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Keywords:

  • peatland;
  • methane;
  • respiration;
  • photosynthesis;
  • carbon balance;
  • subarctic palsa mire

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

[1] Carbon dioxide and methane fluxes in a palsa mire characterized by dry (palsa) and wet surfaces in a subarctic zone in Finland were measured using a static chamber technique during two climatically different years, 1998 and 1999. Each of the 24 collars was individually studied for water table level, peat temperature, pH, vegetation, frost depth, CO2 exchange and CH4 fluxes. The annual gaseous carbon balance varied from −36.9 g C m−2 to −138.6 g C m−2 (uptake) on wet surfaces. During both years, palsa surfaces with shrub vegetation were sinks for carbon, whereas palsa surfaces with sparse vegetation lost carbon to the atmosphere. In 1999, a wet year, a lower NEE resulted mainly from a decrease in photosynthesis. The annual emissions of CH4 ranged from 1.0 g CH4-C m−2 in the palsa to 24.7 g CH4-C m−2 at the palsa margin. On wet surfaces, photosynthesis, respiration and CH4 fluxes were tightly linked. Annual NEE and CH4 fluxes were close to the values reported for boreal peatlands (fens).

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

[2] Carbon accumulates as peat in waterlogged soils where carbon uptake in photosynthesis exceeds the gaseous carbon production in respiration and anaerobic decomposition processes. Peatlands are the most important carbon stores among soil ecosystems, containing 270–455 Pg of carbon [Gorham, 1991; Turunen et al., 2002], which is 35–60% of the carbon stored in atmospheric CO2. Although they represent a huge reservoir of carbon, northern peatlands correspond to one third of the total CH4 emissions from wetlands to the atmosphere [Bartlett and Harriss, 1993].

[3] Possible changes in peatland CO2 and CH4 dynamics caused by global warming could have major impacts on atmospheric gas composition. Therefore, during recent years, peatlands have received special attention in biogeochemical research. While most peatlands are located in tropical, boreal and arctic regions, research has focused on boreal peatlands, where the dynamics of methane and carbon dioxide have been extensively studied [e.g., Alm et al., 1997; Granberg et al., 1997; Waddington and Roulet, 2000]. Climate change in polar regions is expected to be the greatest and most rapid of any region on earth [Intergovernmental Panel on Climate Change, 2001]. For most of the year, arctic and subarctic peatlands remain frozen under a snow cover. Snow acts as insulation from cold, but also as a form of water storage. Biological processes are most active in the unfrozen uppermost peat layer during the short summer months. Global warming could lead to changes in the time of the snowmelt and thawing could evoke unpredictable, irreversible changes in the hydrology and carbon balance of arctic peatlands. One important question for the carbon balance is the possible changes in the length of the carbon-fixing growing season and the length of the cold period when only decomposition occurs [Oechel and Vourlitis, 1997].

[4] Hydrology plays a crucial role in the carbon balance of peatlands. Studies of natural boreal peatlands in Finland and Sweden have shown that a single warm and dry summer may turn a natural boreal peatland into a source of carbon [Alm et al., 1999a; Waddington and Roulet, 2000]. According to Oechel et al. [1993, 1995], the arctic tundra in Alaska has been converted from a sink to a source of carbon during the past 2 decades, and this change has been linked to climatic warming causing drying of soils.

[5] The morphology of subarctic peatlands, their hydrology, temperature regimes, nutrient availability and botanical characteristics can be very patterned. This is very true for palsa mires, a special type of northern aapa mire in subarctic regions. Palsas are peat mounds with permafrost cores. The active layer is dry and covered with moss and shrub vegetation, even though palsas originate from wet surfaces characterized by moss and sedge vegetation. The carbon dynamics of various surfaces experiencing differences in seasonal thaw can reflect in different extents to the changes in hydrology and temperature. While studies of the CH4 and CO2 fluxes on palsa mires have been conducted [Svensson and Rosswall, 1984; Bubier et al., 1995, 1998; Svensson et al., 1999], only limited data are available to show both the annual CO2 balance and CH4 dynamics of the various surfaces [Bubier et al., 1995, 1998].

[6] During a 2-year period, the dynamics of CO2 and CH4 in a subarctic palsa mire in northern Finland were studied. The annual photosynthesis, net ecosystem CO2 exchange and the CH4 fluxes on the wet and dry surfaces of the mire were determined. The 2-year study period had different climatic conditions, which allowed us to consider the sensitivity of mire carbon dynamics to changes in temperature, water balance and seasonal global radiation as the driving force for photosynthesis and methane emissions. This study was part of the CONGAS project (Biospheric Controls on Trace Gas Fluxes in Northern Wetlands) funded by the European Union.

2. Materials and Methods

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

2.1. Study Site

[7] Gas fluxes were studied intensively during the summer of 1998 and 1999, and occasionally during the late autumn and winter to permit calculations of gaseous carbon budgets for the whole year.

[8] The study site (69°49′N, 27°10′E, 295 m. a.s.l.) (Figure 1a) is in the western part of a large palsa mire, Vaisjeäggi. The climate of this northernmost part of Finland is subarctic. Weather data (from 1962) are available from Kevo weather station located 10 km southwest of the study site. The mean air temperature for 1990–2000 was −1.2°C, and annual mean precipitation 456 mm (40% as snow) (Table 1). The length of the average snow-free period is 155 days, and on average there are 146 days when the maximum temperature is below zero and 230 days when the minimum temperature is below zero [Finnish Meteorological Institute, 1991]. Vaisjeäggi, with its exposed landscape, is located 188 m higher than the Kevo weather station; therefore the air temperature is somewhat lower during summer compared to the Kevo weather station (Table 1).

image

Figure 1. Location of (a) research area and (b) the collars at experimental area at Vaisjeäggi; T1–T4, transects; WS, weather station; PS, peat surface level measurement pole. Only the collars without permanent shading are included to this study.

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Table 1. Climatological Statistics From Kevo Weather Station
YearPrecipitation,a mm (Min.–Max.)Temperature,b °C (Min.–Max.)Global radiation, MJ m−2 (Min.–Max.)
1990–2000199819991990–2000199819991990–200019981999
  • a

    Source of climatological data for Kevo is the Finnish Meteorological Institute [2000].

  • b

    The temperatures measured at the site are calculated in a similar way (average of hours 2, 8, 14 and 20) as those at the synoptic weather station at Kevo. Monthly average temperatures measured at the study area are shown in square brackets.

