoir (lat. 52°0′ to 52°12′N, long. 75°30′ to 76°12′W) in the James Bay region of northern Quebec, Canada. From 2006 to 2009, the mean annual air temperature was −0.2 °C and total annual precipitation was 682 mm, based on records taken from a Hydro-Québec weather station (Lac Aunaukach) near EM-1. From May to October, the mean air temperature was 12 °C with a total precipitation of 512 mm, and from November to April, the mean air temperature was − 9.7 °C with a total precipitation of 170 mm. Land-cover in this region includes mixed boreal forests (~60%), mainly composed of mature black spruce (Picea mariana), peatlands (~15%) and lakes, rivers and exposed bedrock (~ 25%). Water impoundment for the 603-km2 EM-1 reservoir started in November 2005 and was completed by May 2006; 49% of the flooded area was mature and recently burned forests, 18% was peatlands, 25% was lakes and rivers, and 8% was exposed bedrock (Teodoru et al., 2011). The average water depth of the reservoir is 11 m and the annual drawdown is 9 m; 46% of water depth in the reservoir is between 0 and 10 m, 39% between 10 and 20 m, and 15% between 20 and 57 m. The ice-free water temperature at 11 m averaged 14.7 °C and ranged between 6.5 and 19.2 °C. The mean ice-free dissolved O2 at the average depth of 11 m was 7.4 mg L− 1 and that in the top 0.5 m depth was 8.4 mg L−1 (C. Teodoru and A. Tremblay, unpublished data).

2.2. Soil and litter sampling In July and August 2008, five soil cores were collected at the forest site, randomly from each of four 30 × 30 m plots within a 4-ha area. After removing the moss layer, the cores were 10 × 10 cm and 15-cm deep to obtain most of the organic layer. Samples of fresh vegetation that include needles and twigs of black spruce, the dominant shrub (Rhododendron spp.), feather and Sphagnum mosses and lichens were collected. At the peatland site, five sampling locations were randomly identified (two hummocks and three hollows), and two peat cores, 10 × 10 cm and 15-cm deep, were collected after discarding surface moss layers. Samples of leaves and twigs of ericaceous shrubs (Chamaedaphne and Rhododendron spp.), sedges (Carex spp.) and Sphagnum mosses were obtained. Large roots in soils were removed and the soils were manually homogenized within the study site. Peat cores were divided into 0 to 5cm and 5 to 15-cm segments, referenced to the top of the cores. Before the start of laboratory incubations, soils were refrigerated at 4 °C and vegetation samples were frozen to avoid decomposition of the labile substrate. 2.3. Design of the incubation experiments

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Table 1 Description of soil and litter substrates used in this study. Substrate type

FS PS-1 PS-2 BL FL

PL

a b

Composition

FH layer of forest soil Surface layer (0–5 cm) of peat soil Subsurface layer (5–15 cm) of peat soil Needle and branch of black spruce litter Forest ground litter Leaf and branch of shrub (R. groenlandicum) Lichen Sphagnum moss Feather moss Peatland ground litter Leaf and branch of shrub (C. calyculata) Stem of sedge (Carex spp.) Sphagnum moss

Mass (g) and content (in parentheses, %)

C:N ratio

Total

Water

C

N

50.0 (100) 50.0 (100) 50.0 (100) 20.0 (100) 20.0 (100) 6.2a 4.2a 9.2a 0.4a 20.0 (100) 4.8b 2.2b 13.0b

41.1 (82.2) 46.3 (92.6) 46.4 (92.8) 8.5 (42.5) 14.8 (74.0)

4.2 (8.4) 1.7 (3.4) 1.7 (3.4) 6.0 (30.0) 2.5 (12.5)

0.10 (0.20) 0.03 (0.06) 0.03 (0.06) 0.08 (0.40) 0.04 (0.20)

39.8 67.9 55.6 75.2 65.9

16.6 (83.0)

1.7 (8.5)

0.03 (0.15)

49.2

Contribution to the total mass of the forest vegetation (Lemieux, 2010). Contribution to the total mass of the peatland vegetation (Pelletier et al., 2011).

replaced by ambient air, and no replacement of air was required for the anoxic treatment. To initiate the non-flooded incubations, the weighed soil or litter was placed in a 150-mL plastic container over a layer of glass wool

that allowed for aeration, and the container was then placed inside a 1-L plastic container to which a rubber septum was installed for gas sampling. Fifteen milliliters of distilled water was added on the top of the material to keep them moist, and water drained from the material

Fig. 1. CO2 production rates (mg C g−1 C d−1) in each treatment for substrate types. The upper two figures include soil types and two lower figures include litter types. Note that figures are grouped with the same temperature (4, 12 or 22 °C) and either flooded or non-flooded treatment. Treatments are FA: flooded–anoxic; FSO: flooded–suboxic; and NFO: non-flooded–oxic.