January30 (12–51)3218−13.1 (−20.5–−8.7)−12.8 [n.d.]−20.5 [−16.4]2 (1–3)22
May25 (6–36)2592.9 (0.7–5.6)3.0 [n.d.]1.3 [1.2]504 (438–580)457508
June65 (7–115)83729.4 (6.2–12)7.8 [5.6]12.0 [11.3]499 (399–589)514522
July59 (27–129)558212.8 (10.7–14.2)14.2 [12.3]13.3 [11.3]458 (376–531)472448
August62 (28–161)3116110.8 (8.5–13.2)10.1 [8.8]8.5 [7.3]321 (244–405)347268
September33 (5–66)48275.7 (2.5–7.9)5.1 [3.5]7.8 [7.0]164 (110–200)142154
October48 (9–97)5871−1.0 (−8.4–3.8)−0.2 [−1.5]1.7 [0.6]56 (43–69)5051
Average456 (296–597)489597−1.2 (−3.3–0.2)−3.3 [n.d.]−1.8 [n.d.]2561 (2144–2798)25772531

[9] The site is on bedrock with a peat layer of 4.5 m at the center of the experimental site and 3.5 m at the palsa margin. The study site had different surfaces based on morphological, hydrological and vegetation characteristics. Four study transects were set up on the mire to encompass the various functional surfaces (Table 2). Transects T1 and T2 were located on the wet surfaces, transect T3 was at a wet palsa margin south of a steep degrading palsa wall, and transect T4 was on the palsa top (Figure 1b). The studied palsa was elevated over 1.5 m from the mire surface. The palsa margin was partially flooded during early summer. On the wet transects, Sphagnumlindbergii dominated in some plots, with a mixture of Sphagnumlindbergii and Sphagnumriparium in others. The most common vascular plants were Eriophorum angustifolium, Eriophorum russeolum, Vaccinium microcarpum and Carex limosa. At the palsa margin, Sphagnum riparium occupied the ground layer, and vascular plants included E. angustifolium and E. russeolum. On the palsa, the vegetation consisted of Vaccinium vitis-idaea, Betula nana, Empetrum nigrum, Rubus chamaemorus, Ledum palustre, Dicranum polysetum, Andromeda polifolia and lichens like Cladina rangiferina and Cladonia species. On the top of the palsa, there were spots without vegetation coverage and some cracks, as some of the palsa edges were breaking down.

Table 2. Water Content, Bulk Density, pH, and Contents of C, P and N (mg/g) in the Upper Soil Profiles of Various Transects at Vaisjeäggi mire in 1998
 Transect 1Transect 2Transect 3Transect 4
0–10 cm10–20 cm20–30 cm0–10 cm10–20 cm20–30 cm0–10 cm10–20 cm20–30 cm0–10 cm10–20 cm
  • a

    pH from palsa was measured from peat water slurry (n = 18).

Peat water content, %96.295.694.995.995.595.696.987.587.965.274.1
Bulk density (dry), g/L27.834.239.429.835.139.217.4101.595.9100.9150.7
Bulk density (wet), g/L717.1781.0776.1736.8783.5902.3552.6810.5795.8290.3582.1
pH, in collars3.9n.d.n.d.4.0n.dn.d.4.0n.d.n.d4.0an.d.
(range)(3.6–4.2)n.d.n.d.(3.8–4.2)n.d.n.d.(3.9–4.0)n.d.n.d.(3.8–4.4)n.d.
C (dry weight), mg/g419.0429.0432.0427.2438.7437.9465.1453.3469.6n.d.n.d.
P (dry weight), mg/g0.380.430.540.470.330.260.930.670.75n.d.n.d.
N (dry weight), mg/g6.839.3712.017.208.097.248.156.6511.45n.d.n.d.

[10] There were some differences in the contents of total carbon, total nitrogen and total phosphorus between transects. Carbon and phosphorus contents were highest at the palsa margin (Table 2). The nitrogen content, peat pH (Table 2), and vegetation composition (see above) suggested that the palsa margin was minerotrophic mesotrophic and the other wet surfaces were ombrotrophic or minerotrophic oligotrophic [Eurola et al., 1995].

2.2. Weather and Soil Physical-Chemical Characteristics

[11] Local weather data were obtained from a Vaisala MAWS weather station in 1998 from July 9 to October 7 and in 1999 from July 2 to September 29. Photosynthetically active radiation (PAR) was measured with a Li-190SA Quantum sensor, air temperature at a height of 1.5 m with a QMH101 sensor and precipitation with a tip bucket rain gauge (QMR101). The radiation and temperature data were measured every 10 min, from which the mean hourly values with standard deviations were calculated. There were additional HOBO loggers storing air and peat temperatures during the winter. These data, in addition to the data from a nearby (2 km distance) weather station set up for the LAPP project [Lloyd et al., 2001], were used to supplement some data gaps in our weather station data.

[12] The changes in the vertical position of the peat surface were examined using wooden poles fastened to the mire bottom at transects T1, T2 and T3 [see Roulet, 1991]. Peat pH was measured in situ from the collars of transects T1, T2 and T3 with a portable pH meter WTW 302 equipped with a Hamilton Flustrode probe and a temperature probe. The results shown here were measured in 1998 on August 10 from a depth of 8 cm (Table 2). Samples to determine peat bulk density and nutrient concentrations were taken with a volumetric peat sampler (8 cm × 8 cm). These samples were dried for 48 hours at +60°C. Total contents of N and P were measured using a FIA technique after digestion with sulphuric acid, and the total C content was determined using a Leco carbon analyzer at the University of Lund. Peat properties and nutrient contents are shown in Table 2.

2.3. Growth of Vegetation

[13] The stems of vascular plants were counted from each collar in August 1998 and 1999. Growth of Sphagnum at transects T1, T2 and T3 was measured as length increment. In 1998, there were five replicate growth wires with a brush shape inserted near a collar at each transect. Two growth wires were also inserted in 1999 to all collars at transects T1, T2 and T3. The biomass of Sphagnum was measured in autumn from samples of 10 cm × 10 cm × 4 cm (n = 3) at each transect to determine the bulk density of new moss biomass. Aboveground biomass of vascular plants was measured by harvesting plants from an area of 25 cm × 25 cm (n = 3). The dry biomass was measured after drying at 60°C. The factor used to convert dry mass to carbon was 0.50 for vascular plants, and for mosses the carbon content of the 0–10 cm layer was used (Table 2). In 1999, the growth patterns of Carex and Eriophorum were measured with a ruler from 10 plants marked with nylon rings at transect T1. The growth measurements were carried out every third week.

2.4. Measurement of CO2 and CH4 Fluxes

[14] Boardwalks were constructed to prevent any disturbance to peat gas storage during measurements or damage to vegetation when regularly visiting the site. The collars (56 × 56 cm) were permanently inserted into the peat to a depth of 15–30 cm in the second week of June, 1998. The upper part of the collars had a groove for the water seal needed for the chamber measurements. The upper parts of the collars were kept flush with the peat surface. Transects T1 and T2 both had 18 collars, with six collars at the palsa margin (T3) and on the palsa (T4) (Figure 1b). Adjacent to each of the collars were perforated tubes inserted into the peat to measure water table depth. Frost depth and peat temperatures were also measured regularly near the collars. Every second collar was covered permanently with tent reducing radiation, these manipulated collars are not included to this study.

[15] During intensive study periods, CH4 fluxes were measured weekly and CO2 fluxes once or twice each week. In 1998, the intensive measurements started in the third week of June and continued until October 10. In 1999, the first flux measurements were made in the second week of June and the intensive study period lasted until September 29. The winter fluxes were measured from April 12–16, 1999 and May 9–10, 2000.

[16] The transparent closed chamber was made of 1.6 mm polycarbonate (60 cm × 60 cm × 25 cm) and was used in the CO2 exchange measurements. The chamber had an automatically driven cooler to prevent temperature increase and a muffin fan ensured mixing of the air during the measurement [Alm et al., 1997]. The CO2 concentration in the chamber was measured with a portable infrared gas analyzer (Li-6200 Portable Photosynthesis System, LI-COR, inc., Lincoln, Nebraska, USA) during an interval of 90 to 120 s and was equipped with a pump circulating all sample air (1.3 L min−1) through a desiccant to the CO2 analyzer. The net CO2 exchange (NEE) was measured at ambient illumination and at an illumination that was 70% of the ambient (the chamber was shaded with a portable dome shaped tent, area 2.3 m−2 and height 1.0 m, made of hessian sackcloth). These measurements at reduced PAR completed the radiation data needed to establish the light response curves between photosynthesis and PAR [Bubier et al., 1998; Tuittila et al., 1999]. Immediately after the CO2 measurement, the transparent chamber was removed from the collar for 1 min, reinserted and darkened with an opaque cloth. The total release of CO2 (Rtot) was then measured with the same IR analyzer. The PAR-sensor (LI-190SA) and radiation shielded temperature sensor were inside the chamber and these data were logged along with time and CO2 concentration data. During the CO2 flux measurements, soil temperatures were measured near the collars with a multichannel temperature probe “Hessu Kevo,” or with a Fluke 52 thermometer at the surface and at depths of 5, 10, 15 and 20 cm. The depth of the frost layer was measured with a metal rod (diameter 5 mm). The water table was measured from the perforated pipes with a ruler. The snow depth was measured with a carbon fiber rod. All depth measurements were fixed to the peat surface.

[17] A dark, closed aluminum chamber (56 cm × 56 cm × 20 cm) was used for methane flux measurements. There was a battery-operated fan in the chamber and the chamber was equipped with a capillary tube to retain atmospheric pressure inside the chamber when sampling. During the 12-min measuring period, four samples were taken into gas tight plastic syringes [Nykänen et al., 1998] from the chambers at the wet transects. For the palsa, a measuring period of 20 min was used. Separate syringes were used for the palsa and wet transects because of the great difference in the methane fluxes. The syringes were left open for at least 24 hours before the next sampling. The CH4 concentrations were analyzed within 24 hours with an HP 5890 gas chromatograph with an FI-detector, HayeSep Q 1/8″ × 1.8 m column and a manual sampling valve with 0.5 mL loop. The peak areas were analyzed with an HP integrator 3369. The gas standard applied had 9.89 ppm ± 3% CH4 and 401 ppm ± 3% CO2 in synthetic air (Aga, Sweden). The CV of analyses of repeated runs of standards was less than 0.5%. Methane flux was calculated from the linear slope of the change in concentration over time, thus CH4 from ebullition is not included in the flux rates. The same environmental background variables for the CO2 fluxes were measured for CH4, except that the temperatures in the peat were measured at depths of 0, 2, 5, 10, 15, 20, 25, 30, 35, 40 and 50 cm. Winter fluxes of CH4 and CO2 were measured on April 16, 1999, and on May 10, 2000, using a snow profile method [Sommerfeld et al., 1993; Alm et al., 1999b]. In 1999, measurements were made from 48 snow profiles. On the palsa, chambers were also used during late winter 2000.

2.5. Modeling Gaseous Carbon Fluxes

[18] Statistical analyses were performed using SPSS for Windows [SPSS, Inc.]. Analyses of variance, correlation and regression analyses were used to study the relationships between the CH4 fluxes and the environmental parameters.

[19] The annual CO2 flows were reconstructed in a similar way as previously applied for carbon balance studies on peatlands using the chamber technique [Alm et al., 1997; Tuittila et al., 1999]. The net ecosystem CO2 exchange (NEE) in situ at prevailing radiation and at reduced radiation (shading) were calculated from the linear change in the CO2 concentration logged at 5-s intervals during the measuring period of 90–120 s. When plotting the changes in the CO2 concentration against time, the best fit period (the highest r2) of six measurements made during 30 s was used. For the respiration measurements, similar measuring periods were used as above.

[20] Negative NEE values were used when the CO2 fixation by the vegetation exceeded the total respiration of vegetation and soil. With this approach, respiration had positive values. Similarly, the CH4 emissions were positive, and uptake from atmosphere to soil had negative values.

[21] The gross photosynthesis (PG) was calculated using data obtained with the full (NEEf) and reduced radiation (NEEr). PG is the result of subtracting respiration (Rtot) from NEE, i.e. PG1 = NEEf − Rtot, and PG2 = NEEr − Rtot. Using this approach, two PG:s were obtained at every measuring event. In these two measurements, other environmental conditions except radiation were constant, which strengthened the modeling. The Rtot is the sum of CO2 produced by plant dark respiration, and by respiration of microbes and soil fauna.

[22] PG was assumed to be dependent on solar irradiation (I) in a functional form of a rectangular hyperbola with parameters Qmax (asymptotic maximum value of photosynthesis in optimal light) and half saturation parameter k (amount of radiation when photosynthesis is equation image of maximum). The linear part of equation (1) includes air temperature (Tair) multiplied by a constant (t). CO2 release (Rtot) was related to air temperature with a logarithmic model. For CO2, modeling air temperature was selected because it had the best correlation with the CO2 fluxes (Rtot and PG). The response function for PG with solar irradiation (I) and air temperature (Tair) as independent variables is shown in equation 1, and Rtot with Tair as the independent variable is shown by equation (2).

  • equation image
  • equation image

[23] To account for the high-frequency variation in irradiation, equation (1) was calculated twice with two irradiation values (hourly averaged PAR ± 1 S.D). The mean of these calculations was the hourly PG [Smolander, 1984; Alm et al., 1999a].

[24] During the summer, the net CO2 exchange (NEE) was calculated for every hour using the formula

  • equation image

To avoid negative PG values in modeling, PG in equation (1) was marked as 0 when PAR was below 4 μmol m−2 s−1.

2.6. Reconstruction of the Annual Carbon Balance

[25] The annual CO2 balances were calculated using the sum of three periods. First, the spring fluxes of CO2 (Julian days 152–182) were modeled using combined CO2 data from years 1998 and 1999 at the transect levels. In the calculation of actual annual spring fluxes, weather data for that particular year were used. Since PG and Rtot measurements began in the second half of June, the fluxes for early June were approximated from the models made for late June. During Julian days 152–182, the increase in respiration and photosynthesis rates were assumed to increase linearly from the rates during the winter. During winter, there was no photosynthesis. The CO2 balance during summer was modeled separately for both years using data from Julian days 183–280 from individual collars. The CO2 emissions for the late autumn and winter (Julian days 281–151) were calculated from the flux measurements carried out with the snow gas gradient method on the transects (see above). The CH4 fluxes for Julian days 162–258 were calculated for every collar by summing the weekly mean fluxes multiplied by the hours of the week. Late autumn fluxes (Julian days 259–270) were extrapolated from measurements made at the transect level during both years, similarly the fluxes for Julian days 271–167 were extrapolated from the measurements made with the snow gas gradient technique at the transect level during winter.