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across the glass wool was then removed. The 1-L containers that contained the 150-mL containers were securely closed, which were filled with ambient air (~21% O2 concentration). The anoxic/suboxic/oxic treatments were: (1) anoxic treatments in the flooded incubators (~ 0% O2 in the headspace) by purging the water with O2-free N2 gas and headspace air replaced with O2-free N2 gas; (2) suboxic treatments in the flooded incubators (~50% saturation of ambient O2 in the headspace) which had a headspace filled with ~ 50% ambient air and ~ 50% O2-free N2 gas; and (3) oxic treatments under ambient air in the non-flooded incubators (~100% saturation of ambient O2 in the headspace). Treatments are defined by flooding (flooded or non-flooded), O2 level (anoxic, suboxic or oxic) and temperature (4, 12 or 22 °C). The total number of incubators was 162 (flooded anoxic: 54; flooded suboxic: 54; and non-flooded oxic: 54) for three O2 and three temperature treatments and six substrate types with three replicates per treatment (n = 3). All incubators were wrapped in aluminium foil and then transferred to their respective temperature-controlled incubation chambers. The incubations were divided into three batches by temperature treatment, and the 54 incubators of each batch were treated with either flooded or non-flooded. The batch sequence of temperatures was 12, 4 and 22 °C with a four-week interval between the batches. Sørensen (1998) and Stenberg et al. (1998) have shown that a storage time of less than three months did not much affect C and N mineralization of soils, compared to that without storage before incubation. As samples were either

refrigerated or frozen for a maximum of 8 weeks, we assumed that the effect of batch interval on the results of incubation was negligible. Incubations were conducted for 16 weeks, and each week, the O2 concentration within the incubators was adjusted to return it to initial conditions by flushing the flooded incubators with O2-free N2 gas and opening the non-flooded incubators for 10 min and then resealing. Using an air density of 1.29 kg m−3 and the air space of incubators (maximum 0.5 L for flooded and 1 L for non-flooded), O2 amounts can be calculated as 67 and 134 g in the incubators for flooded suboxic and non-flooded treatments, respectively. These O2 amounts are larger than the total C (1.7–6.0 g), produced as CO2 (7.9–28.0 g) in the substrates (Table 1). Fifteen milliliters of water was collected from the flooded incubators every three weeks and replaced with distilled water after purging with O2free N2 gas. Fifteen milliliters of distilled water was added to the nonflooded incubators to maintain moist conditions for soils and litter. 2.4. Gas sampling and analyses Fifteen-milliliter gas samples were taken from the headspace of the Mason jars or plastic containers at 0, 4, 8 and 24 hour intervals after the O2 concentration was established, and the same volume of O2-free N2, mixture of half O2-free N2 and half ambient air, and ambient air was injected to preserve 0, 50 and 100% ambient O2 concentrations in the headspace, respectively. After the initial gas sampling (week 0), sampling continued at weeks 1, 2, 3, 4, 6, 8, 12 and 16. The gas samples

Fig. 2. CH4 production rates (μg C g−1 C d−1) in each treatment for substrate types (a negative value indicates CH4 consumption). The upper two figures include soil types and two lower figures include litter types. Note that figures are grouped with same temperature (4, 12 or 22 °C) and either flooded or non-flooded treatment. Treatments are FA: flooded–anoxic; FSO: flooded–suboxic; and NFO: non-flooded–oxic.

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were transferred to 12-mL crimped, evacuated borosilicate glass vials with gray butyl septa and stored for a maximum of one week before analysis. The gas samples were analyzed for CO2 and CH4 concentrations using a Shimadzu Mini-2 gas chromatograph and a MTN-1 methanizer (Shimadzu Ltd., Kyoto, Japan) equipped with a 183-cm Porapak-Q column (80/100 mesh) and a flame ionization detector (FID). Nitrogen was used as the carrier gas for CO2 (with a flow rate of 30 mL min−1), and column and detector temperatures were fixed at 50 and 100 °C, respectively. Hydrogen was used as the carrier gas for CH4 (with a flow rate of 10 mL min− 1) and the column, packed with Shimalite, was kept at an operating temperature of 375 °C. Certified gas standards of CO2 (450 ppmv) and CH4 (1.8 ppmv) were used for calibration. Each day, repeated standards (at least ten standards at the start of the run and then one standard between twenty sample runs) were used. The production rate was determined as the linear change in gas concentrations between sampling periods. If the rate of change was greater than the variation in gas standards (i.e. the change rate N the detected level of variation), it was accepted as an increasing or a decreasing rate of gas production: the mean (± standard deviation) of R2 of the change rates was 0.60 ± 0.34. Less than 10% of CO2 and CH4 production rates

385

were rejected. Production rates from the three replicates were averaged and reported relative to the initial organic C mass of the incubated substrates: mg C g−1 C d−1 for CO2 and μg C g−1 C d−1 for CH4. The rates of CO2 and CH4 exchange in a sampling week were multiplied by the number of days between sampling to obtain an estimate for the 16-week total production. Methane exchange rates were treated as production when N0 and if the exchange rates were b0, they were net consumption. Total production and consumption were expressed in mg C g−1 C for CO2 and μg C g−1 C for CH4. 2.5. Statistical analyses Differences in CO2 and CH4 production rates between the flooded and the non-flooded treatments were identified by calculating the average ratios of the production rates from one treatment to the other treatment at the same temperature. Student's t-test was conducted to determine the flooding effects on CO2 and CH4 production rates. Twoway repeated measures analysis of variance (ANOVA) was applied to the O2 concentration and temperature effects on the production rates for the flooded incubations. The Bonferroni post-hoc test was used to establish the interaction of O2 and temperature treatments. One-way