[26] Standard error and 95% confidence limits were calculated according to Heikkinen et al. [2002b] for variation in the gaseous carbon balance (GCB) at various transects: GCB ± t × S.E (n = 3, t = 4.3; n = 9, t = 2.3). The standard error (S.E.) for the growing season flux estimate was calculated from SD = (var (GCB))1/2, where

  • equation image

Winter and spring fluxes were excluded from uncertainty analysis.

3. Results

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

3.1. Snow Depth and Frost Thawing

[27] The palsa margin, transect T3, had the thickest snow cover. The snow depth decreased gradually when moving from the palsa margin. The snow depth on April 14, 1999, was 46 cm (range 38–55 cm); 47 cm (range 40–52 cm), 69 cm (range 47–81 cm) and 12 cm (range 2–26 cm) at transects T1, T2, T3 and T4, respectively.

[28] During both winters, the frost penetrated to a depth of 50–60 cm in transects T1 and T2. The frost disappeared faster in 1999 than in 1998 (Table 3). In 1998, the average disappearance of frost at T1 occurred on Julian day 215, but in 1999, it disappeared on average by Julian day 195 (Figure 2, Table 3). The faster melt in 1999 was associated to the high temperature in June 1999. The Julian day when the frost disappeared correlated negatively with water table (r = −0.581 p = 0.00, n = 37); that is, on peat surfaces with a high water table, the frost disappeared earlier. The waterlogged palsa margin (T3) had only a thin ice layer on the peat surface in early spring. The palsa had a frozen core year round, but melting of the active layer was faster in 1999 than in 1998 (Table 3, Figures 2c and 2d).

image

Figure 2. (a) Precipitation and (b) air temperature; (c) adjustment of peat surface (first measurement = 0 cm), left axis (T1, bold solid line; T2, bold dotted line; T3, bold long-dashed line) and (d) average level of frost, right axis (T1, solid line; T2, dotted line; T4, dash-dotted line); (e, f) average of photosynthetically active radiation (PAR) during day hours (solid line); (g) growth of Sphagnum (T1, solid line; T2, dotted line; T3, long-dashed line); (h) growth pattern of vascular plants, right axis (thick solid line). Left-hand column refers to 1998; right-hand column refers to 1999.

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Table 3. Seasonal Mean Water Table and Soil Temperature at a Depth of 10 cma
TransectNWT (cm)Temp. °C (10 cm)Frost (cm)Day of frost escapeSpeed of frost melting (mm/day)
MeanS.E.Min.Max.MeanS.E.Min.Max.MeanS.E.Min.Max.MeanS.E.Min.Max.MeanS.E.Min.Max.
  • a

    Average depth of frost layer during Julian days 186–200, Julian day of frost escape and speed of frost disappearance. Data are from 1998 and 1999; n.d., not measured; p.f., permafrost.

1998
19−4.10.4−5.6−2.58.00.17.68.4−34.30.44−42−28.02151.62102253.60.22.34.2
29−6.10.5−8.2−3.97.60.26.98.8−36.00.40−42−30.52154.41912404.20.33.15.5
33−1.91.2−4.1−0.110.10.39.510.40000184n.d.184184n.dn.dn.d.n.d.
43n.d.n.d.n.d.n.d.6.80.95.08.0−26.51.91−35−17.5p.f.n.d.n.d.n.d.2.70.61.95.5
Mean −4.60.4−8.2−0.17.90.25.010.4−34.30.39−42−17.52142.71842403.70.21.95.5
 
1999
19−2.60.4−4.7−0.98.20.27.29.5−41.40.48−49−351954.31742088.30.86.014.0
29−5.70.8−9.3−2.47.30.45.98.7−37.50.48−46−302084.01882248.00.93.611.3
33−4.21.3−6.5−2.19.50.19.29.70000n.dn.d.n.dn.dn.dn.dn.d.n.d.
43n.d.n.d.n.d.n.d.6.90.56.17.7−33.90.95−43−24p.f.n.d.n.d.n.d.3.20.52.64.0
Mean −4.20.5−9.3−0.97.80.25.99.7−38.30.40−49−242023.21742247.40.62.614

3.2. Temperature, Radiation and Hydrology in Summer 1998 and 1999

[29] Monthly mean temperature, precipitation and global radiation in 1998, 1999 and 1990–2000 measured at Kevo weather station are presented in Table 1. The summer of 1998 and 1999 differed in their weather conditions. June 1999 was 4.2°C warmer than June 1998, which was 1.6°C colder than the 10-year average. July 1999 was 0.9°C colder than July 1998, but both months were warmer than the 10-year average. The temperature in August 1998 was 0.7°C colder and in 1999 it was 2.3°C colder than average (Table 1, Figures 2a and 2b). The average temperatures from January to May 1999 were the coldest during the period of 1990–2000. Precipitation in June 1998 was 18 mm greater than the 10-year average, whereas the other summer months in 1998 were dryer than average (Table 1). May and June were drier in 1999 than in 1998, whereas July and August in 1999 had a higher precipitation than in 1998. The precipitation in July and August 1999 exceeded the 10-year average for these months. In August 1999, precipitation was 5 times that of August 1998 and was also the greatest during the period of 1990–2000. There also were differences in the global radiation between these two years (Table 1). In 1999, the global radiation in May and June was higher than in 1998, and also higher than the long-term average. However, July and August 1999 had a lower global radiation than 1998, and in July and August 1999, the radiation was lower than the 10-year average (Table 1).

[30] The water table and peat temperature in 1998 and 1999 reflected the weather differences between these years (Table 3). Except for transect T3, the average water table was higher in 1999 than 1998. The average temperature at a depth of 10 cm correlated positively with the average water table (r = 0.36; p = 0.000, n = 42). In June and July, peat at a depth of 10 cm was warmer in 1999 than in 1998, whereas in August and September it was colder in 1999. However, there was no statistical difference in the mean peat temperature between summer 1998 and 1999 at a depth of 10 cm (Table 3).

[31] At the depth of 10 cm, the palsa (T4) had a lower temperature than the wet transects T1 and T2 (Figures 3o and 3p, Table 3). However, the palsa margin (T3) had the highest peat temperature, and was on average 2°C higher than the other transects (Table 3, Figure 3). The thicker snow cover probably accounted for the high peat temperature at the palsa margin (see above). The thin frost layer disappeared within a few days in June at T3, and especially in 1999 the peat at the palsa margin warmed up rapidly in early summer (Figure 3).

image

Figure 3. (a, b, e, g, i, j, m, n) Net ecosystem exchange (NEE) with full light (solid circles) and reduced light (solid triangles) and Rtot in total darkness (open circles); (c, d, g, h, k, l, o, p) fluxes of CH4 (solid circles), soil temperature during measurements at depth of −10 cm (thick line) and level of water table (thin line) in T1 (Figures 3a–3d), T2 (Figures 3e–3h), T3 (Figures 3i–3l) and T4 (Figures 3m–3p). Negative values indicate uptake of carbon (CO2-C or CH4-C) from atmosphere to soil by the ecosystem; positive values indicate release of gaseous carbon.