Fig. 3. Total CO2 production (mg C g−1 C) and total CH4 production and consumption (μg C g−1 C) throughout the 16-week incubation for substrate types under different treatments of flooding, oxygen (O2) level and temperature (FA4: flooded–anoxic at 4 °C; FA12: flooded–anoxic at 12 °C; FA22: flooded–anoxic at 22 °C; FSO4: flooded–suboxic at 4 °C; FSO12: flooded– suboxic at 12 °C; FSO22: flooded–suboxic at 22 °C; NFO4: non-flooded–oxic at 4 °C; NFO12: non-flooded–oxic at 12 °C; and NFO22: non-flooded–oxic at 22 °C). Means of total CO2 production and total CH4 production and consumption by each of flooded (FA: flooded anoxic; FSO: flooded suboxic; F: all of FA + FSO) and non-flooded (NF) conditions are provided (±standard error in either flooded or non-flooded conditions). Top three figures are for total CO2 production; middle three are for total CH4 production; and low three are for total CH4 consumption. The exact values of production, standard errors of treatments or flooded/non-flooded conditions are present in Table 2.

386 Table 2 Total CO2 production (mg C g−1 C) and total CH4 production and consumption ( μg C g−1 C) throughout the 16-week incubation (±standard error of three replicates) for substrate types under different treatments of flooding, oxygen (O2) level and temperature (FA4: flooded–anoxic at 4 °C; FA12: flooded–anoxic at 12 °C; FA22: flooded–anoxic at 22 °C; FSO4: flooded–suboxic at 4 °C; FSO12: flooded–suboxic at 12 °C; FSO22: flooded–suboxic at 22 °C; NFO4: non-flooded–oxic at 4 °C; NFO12: non-flooded–oxic at 12 °C; and NFO22: non-flooded–oxic at 22 °C). Means of total CO2 production and total CH4 production and consumption by each of flooded (FA: flooded anoxic; FSO: flooded suboxic; all: FA + FSO) and non-flooded conditions are provided (±standard error in either flooded or non-flooded conditions). Treatment FA4

Mean of treatment FA12

FA22

FSO4

FSO12

FSO22

NFO4

NFO12

NFO22

Flooded FA (n = 3)

CO2 production FS 12.9 ± 1.2 PS-1 15.8 ± 2.1 PS-2 11.4 ± 1.0 BL 16.4 ± 4.9 FL 26.3 ± 3.8 PL 48.6 ± 2.8

7.1 ± 0.5 4.8 ± 1.4 8.5 ± 1.0 14.2 ± 5.4 22.7 ± 2.4 31.3 ± 10.9

9.2 ± 2.7 14.2 ± 1.4 14.4 ± 3.2 15.5 ± 1.8 20.8 ± 4.1 33.6 ± 5.6

9.0 ± 0.6 17.5 ± 1.1 10.0 ± 1.5 7.7 ± 3.4 36.8 ± 2.3 71.4 ± 2.1

7.0 ± 2.2 12.4 ± 7.8 8.6 ± 2.6 12.0 ± 4.9 14.4 ± 7.3 23.3 ± 15.9

12.1 ± 2.8 28.7 ± 4.6 36.0 ± 1.9 17.4 ± 2.9 44.8 ± 3.2 45.3 ± 10.2

5.1 ± 0.8 10.6 ± 1.2 9.9 ± 1.9 18.0 ± 6.6 31.5 ± 2.3 46.8 ± 8.4

14.7 ± 1.2 13.1 ± 2.7 12.2 ± 2.5 25.3 ± 9.6 27.0 ± 7.3 76.2 ± 24.3

29.3 ± 2.6 33.3 ± 9.4 31.8 ± 4.8 39.5 ± 9.1 59.9 ± 10.2 84.5 ± 19.9

CH4 production FS 5.7 ± 3.3 PS-1 4.6 ± 2.7 PS-2 17.1 ± 6.3 BL 1.7 ± 1.7 FL 11.5 ± 1.5 PL 14.7 ± 8.8

7.0 ± 2.0 30.4 ± 8.8 406 ± 142 5.9 ± 6.1 3.8 ± 3.1 11.1 ± 10.9

322 ± 275 2830 ± 461 8158 ± 3519 48.9 ± 43.1 4.5 ± 3.5 4.9 ± 3.7

2.1 ± 1.9 4.2 ± 5.0 8.8 ± 11.1 1.0 ± 0.4 4.4 ± 2.4 8.1 ± 3.6

3.1 ± 2.1 11.6 ± 8.3 6.2 ± 3.5 2.2 ± 3.0 10.0 ± 4.3 22.3 ± 10.5

86.9 ± 39.0 44.3 ± 22.7 457 ± 92.2 10.4 ± 3.8 5.8 ± 3.1 20.7 ± 5.5

2.2 ± 1.7 18.9 ± 7.9 13.6 ± 13.9 1.1 ± 0.8 5.5 ± 1.9 3.6 ± 11.7

0 7.6 ± 6.8 0 1.3 ± 0.8 7.9 ± 8.0 2.2 ± 7.6

23.6 ± 10.0 38.8 ± 21.5 85.8 ± 13.3 18.0 ± 8.9 40.7 ± 24.0 64.0 ± 25.2

112 ± 93.5 955 ± 158 2860 ± 1 222 18.8 ± 16.9 6.6 ± 2.7 10.2 ± 7.8

−0.0 ± 0.2 0 0 −0.4 ± 1.1 −0.2 ± 0.8 −1.6 ± 14.8

−0.1 ± 0.1 0 0 0 −1.7 ± 0.8 −1.1 ± 0.8

−1.1 ± 1.0 −0.3 ± 0.4 −13.5 ± 11.3 −0.9 ± 0.5 −1.6 ± 1.6 −0.3 ± 0.5

−0.0 ± 0.2 −0.9 ± 0.7 0 −0.8 ± 0.5 −0.3 ± 2.0 0

−2.9 ± 1.6 −0.8 ± 0.6 0 −1.5 ± 1.5 −11.3 ± 12.4 −2.0 ± 5.9

−8.6 ± 6.4 −17.2 ± 12.0 −31.3 ± 9.1 −6.3 ± 2.7 −5.3 ± 6.9 −15.5 ± 6.9

0 −1.4 ± 3.1 0 0 0 0

−1.6 ± 1.8 −3.2 ± 2.9 −1.3 ± 1.1 −0.7 ± 0.9 −0.5 ± 0.4 −0.8 ± 5.0

CH4 consumption FS −0.1 ± 0.5 PS-1 −5.7 ± 5.0 PS-2 0 BL −1.2 ± 0.8 FL 0 PL 0

−4.6 ± 4.6 −4.0 ± 3.8 −4.0 ± 3.4 −0.6 ± 0.8 −1.3 ± 0.5 −0.9 ± 0.2

9.8 ± 1.5 11.6 ± 1.6 11.4 ± 1.7 15.4 ± 4.1 23.3 ± 3.4 37.8 ± 6.5

FSO (n = 3)

All (n = 6)

Non-flooded (n = 3)

9.4 ± 1.9 19.5 ± 4.5 18.2 ± 2.0 15.7 ± 3.7 32.0 ± 4.3 46.7 ± 9.4

9.6 ± 1.7 15.6 ± 3.1 14.8 ± 1.9 15.5 ± 3.9 27.6 ± 3.9 42.3 ± 7.9

16.4 ± 1.5 19.0 ± 4.4 18.0 ± 3.1 27.6 ± 8.4 39.5 ± 6.6 69.2 ± 17.5

30.7 ± 14.4 20.1 ± 12.0 157 ± 35.6 4.5 ± 2.4 6.7 ± 3.3 17.1 ± 6.5

71.2 ± 53.9 488 ± 84.8 1509 ± 529 11.7 ± 9.7 6.7 ± 3.0 13.6 ± 7.2

8.6 ± 3.9 21.8 ± 12.1 33.2 ± 9.1 6.8 ± 3.5 18.0 ± 11.3 23.3 ± 14.8

−0.4 ± 0.4 −0.4 ± 0.4 −4.5 ± 3.8 −0.6 ± 0.3 −1.2 ± 1.5 −0.5 ± 0.4

−1.0 ± 1.1 −1.8 ± 1.7 −2.9 ± 2.5 −0.7 ± 0.6 −0.9 ± 0.9 −0.6 ± 2.7

−3.8 ± 2.7 −6.5 ± 5.2 −10.4 ± 3.0 −2.6 ± 1.4 −5.5 ± 6.4 −5.8 ± 4.2

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Substrate type

Y. Kim et al. / Science of the Total Environment 511 (2015) 381–392

repeated measures ANOVA was applied to the temperature effects on the production rates for the non-flooded incubations. Before analyses commenced, all CO2 and CH4 data were adjusted using logarithmic transformation, because the data were not normally distributed (based on the Shapiro–Wilk test, P b 0.05). For CH4, all data were separated into net production and net consumption (i.e. N 0 and b0 of the production rate, respectively), and then missing data were filled with zero to allow statistical analysis. Logarithmic transformation was carried out after (1) negative values in net consumption were changed into positive ones by multiplying by −1 and (2) + 0.1 was applied to the entire production and consumption rates. The Q10 values were the ratios of CO2 production at 22 °C and 12 °C incubations calculated using the total production over 16 weeks. The significance level set at P = 0.05 and PASW Statistics 18 for Windows was used.

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cumulative exchange over the 16-week incubation and the effects of treatments. 3.1. Temporal patterns of CO2 and CH4 exchange Carbon dioxide production rates peaked between weeks 2 and 3 in samples incubated at 22 °C; the pattern was less pronounced at 12 °C and negligible in all substrates incubated at 4 °C (Fig. 1). Higher rates generally occurred in the non-flooded–oxic 12 and 22 °C followed by the flooded suboxic treatments, and rates under the flooded treatments were slower than under the non-flooded treatments. Methane production rates in several of the flooded–anoxic treatments of soils (FS, PS-1 and PS-2) clearly increased over time at 12 and 22 °C, whereas CH4 consumption often occurred in the nonflooded incubations (Fig. 2).

3. Results

3.2. Total CO2 and CH4 exchange

We first examine the patterns of CO2 and CH4 exchange using the nine individual sampling dates, followed by comparisons of the

The largest and smallest total CO2 production mainly occurred in the non-flooded–oxic at 22 °C treatment and in the flooded–anoxic at 12 °C

Fig. 4. Mean CO2 and CH4 production rates (mg C g−1 C d−1 and μg C g−1 C d−1, respectively) in each treatment during 16-week incubation for substrate types. CH4 production rates are divided into net production (i.e. N0) and net consumption (i.e. b0), which occurred under both flooded and non-flooded conditions. Top three figures are for CO2 production rates; middle three are for CH4 net production rats; and low three are for CH4 net consumption rates. The exact values of rates, standard errors of treatments and the results of analysis to evaluate the effects of flooding, oxygen (O2) level and temperature of a substrate type are present in Table 3. F: flooded; NF: non-flooded; FA: flooded anoxic; and FSO: flooded suboxic.