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[32] There were differences in the adjustment of the peat surface between transects T1, T2 and T3. The palsa margin was the most stable, whereas transect T1 was the most labile (in 1999 the peat surface sunk down to 17 cm from its spring level (Figures 2c and 2d)). Faster frost thawing and/or lower precipitation in early summer 1999 were the probable reasons for the greater sinking of the peat surface in 1999 than in 1998 (Figures 2c and 2d). The water table in the wet transects remained high during the whole summers (Figures 3c, 3d, 3g, 3h, 3k, and 3l), as the floating peat likely prevented flooding. In August 1999, when the peat surface rose more than 5 cm in 10 days (Figure 2d), there were some collars submerged after a period of heavy rain. In 1998, transects T1 and T2 had the lowest water table in the early summer (Julian Day 190) although there later was a subsequent dry period (Figures 2a, 3c, and 3g). In contrast to T1 and T2, the water table at the palsa margin decreased during the whole summer (Figures 2c, 2d and 3).

3.3. Measured Carbon Dioxide Exchange

[33] In 1998, respiration was generally highest before Julian day 220 and NEE (uptake of CO2) later reached its maximum (Figures 3a, 3e, 3i, and 3m). In 1999, both the respiration and NEE had their maximum before Julian day 220 (Figures 3b, 3f, 3j, and 3n). This difference between early summer 1998 and 1999 was not associated with the daytime PAR (Figures 2e and 2f). Global radiation and PAR reached a maximum in early summer (May and June, Table 1, Figures 2e and 2f) causing rapid warming of the soil and a high respiration rate, which was further favored by the low water table maintained by the seasonal thaw. The increase in photosynthesis when plants emerged after thawing was probably compensated for by the increase in the respiration. The increase in NEE was linked to the growth of vegetation (Figures 2g and 2h). In 1999, the decrease in the NEE (Figure 3b, 3f, 3j, and 3n) occurred after the vascular plants reached their maximum length (Figure 2h), and even tough growth of Sphagnum continued (Figure 2h). The ecosystem started to lose carbon (positive daily NEE values) in autumn when PAR values started to decrease (Figures 2e and 2f), but soil temperature was still above zero (Figures 3a, 3b, 3e, 3f, 3i, 3j, 3m, and 3n).

3.4. Modeling of CO2 Exchange

[34] NEE was calculated from PG and Rtot separately for individual collars (examples shown in Figures 4a and 4b). The gas balance calculation based on the individual collars provided the possibility to more closely study the factors affecting gas exchange. The fit between PG and PAR (light response curve) was generally good, including values taken from the collar on the palsa with only sparse vegetation (Table 4). The fit between respiration and chamber temperature was generally good except for on the palsa margin in 1998 (Table 4).

image

Figure 4. Original measured CO2-fluxes (collars 14, 30 and 45) and those based on modeling shown by lines. Measured fluxes made before J.D. 182 are not used in the model, but are shown here. (a) PG and PAR and (b) Rtot and temperature. Models for photosynthesis and respiration:

  • Collar 14, transect 1; PG: r2 = 0.87, CO2-C (mg m−2 h−1) = (−104.16 × PAR)/(270.49 + PAR) + 41.41 + −4.94 × Tair, Rtot: r2 = 0.69, Ln CO2-C (mg m−2 h−1) = 1.04 + 0.16 × Tair.
  • Collar 30, transect 2; PG: r2 = 0.86, CO2-C (mg m−2 h−1) = (−352.84 × PAR)/(160.11 + PAR) + 81.33 + −0.49 × Tair. Rtot: r2 = 0.71, Ln CO2-C (mg m−2 h−1) = 3.19 + 0.07 × Tair.
  • Collar 45, transect 3; PG: r2 = 0.74, CO2-C (mg m−2 h−1) = (−454.85 × PAR)/(325.17 + PAR) + 62.27 + −0.46 × Tair, Rtot: r2 = 0.86, Ln CO2-C (mg m−2 h−1) = 3.46 + 0.07 × Tair.

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Table 4. Photosynthesis and Respiration Models for the Various Transects in 1998 and 1999
TransectNaYearr2Qmax, mg CO2-C m−2 h−1K, μ mol m−2 h−1b0, mg CO2-C m−2 h−1tb1, mg CO2-C m−2 h−1
MeanbS.E.MeanS.E.MeanS.E.MeanS.E.MeanS.E.MeanS.E.
  • a

    Number of microsites (collars).

  • b

    Mean and S.E. are calculated from the models made separately for every collar in a transect.

Photosynthesis Models: PG = Q × I/(I + k) + b0 + t × Tair
1919980.900.01−206.123.1271.9113.0−21.120.16.44.1  
1919990.740.06−194.663.5369.0471.3−41,549.410.53.8  
2919980.870.01−329.044.9248.692.2−61,761.77.95.3  
2919990.860.02−308.969.4454.3302.1−30.930.917.94.7  
3319980.820.04−375.554.4315.2156.4−43.443.4−5.78.7  
3319990.710.06−295.989.5358.9397.2−43.243.28.97.4  
4319980.590.09−326.9145.7182.4211.2−173.6173.64.59.6  
4319990.600.17−366.067.2348.3396.3−44.744.7−2.25.8  
 
Respiration Models: LnRtot = b0 + Tair * b1
1919980.770.03    2.100.26  0.110.01
1919990.700.06    1.820.18  0.100.01
2919980.790.03    3.140.12  0.070.01
2919990.770.04    2.900.08  0.080.00
3319980.160.08    3.180.24  0.050.02
3319990.690.19    3.070.16  0.060.01
4319980.520.04    2.960.26  0.080.02
4319990.760.08    3.180.68  0.070.02

[35] The NEE of individual collars during the intensively measured seasons (summer) ranged from a release of 25.3 g C m−2 at the sparsely vegetated palsa collar to uptake of 201.5 g C m−2 at the palsa margin (Table 5). The NEE average in 1998 and 1999 differed (p < 0.05, T-test) on transect 3, from −175.2 g C m−2 in 1998 and −100.2 g C m−2 in 1999 (Table 5). In general, the average carbon accumulation was lower in 1999 (NEE − 83.5 g C) than in 1998 (NEE − 106.9 g C) (Table 5). The difference was mainly due to the lower PG in 1999, and the difference in respiration between these years was minor (Table 5). In contrast to the wet transects, respiration on the palsa was clearly higher in 1999 than in 1998, whereas the PG value was similar in both years (Table 5).

Table 5. PG, Rtot, and NEE (Julian Days 183–280) and CH4 Fluxes and Gaseous Carbon Balance for the Summer Season (Julian Days 168–258) Calculated for 1998 and 1999a
 NPG g C m−2Rtot g C m−2NEE g C m−2CH4 g C m−2Carbon balance g C m−2 (S.E. and 95% Confidence limits)
MeanS.E.Min.Max.MeanS.E.Min.Max.MeanS.E.Min.Max.MeanS.E.Min.Max.MeanS.E.LowerUpper
  • a

    Different letters after the mean indicate statistically significant differences (P > 0.05) between the transects (Tukey, HSD) whereas an asterisk indicates statistically significant differences of mean along the same transect (T-test, P > 0.05) between 1998 and 1999.