388 Table 3 Mean CO2 and CH4 production rates (mg C g−1 C d−1 and μg C g−1 C d−1, respectively) during 16-week incubation (±standard error in each treatment) and Student's t-test to evaluate the effects of flooding and one-way or two-way repeated measures analysis of variance (ANOVA) to evaluate the effects of oxygen (O2) level and temperature on each treatment of a substrate type. CH4 production rates are divided into net production (i.e. N0) and net consumption (i.e. b0), which occurred under both flooded and non-flooded conditions. F: flooded; NF: non-flooded; A: anoxic; and SO: suboxic. Flooded & non-flooded (n = 81)

Flooded (n = 54)

Flooding

O2 level

F CO2 production FS PS-1 PS-2 BL FL PL

NF

P

A

Non-flooded (n = 27) Temperature SO

P

4 °C

Temperature 12 °C

22 °C

P

4 °C

12 °C

22 °C

P

0.08 ± 0.01 0.13 ± 0.01 0.12 ± 0.01 0.14 ± 0.01 0.26 ± 0.02 0.42 ± 0.03

0.16 ± 0.02 0.19 ± 0.02 0.18 ± 0.02 0.27 ± 0.03 0.39 ± 0.05 0.79 ± 0.11

b0.01 0.04 0.04 b0.01 b0.01 b0.01

0.08 ± 0.01 0.10 ± 0.01 0.10 ± 0.01 0.15 ± 0.02 0.23 ± 0.02 0.41 ± 0.05

0.08 ± 0.01 0.16 ± 0.02 0.15 ± 0.02 0.13 ± 0.01 0.28 ± 0.03 0.42 ± 0.04

0.93 b0.01 0.03 0.57 0.27 0.60

0.07 ± 0.01 0.11 ± 0.01 0.08 ± 0.01 0.13 ± 0.02 0.24 ± 0.02 0.52 ± 0.05

0.07 ± 0.01 0.08 ± 0.01 0.09 ± 0.01 0.12 ± 0.01 0.19 ± 0.03 0.30 ± 0.04

0.09 ± 0.01 0.20 ± 0.02 0.21 ± 0.03 0.17 ± 0.03 0.34 ± 0.05 0.44 ± 0.06

0.30 b0.01 b0.01 0.42 0.08 b0.01

0.05 ± 0.01 0.09 ± 0.01 0.09 ± 0.01 0.15 ± 0.01 0.25 ± 0.03 0.43 ± 0.04

0.14 ± 0.02 0.14 ± 0.02 0.13 ± 0.02 0.24 ± 0.03 0.29 ± 0.04 0.82 ± 0.20

0.30 ± 0.04 0.34 ± 0.03 0.31 ± 0.02 0.42 ± 0.07 0.64 ± 0.08 1.11 ± 0.23

b0.01 b0.01 b0.01 b0.01 b0.01 0.03

CH4 net production FS 0.47 ± 0.24 PS-1 3.24 ± 1.75 PS-2 8.71 ± 3.94 BL 0.09 ± 0.03 FL 0.08 ± 0.01 PL 0.14 ± 0.01

0.18 ± 0.04 0.36 ± 0.09 0.65 ± 0.13 0.14 ± 0.03 0.27 ± 0.07 0.38 ± 0.13

0.83 0.43 0.50 0.30 0.06 0.27

0.77 ± 0.52 7.99 ± 4.24 15.42 ± 7.16 0.14 ± 0.06 0.08 ± 0.01 0.13 ± 0.01

0.23 ± 0.12 0.15 ± 0.04 1.05 ± 0.47 0.04 ± 0.01 0.08 ± 0.01 0.15 ± 0.02

0.67 0.01 b0.01 0.04 0.27 0.94

0.04 ± 0.01 0.05 ± 0.01 0.16 ± 0.06 0.01 ± 0.00 0.08 ± 0.01 0.12 ± 0.02

0.06 ± 0.03 0.18 ± 0.06 1.95 ± 0.77 0.04 ± 0.01 0.07 ± 0.01 0.16 ± 0.02

1.09 ± 0.55 7.55 ± 3.99 20.54 ± 9.30 0.21 ± 0.08 0.08 ± 0.01 0.13 ± 0.02

0.02 b0.01 b0.01 0.03 0.25 0.64

0.08 ± 0.02 0.16 ± 0.03 0.42 ± 0.23 0.15 ± 0.10 0.10 ± 0.02 0.04 ± 0.01

0 0.14 ± 0.05 0 0.02 ± 0.00 0.28 ± 0.00 0.03 ± 0.02

0.25 ± 0.05 0.62 ± 0.14 0.88 ± 0.11 0.16 ± 0.03 0.43 ± 0.13 0.72 ± 0.19

b0.01 0.03 b0.01 0.02 b0.01 b0.01

CH4 net consumption FS −0.10 ± 0.03 PS-1 −0.16 ± 0.05 PS-2 −0.28 ± 0.07 BL −0.07 ± 0.02 FL −0.13 ± 0.05 PL −0.13 ± 0.03