Year 1998
19−150.716.7−89.2−230.857.7a8.028.9102.0−93.0ac9.2−57.4−138.07.8a1.43.314.4−85.213.7−54.3−116.1
29−213.311.5−163.7−260.6103.9b8.271.6150.1−109.5abc6.9−73.7−136.011.7a*1.34.819.3−97.813.1−68.1−127.4
33−254.2*15.9−237.9−286.079.0ab6.366.486.0−175.2b*14.4−151.9−201.512.9a2.18.815.8−162.315.7−94.8−229.8
43−171.983.2−16.6−301.599.0ab30.141.9144.2−72.9c53.225.3−157.30.1b0.00.00.2−72.868.2220.3−366.0
 
Mean −189.813.8−16.6−301.582.86.828.9150.1−106.99.225.3−201.58.91.10.019.3−104.5   
 
F = 2.953   4.622   4.646   8.88       
P = 0.057   0.013   0.013   0.001       
 
Year 1999
19−115.5a10.4−70.8−161.939.1a5.316.661.3−76.4ab7.4−41.4−104.17.9a1.42.616.5−68.59.5−47.0−90.1
29−186.5b9.1−140.3−219.287.0bc5.655.7109.6−99.5a4.9−73.8−120.216.7b*1.311.121.6−82.88.8−63.0−102.6
33−182.3ab*21.7−144.3−219.582.1abc7.071.095.0−100.2ab*14.9−73.3−124.522.7c5.115.232.4−77.511.3−28.8−126.1
43−160.6ab78.5−10.9−276.4120.5bc45.838.9197.3−40.1b34.228.0−79.10.2ad0.10.10.4−39.973.3275.4−355.1
 
Mean −156.111.9−10.9−276.472.68.116.6197.3−83.56.528.0−124.512.11.70.132.4−67.2   
 
F = 3.15   7.43   4.41   20.16       
P = 0.048   0.002   0.016   0.000       

3.5. CO2 Exchange

[36] The Rtot correlated with the average water table during the summer (Figure 5a), increasing with the decreasing water table (r = −0.58, n = 42, p = 0.000), and this was true also for PG in 1999 (Figure 5b) (r = 0.62, n = 21, p = 0.003). In 1998, the PG correlated poorly with WT (r = 0.10, n = 21, p = 0.704).

image

Figure 5. Seasonal (a) Rtot and (b) PG with average water table. (c) Rtot with PG. Closed symbols represent year 1998, open symbols represent year 1999. T1, circle; T2, triangle; T3, square; T4, diamond.

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[37] Photosynthesis and respiration were closely linked. There was a correlation (r = −0.80 in 1998 and r = −0.86 in 1999) between the modeled PG and Rtot (Figure 5c). Rtot was 40–58% of the PG.

[38] The PG and Rtot were lower in 1999 than in 1998 (Table 5), but the fluxes in 1998 and 1999 correlated with each other (Figures 6a and 6b). Following this, the NEE:s also correlated with each other (r = 0.88, n = 24, p = 0.000).

image

Figure 6. Seasonal (a) PG and (b) Rtot and (c) CH4 fluxes from years 1998 and 1999. T1, circle; T2, triangle; T3, square; T4, diamond.

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3.6. CH4 Fluxes

[39] Especially in 1999, the first CH4 fluxes measured in early summer were higher than subsequent fluxes (Figures 3c, 3d, 3g, and 3h), likely due to the release of CH4 from melting peat. After this initial burst, CH4 fluxes increased gradually until Julian day 190, after which occasional high emissions of CH4 occurred. The CH4 fluxes were high until the late autumn. Methane fluxes at the palsa margin differed from this pattern, as high CH4 bursts occurred regularly up to end of July (Figure 3). In the wet transects, there was a correlation between seasonal PG and CH4 fluxes (Figure 7a). The hourly average for winter CH4 fluxes was 0.17, 0.27, 0.20 and 0.27 mg CH4-C h−1 m−2 at transects 1, 2, 3 and 4, respectively. The CH4 fluxes at the palsa (T4) were extremely low during the summer (Figures 3o and 3p). However, during late winter the palsa released 0.27 mg CH4-C h−1 m−2 in April 1999 and 0.27 mg CH4-C h−1 m−2 in May 2000.

image

Figure 7. Seasonal CH4 fluxes with (a) PG, (b) Rtot and (c) NEE. Closed symbols represent year 1998, open symbols represent year 1999. T1, circle; T2, triangle; T3, square; T4, diamond.

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[40] During the summer, the cumulative fluxes of CH4 from individual collars on wet surfaces ranged from 2.6 to 32.4 g CH4-C m−2, whereas fluxes from the palsa were low, ranging from 0 to 0.4 g CH4-C m−2 (Table 5). The differences in fluxes between transects were statistically significant (p < 0.05) only between transect 4 and the wet transects 1, 2 and 3 in 1998. In 1999, fluxes at transect 1 and 2 also differed significantly, along with transect 4 and wet transects 1, 2 and 3. The CH4 fluxes in 1998 and 1999 clearly correlated (Figure 6c), and the average CH4 fluxes were higher in 1999 than in 1998 (Table 5).

3.7. Interactions Between Respiration, Photosynthesis and Methane Release

[41] During the summer, the CH4 fluxes correlated both with photosynthesis (Figure 7a) and NEE (Figure 7c). At the same PG level, the CH4 fluxes during the wet summer of 1999 were higher than those in the dryer summer of 1998 (Figure 7a). On the wet surfaces, the CH4 emission correlated with respiration (Figure 7b). This correlation is explained partly by the close link between PG and Rtot (Figure 5c), but also indicates that there was a coupling between overall carbon mineralization and methanogenesis. Qmax (asymptotic maximum value of photosynthesis) reflects the amount of green biomass and therefore increased with the number of vascular plants (Figure 8a), total respiration (Figure 8b) and CH4 release (Figure 8c). Qmax correlated more clearly with the CH4 emissions than the number of vascular plant stems (Figure 8d). The lowest and highest Qmax values were found on the palsa, from −23 mg CO2-C m−2 h−1 on sparsely vegetated surface to −610 mg CO2-C m−2 h−1 on the densely vegetated Empetrum nigrum surface. However, on the dry palsa, Qmax was not associated with the CH4 emissions (Figure 8d).

image

Figure 8. Scatter of Qmax and number of vascular plants per square meter for (a) all transects, (b) Rtot (wet transects), (c) CH4 flux (wet transects) and (d) CH4 fluxes and number of vascular plants per square meter (wet transects). Data are from years 1998 (solid symbols) and 1999 (open symbols). The slope for regression line explaining seasonal Rtot at wet transects is −0.27 × Qmax + 5.27 (r2 = 0.63, p = 0.000, n = 21) in 1998 and −0.28 × Qmax − 7.23 (r2 = 0.65, p = 0.000, n = 21) in 1999, and, respectively, for summer time CH4 by Qmax on wet transects was −0.031 × Qmax + 1.106 (r2 = 0.49, p = 0.000, n = 21) for year 1998 and −0.055 × Qmax − 1.96 (r2 = 0.31, p = 0.001, n = 21) for year 1999.