−0.07 ± 0.02 −0.52 ± 0.24 −0.56 ± 0.19 −0.14 ± 0.07 −0.19 ± 0.09 −0.62 ± 0.35

0.58 0.05 0.03 0.29 0.22 0.02

−0.10 ± 0.03 −0.17 ± 0.07 −0.44 ± 0.17 −0.07 ± 0.03 −0.11 ± 0.07 −0.08 ± 0.03

−0.11 ± 0.08 −0.14 ± 0.05 −0.20 ± 0.07 −0.08 ± 0.02 −0.14 ± 0.06 −0.18 ± 0.04

b0.01 0.01 0.58 0.26 0.18 0.39

−0.03 ± 0.01 −0.27 ± 0.13 −0.07 ± 0.00 −0.09 ± 0.05 −0.39 ± 0.15 −0.19 ± 0.03

−0.14 ± 0.04 −0.10 ± 0.03 −0.31 ± 0.08 −0.07 ± 0.02 −0.11 ± 0.05 −0.15 ± 0.08

−0.04 ± 0.01 −0.23 ± 0.13 0 −0.02 ± 0.01 −0.23 ± 0.16 −0.07 ± 0.02

−0.09 ± 0.03 −0.72 ± 0.36 −0.56 ± 0.19 −0.24 ± 0.11 −0.20 ± 0.12 −0.90 ± 0.49

0 −0.08 ± 0.03 0 0 −0.01 ± 0.00 0

b0.01 0.01 b0.01 0.02 0.05 0.02

−0.03 ± 0.03 −0.12 ± 0.01 0 −0.06 ± 0.03 −0.02 ± 0.01 −0.09 ± 0.04

0.02 0.10 0.04 0.02 0.12 0.96

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Substrate type

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or flooded–suboxic at 12 °C treatment, respectively (Fig. 3; Table 2). The incubations resulted in treatment effects of non-flooded N flooded; flooded suboxic N flooded anoxic; 22 or 4 N 12 °C under the flooded treatments and 22 N 12 N 4 °C under the non-flooded treatments. Litter produced more CO2 than soils under both flooded and non-flooded conditions. The Q10 values (ratios of 22 to 12 °C) averaged 2.0 ± 0.9 (standard deviation) for all substrates (flooded: 2.0 ± 1.0; non-flooded: 2.0 ± 0.6), 1.7 ± 0.7 for the forest substrates (flooded: 1.6 ± 0.8; nonflooded: 1.9 ± 0.3) and 2.3 ± 1.0 for the peatland substrates (flooded: 2.4 ± 1.1; non-flooded: 2.1 ± 0.8) (Table 2). Overall, the largest CH4 production occurred in the flooded–anoxic at 22 °C treatment, while no specific treatment accounted for the smallest production (Fig. 3; Table 2). In general, treatment effects on CH4 production were flooded N non-flooded; anoxic N oxic; and 22 N 12 = 4 °C under flooded condition and 22 N 4 N 12 °C under nonflooded conditions. Soils, especially peat soils, exhibited larger CH4 production than litter within both flooded and non-flooded conditions. Dominant treatment effects on CH4 consumption were non-flooded N flooded and soils N litter of the non-flooded treatment.

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3.3. Treatment effects of CO2 and CH4 exchange Most of the flooded anoxic treatments resulted in slower CO2 production rates than the flooded suboxic and non-flooded treatments, and O 2 level under the flooded treatments much increased the rates for peat soils (PS-1 and PS-2) (Fig. 4; Table 3). Significant effects by O2 level (P b 0.01) existed in PS-1 under flooded conditions. In general, the influence of CO2 production rates was 22 N 12 N 4 °C for the non-flooded treatments and 22 N 4 N 12 °C for the flooded treatments; the latter unexpected pattern may result from increased leaching of DOC at 12 °C under flooded conditions. Temperature caused significant increases in CO2 production rates (P b 0.01) under flooded conditions only for peatland substrates (PS-1, PS-2 and PL), while temperature accounted for strong changes (P ≤ 0.03) for all substrates under non-flooded conditions (Table 3). Under flooded conditions, a significant interaction between O2 level and temperature (P ≤ 0.03) was found in all substrate types except BL. Litter types, especially PL, showed faster rates of CO2 production than soil types under both the flooded and non-flooded treatments.

Fig. 5. Mean ratios of CO2 production rates (n = 9) in the flooded treatments to the non-flooded treatments for substrate types. Error bars indicate standard error of the mean, and dashed lines indicate y = 1. Note that soil substrate types are on the left side, litter substrate types are on the right side and the scale of y-axis is different for FS. X-axis labels for treatments are FA4: flooded–anoxic at 4 °C; FA12: flooded–anoxic at 12 °C; FA22: flooded–anoxic at 22 °C treatment; FSO4: flooded–suboxic at 4 °C; FSO12: flooded–suboxic at 12 °C; and FSO22: flooded– suboxic at 22 °C.

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Fig. 6. Mean ratios of CH4 production rates (n = 9) in the flooded treatments to the non-flooded treatments for substrate types. Error bars indicate standard error of the mean, and dashed lines indicate y = 1. Note that soil substrate types are on the left side, litter substrate types are on the right side and the scale of y-axis is different for FS, PS-1 and PS-2. X-axis labels for treatments are FA4: flooded–anoxic at 4 °C; FA12: flooded–anoxic at 12 °C; FA22: flooded–anoxic at 22 °C; FSO4: flooded–suboxic at 4 °C; FSO12: flooded–suboxic at 12 °C; and FSO22: flooded–suboxic at 22 °C.

Under flooded conditions, strong effects by O2 level (P ≤ 0.04) occurred in PS-1, PS-2 and BL for CH4 net production and significant effects (P ≤ 0.01) were shown in FS and PS-1 for net consumption (Fig. 4; Table 3), with the smaller O2 level generally supporting larger production or consumption rates in soils. Warmer temperatures accounted for significantly larger CH4 production rates (P ≤ 0.03) for PS-1, PS-2 and BL under flooded conditions and all substrates under non-flooded conditions. For CH4 net consumption, FS, PS-2 and BL demonstrated an influence of temperature (P ≤ 0.02) under flooded conditions, while most substrate types (except FL) showed a significant effect of temperature (P ≤ 0.02) under the non-flooded treatments, as the lower or middle temperature led to higher CH4 consumption rates (Fig. 4; Table 3). There was a strong interaction between O2 level and temperature (P ≤ 0.05) for CH4 net production in all the flooded substrates except FS and BL. Soils, particularly peat, exhibited faster rates of CH4 production and consumption than litter under both flooded and non-flooded conditions. 3.4. Comparison of CO2 and CH4 exchange rates between flooded and nonflooded conditions The ratios of CO2 production rates in the flooded to non-flooded treatments averaged 0.76 ± 0.44 (standard deviation) and were N1 in

7 out of 36 cases, all at 4 °C (Fig. 5). The flooded suboxic incubations produced generally larger ratios than the flooded anoxic incubations and the ratios were commonly 4 N 12 N 22 °C among substrate types, with the ratios closer to 1 in soils than in litter. The ratios of CH4 production rates in the flooded to non-flooded treatments averaged 1.32 ± 13.85 (standard deviation) and were N 1 in 14 out of 36 cases (included in all three temperatures) (Fig. 6). Higher ratios were found at 22 °C in all soils and BL, but no clear difference in the ratios was observed between flooded anoxic and suboxic incubations. 4. Discussion The rapid production of CO2 at the beginning of the incubation, followed by a decline (Fig. 1), is similar to those obtained in other studies (e.g. Côté et al., 2000; Dalias et al., 2001; Keller et al., 2004) and attributed to the rapid mineralization of the labile fraction of organic matter (Scanlon and Moore, 2000; Guérin et al., 2008). The dramatic increases in CH4 production in soils over the incubation under anoxic conditions (Fig. 2) are probably related to lag time for the microbial consumption of electron acceptors and to the later development of methanogen populations (Blodau and Moore, 2003b). Thus, CO2 is the

Y. Kim et al. / Science of the Total Environment 511 (2015) 381–392

dominant gas of C mineralization in short-term experiments, as shown by CO2:CH4 production ratios averaging 4246 ± 6193 (standard deviation) (Fig. 3; Table 2). The exceptions are the flooded anoxic 22 °C FS, PS-1 and PS-2, the flooded anoxic 12 °C PS-2 and the flooded suboxic 22 °C PS-2 treatments, where CH4 production released between 0.3 and 8.0 mg C g−1 during the incubation. This study provides an insight into the influence of temperature on microbial mechanisms of anaerobic CO2 and CH4 production. As a terminal step in microbial respiration under anoxia, methanogenic microbes lead to CH4 production through the reduction of an alternate electron acceptor and this consequently generates CO2 (Segers and Kengen, 1998). Under flooded conditions, CO2:CH4 production ratios were 4 N 12 N 22 °C (4027 ± 4661 N 797 ± 1809 N 152 ± 2693) (Fig. 3; Table 2), showing that warmer conditions promoted CH4 production compared to CO2. Dissolved organic matter (DOM) in the water could play a role as an alternative electron acceptor for CO2 production under anoxic conditions (Heitmann et al., 2007). The DOC analysis from the same experiments (Kim et al., 2014) showed that large amounts of DOC were released from the flooded anoxic treatments (means of 5–143 mg C g−1 C for 15 weeks in all substrates) compared to non-flooded treatments (means of 1–17 mg C g− 1 C), and these DOC amounts could be related to CO2 production. There was a significant positive correlation (P b 0.05) between cumulative DOC and CO2 production among substrates under flooded anoxic conditions at weeks 3, 6 and 12. Larger DOC production under flooded conditions may have promoted anaerobic CO2 production, and at 4 °C, anaerobic CO2 production outweighs aerobic production (Fig. 5). Our measurements support the first hypothesis that C mineralization, dominated by CO2 production under flooded conditions, is slower than under non-flooded conditions (Fig. 5). In contrast, Oelbermann and Schiff (2008) reported flooding:non-flooding CO2 production ratios of 1.6 and 1.8 for a forest organic soil and litter, respectively, and a ratio of 3.8 was reported by Blodau and Moore (2003a) for peat soil. These differences probably reflect variations in experimental conditions, such as water depth, O2 level and temperature (e.g. 8 cm vs. 1 to 2 cm; suboxic or anoxic conditions vs. oxic conditions; and 4 to 22 °C vs. 12 to 22 °C). Increases in the ratios of CH4 production rates in flooded to non-flooded conditions (Fig. 6) are similar to other studies in boreal regions (e.g. Kelly et al., 1997; St. Louis et al., 2000; Huttunen et al., 2002; Oelbermann and Schiff, 2008). The hypothesis that C mineralization under flooded conditions occurs faster in the presence of higher O2 concentration is not fully supported by our results. Carbon dioxide production rates in most of the substrate types did not significantly differ between the flooded anoxic and flooded oxic treatments except for peat soils (Fig. 4), and O2 concentration itself might not influence CO2 production rates as significant interactions existed between O2 and temperature treatments. Yavitt et al. (1997) showed a 20% decrease in CO2 production from oxic to anoxic treatments in peat soil incubations, but no statistically significant difference was identified between the two treatments. Methane production showed significant differences in response to O2 level in peat soils and black spruce litter, with larger production under anoxic conditions (Figs. 3 and 4). Moore and Dalva (1997) and Keller et al. (2004) demonstrated that CH4 production was more dominant under anoxic conditions than under oxic conditions. Our results demonstrate that warmer conditions promote more C mineralization under flooded conditions (Figs. 3 and 4), as found by others (e.g. Neff and Hooper, 2002; St. Louis et al., 2003; Paré et al., 2006; Oelbermann and Schiff, 2010). The CO2 Q10 values, determined by our 22:12 °C incubations, are similar to those reported as 1.6 for boreal forest soils (Dalias et al., 2001) and 1.4 to 2.0 for peat soils (Scanlon and Moore, 2000). We found that litter had larger C mineralization than soils, supporting our last hypothesis on the difference in mineralization rates by substrate type. Peatland litter and soils produced larger amounts of CO2 and CH4 than forest litter and soils, under the same