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3.8. Gaseous Carbon Balance

[42] The gaseous carbon balance with error estimates for summer show that all wet surfaces accumulated carbon (Table 5). On an annual basis, all surfaces with vegetation accumulated carbon during 1998 and 1999 (Tables 5 and 6). The NEE during the most active season was lower in 1999 than in 1998, mostly attributable to the smaller photosynthesis in 1999 (Table 5). However, on the sparsely vegetated palsa, there was low carbon accumulation, substantial respiration of CO2 during winter (15.1 g C m−2) (Table 6) and a large release of CO2 during summer (Table 5 and 6). This probably led to a net loss of carbon from some parts of the palsa. High variation on individual flux measurements is reflected to the error estimate for gaseous carbon budget having wide 95% confidence limits (Table 5).

Table 6. Components of the Annual Gaseous Carbon Budget (g C m−2) Calculated for Various Periods Along the Transectsa
(g C m−2)Period19981999
T1T2T3T4T1T2T3T4
  • a

    Measured biomass carbon production of Sphagnum sp. and vascular plants during 1998 and 1999, mean and S.E.

  • b

    For Julian days 183 to 280, PG and Rtot were modeled for individual collars separately and then the averages for the transects were calculated (Table 5). Photosynthesis and respiration for days 152–182 are calculated from models based on combined data from 1998 and 1999 using original weather station data from 1998 to calculate PG and Rtot for 1998. For 1999, the same model was used, but hourly air temperatures were from a Hobo logger and the PAR-radiation was modified from 1998 data by assuming a similar ratio between June 1998 and 1999 radiation as that measured for global radiation (Table 2) at Kevo station. Because NEE and Rtot measurements were started during the second half of June, the increase in the photosynthetic activity of the mire vegetation from winter dormancy to July activity was approximated by linear models, where PG increased with Julian day during days 152–182. The start of photosynthesis was set to the Julian day when PG was zero and photosynthesis reached full activity on Julian day 183. The increase of respiration activity was calculated similarly, but respiration before the Julian day when respiration was zero according to the model was assumed to be similar to that during winter. The palsa is assumed to be frozen and thus having no respiration during days when maximum air temperature is below zero, annually during 146 days [Finnish Meteorological Institute, 1991].

  • c

    Methane data for summer are the average from those summed in Table 5, and for autumn from measurements made during days 259–270 in both years, and for both winters are calculated from the measurements made in April 1999, similar to those for respiration.

PGb152–182−10.9−17.2−0.6−1.9−26.5−25.2−10.9−7.7
183–280−150.7 (16.7)−213.3 (11.5)−254.2 (15.9)−171.9 (83.2)−115.5 (10.4)−186.5 (9.1)−182.3 (21.7)−160.6 (78.5)
PGannual −161.6−230.5−254.8−173.8−142.0−211.7−193.2−168.4
 
Rtot152–1825.99.94.26.515.620.631.313.8
183–28057.7 (8.0)103.1 (8.2)79.0 (6.3)99.0 (30.3)39.1 (11.7)87.0 (5.6)82.1 (7.0)120.5 (45.8)
281–15116.7 (0.18)23.4 (0.16)18.2 (0.12)15.1 (9.2)16.7 (0.18)23.4 (0.2)18.2 (0.12)15.1 (9.2)
Rtotannual 80.3136.4101.3120.671.4131.0131.6149.4
 
NEE annual −81.3−94.1−153.5−53.2−70.6−80.7−61.6−18.9
 
CH4c168–2587.81 (1.37)11.7 (1.33)12.88 (2.10)0.08 (0.05)7.89 (1.40)16.7 (1.32)22.7 (5.10)0.19 (0.10)
259–2700.46 (0.10)1.33 (0.22)0.77 (0.27)0.11 (0.04)0.46 (0.10)1.33 (0.22)0.77 (0.27)0.11 (0.04)
271–1671.08 (0.07)1.73 (0.07)1.26 (0.02)0.77 (0.41)1.08 (0.07)1.73 (0.07)1.26 (0.02)0.77 (0.41)
 
CH4annual 9.314.814.91.09.419.824.71.1
 
Annual gaseous carbon balance −72.0−79.3−138.6−52.2−61.2−60.9−36.9−17.8
 
Annual biomass growth as carbon
Sphagnum biomass 107.7 (19.2)54.5 (15.8)82.8 (6.7)64.7 (6.6)120.2 (30.1)82.9 (19.3)
Biomass of vascular plants 10.6 (3.0)20.2 (1.7)32.0 (−)94.0 (−)14.1 (2.7)30.5 (5.4)20.1 (5.1)85.3 (7.5)
Total aboveground biomass (g C m−2) 118.374.7114.894.078.8150.7103.085.3

3.9. Aboveground Biomass Carbon

[43] At the wet transects (Table 6), Sphagnum sp. accounted for most of the carbon sequestering to aboveground biomass (54.5 g C m−2 to 120.2 g C m−2), whereas carbon bound to aboveground biomass of vascular plants ranged from 10.6 to 32.0 g C m−2. The growth of Sphagnum sp. decreased with the increasing stem count of vascular plants (r2 = 0.32, slope = −0.14, n = 21, p = 0.015). On the palsa (Table 6), the annual aboveground biomass growth was 85.3 to 94.0 g C m−2. There was no statistical relation between the aboveground biomass carbon sequestering and measured carbon balance.

4. Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

4.1. CO2 Dynamics

[44] The wet surfaces were sinks for CO2 and also palsa surfaces with shrub vegetation showed carbon uptake. However, the palsa surfaces with sparse vegetation released carbon (Table 5 and 6). Our CO2 uptake rates on the wet surfaces are comparable to those previously reported for the CO2 uptake during the growing season in northern peatland ecosystems. A high arctic fen in Greenland had an NEE of 96.3 g C m−2 and Rtot of 61.2 g C m−2 [Søegaard and Nordstroem, 1999]. Peatlands in boreal Canada have shown an NEE from 13 ± 24 g C m−2 in a bog to 65 ± 47 g C m−2 in a poor fen with palsas [Bubier et al., 1999]. However, Oechel et al. [1993] and Oechel and Vourlitis [1997] have reported carbon losses of 34 g C m−2 from wet sedge tundra, and a loss of 156 g C m−2 from tussock tundra in Alaska. Boreal peatlands can lose carbon during warm and dry summers when there is a decrease in the water table, causing an increase in respiration and decrease in photosynthesis [Shurpali et al., 1995; Alm et al., 1999a; Waddington and Roulet, 2000]. A decrease of 15 cm in the water table in taiga bog microcosm experiments increased the soil respiration rate by almost three fold [Funk et al., 1994], showing that these soils have a great potential for CO2 loss. In our peatland, there was some decrease in the water table in early summer, when there was still a frost layer preventing the sinking of surface peat with the decreasing water table. Surface adjustment of floating peat [Roulet, 1991; Roulet et al., 1992] is an important feature of subarctic peatlands; lowering of the peat surface ensures that vegetation does not suffer from drying even during a dry summer, allowing good CO2 fixation capacity. However, peat surface lowering can support high CH4 fluxes from waterlogged peat at a high temperature [Bubier et al., 1995]. Thus, peatlands not capable of surface adjustment (peat on permafrost or thin peat layer on mineral soil) are more vulnerable to carbon loss and reduction in CH4 emissions during dry and warm summers. In the present study, the decrease in NEE in 1999 was mainly a result of a decrease in the photosynthesis during the wet and dismal summer (Table 1), not the increase in respiration which was quite similar in 1999 as in 1998. In this study respiration in the summer ranged from 45 to 58% of photosynthesis. This was higher than the one third reported by Bubier et al. [1999] for boreal peatlands in Canada. In contrast to findings at our site, warm and dry summers have caused a decrease in the water table in boreal peatlands leading to carbon loss [Alm et al., 1999a]. A mesotrophic flark fen with thin peat at Kaamanen, 75 km south of our site, has shown carbon uptake during summer, but substantial CO2 emissions during winter. According to the eddy correlation measurements, the site is losing carbon when winter is included into the annual CO2 balance [Aurela et al., 2001a].