391

environmental treatment (Figs. 3 and 4). Oelbermann and Schiff (2008) noted that CO2 production from forest litter was more than from forest soils, and Scanlon and Moore (2000) showed that surface layers of a peat were more productive in CO2 than deeper layers. The presence of a methanogen population is important for CH4 production, with methanogens naturally occurring under anoxic conditions (Merilä et al., 2006). Although methanogen populations could exist in forest organic soils and litter (Angel et al., 2011, 2012), CH4 production was weak during the incubation of forest soils and fresh litter in our study, and Oelbermann and Schiff (2010) showed that flooded litter produced less CH4 than flooded FH-horizon soils. Our study shows how substrate types, soil or litter, can contribute to the total C mineralization from flooded boreal ecosystems. The ELA flooding experiments were unable to identify the contribution of each soil and litter substrate as they did not separate organic substrates (e.g. Kelly et al., 1997; McKenzie et al., 1998; Matthews et al., 2005; Oelbermann and Schiff, 2008, 2010). Our results suggest that inundation would reduce CO2 production but increase CH4 production from forest soils (Figs. 5 and 6). Increased CO2 fluxes from the flooded ecosystems can be attributed to the mineralization of vegetation killed by flooding and senesced ground vegetation that may include fine roots. On the other hand, increased CH4 fluxes over the flooded ecosystems are likely derived from soils rather than vegetation or litter. The size of C pools in soil and vegetation is an important control of the magnitude of increased CO2 and CH4 fluxes; in peatlands, the large C mass in peat could be more important than that in vegetation. The CO2 and CH4 production rates we observed help to explain patterns of the net changes in boreal forest and peatland CO2 and CH4 fluxes before and after inundation. Caution should be exercised with the interpretation of the magnitude of changes from our four-month incubations. For boreal forest and peatland ecosystems, CO2 flux would change from a sink as photosynthesis of terrestrial vegetation ends and flooded soils and fresh litter decompose. Boreal forests consume small amounts of CH4 (b1 g C m−2 yr−1, Ullah et al., 2009), whereas boreal peatlands emit large amounts of CH4 (b 10 g C m−2 yr−1, Pelletier et al., 2007). 5. Conclusions Our incubations revealed the interactions between flooding treatments, O2 levels, temperatures and substrates on CO2 and CH4 production. Flooding decreased CO2 production from soils and litter by 24%, while CH4 production increased by 32%. Within the flooded incubations, the higher O2 concentration (suboxic) treatments increased CO2 production from peat soils, and warmer temperatures (12 and 22 °C) considerably increased the production from peat soils and peatland ground litter. The lower O2 concentration (anoxic) treatments and warmer temperatures increased CH4 production from peat soils and black spruce litter under the flooded treatments. Among the substrate types, peatland ground litter and peat soils, respectively, produced the largest CO2 and CH4 under both flooded and non-flooded conditions. Our measurements of CO2 and CH4 production can provide an estimate of the range of changes in C mineralization rates in boreal soils and litter before and after flooding, and support model parameters and scaling factors for simulating C exchanges from newly flooded boreal landscapes. However, this study may be difficult to apply to the longterm changes in C exchanges from boreal ecosystems in response to flooding. Our study allows a quantification of the interactive influence of O2 and temperature on C mineralization rates in inundated boreal forest/peat litter and soil as well as the application to models such as DNDC (DeNitrification DeComposition) (Kim, 2011). Acknowledgments This study was financially supported by the Canadian Foundation for Climate and Atmospheric Research (CFCAS GR-627) and Hydro-Québec.

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The authors thank A. Tremblay at Hydro-Québec Production for logistic support in the field, and H. Lesage, M. Romer and M. Dalva, McGill University for their help with the laboratory analyses.

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