[45] Similar production of Sphagnum was measured at the study site and is comparable to that of Moore [1989], who reported biomass production rates for hummocks to be 80 g m−2 yr−1 and for lawns 140 g m−2 yr−1 at a subarctic patterned fen. There, biomass production of vascular plants (above ground) was also quite similar: 7 g m−2 in pools, 41 g m−2 in flarks and 93 g m−2 in strings. Bubier et al. [1998] found biomass production ranging from 48 g to 164 g in a boreal peatland. Vascular plants are known to be important components of carbon flow, even though the aboveground biomass is smaller than that of Sphagnum, as up to 78% of the biomass of vascular plants is located belowground [Saarinen, 1996]. Even tough plants are responsible for almost all carbon sequestering, the carbon bound to biomass at these transects was not connected to measured gaseous carbon exchange values.

4.2. CH4 Dynamics

[46] In the carbon balance of peatlands, the CO2 fluxes are quantitatively the most important, but in the radiative forcing, the CH4 fluxes have to be considered because GWP (100-year time horizon) of CH4 is 28 times that of CO2 [Jain et al., 2000]. The annual CH4 emissions at Vaisjeäggi mire (1.0 to 24.7 g CH4-C m−2) were in the range reported for arctic peatlands (0.13–60.6 g CH4-C m−2) by Bartlett and Harriss [1993]. The fluxes from Siberian and European tundra [Christensen et al., 1995] have been lower, whereas those from tundra sites in northeastern Siberia were quite similar to ours [Nakano et al., 2000]. Friborg et al. [2000] reported a CH4-C release of 3.7 ± 0.57 g m−2 between June and September for a high-arctic valley in Greenland. Smaller CH4 emissions from a fen at Kaamanen ranged from, 68 mg CH4-C m−2 d−1 from lawns, 78 CH4-C m−2 d−1 from flarks and 6.0 mg CH4-C m−2 d−1 from strings for period from June to September [Heikkinen et al., 2002a].

[47] Our CH4 fluxes from the subarctic mire surfaces (including dry palsa) are comparable to those recorded during the summer from boreal peatlands in Finland (summer average 6 g CH4-C m−2 for ombrotrophic bogs, 14.3 g CH4-C m−2 for minerotrophic fens and 0.23 g CH4-C m−2 for drained minerotrophic fens) [Nykänen et al., 1998]. The annual mean temperature is 4°–5°C higher and the annual precipitation is on average 280 mm higher, global radiation is higher and the snow-free period is 25 days longer in the boreal zone of Finland than in the subarctic region studied here [Finnish Meteorological Institute, 1991].

[48] The amount of CH4 released during autumn and winter was a substantial proportion of the annual CH4 fluxes on wet transects, and on the palsa with low CH4 fluxes; however, the winter fluxes were even higher than those during summer. The seasonal dynamics of CH4 here differed from that reported for boreal peatlands. On the transects, except for that site close to the palsa, there was a burst of CH4 emissions during thawing similarly to that described by Friborg et al. [1997] for an arctic peatland. The transect near the palsa had seasonal CH4 dynamics similar to boreal peatlands where there is no frost layer during growing season. Melting of frost in July liberates CH4 stored in peat to the atmosphere, a phenomenon occurring earlier at the palsa margin (T3) than at the other transects. After the CH4 burst, there was a steady increase in the CH4 emissions probably as a result of an increase in temperature and new labile carbon from plants both supporting the activity of methanogens. The development of vascular plants also increases the aerenchymal transport of CH4 to the atmosphere [Schimel, 1995]. The temperature below the frost layer (2° to 4°C at a depth of 1 m in August 1999) is sufficient to allow decomposition of organic material to also take place during winter. Also, there may be a liberation of suitable substrates when the frozen organic matter melts [Panikov and Dedysh, 2000]. Bubier et al. [1995] found a similar pattern in CH4 fluxes, continuing at a high level to late autumn on sites with a floating peat surface.

[49] On an annual basis, the amount of CH4-C released was 9.8–15.4% of the NEE in 1998 and 18.1–40.3% in 1999; the highest value found was at the palsa margin. These values, except that at palsa margin in 1999, are close to those reported earlier for boreal peatlands [Alm et al., 1997]. In the wet year, 1999, more of the carbon fixed in photosynthesis was allocated to the CH4 production (see Figure 7a). In addition, the high CH4 release in 1999 at transect 2 could result from the early escape of frost, allowing longer release of CH4 from the whole peat column and also leading to a higher water table due to an adjustment of the peat surface. At the palsa margin, organic matter from degrading peat released from palsa in 1999 could also contribute to the high CH4 emission leading to a high CH4-C/NEE ratio. Small changes in the water table (as here) can cause major changes in CH4 fluxes at sites with high average water tables [Bubier et al., 1995; Nykänen et al., 1998].

4.3. Qmax, Respiration and CH4 Fluxes

[50] It is reasonable that Qmax correlated with respiration and CH4 fluxes. Qmax is a measure of active plant biomass, and is probably connected with the fertility of the site. Our mean Qmax values (−288 mg m−2 h−1 in 1998 and −271 mg m−2 h−1 in 1999) were quite similar to those obtained by the eddy correlation method at a fen close to our site (−294 mg m−2 h−1 in 1999) [Laurila et al., 2001] and those at Kaamanen (−304 mg m−2 h−1) [Aurela et al., 2001a]. There seems to be a close relationship between the leaf area indices, and Qmax [Aurela et al., 2001b], and in general, Qmax is greater on fens than on bogs [Frolking et al., 1998].

4.4. CO2 and CH4 Release During Winter

[51] Alm et al. [1999b] reported an average winter release of 41 g C m−2 (30–76 g C m−2) for boreal bogs and fens in Finland. Our estimate for the respiration on wet surfaces (16.7–23.4 g C m−2) during winter is lower than that in boreal peatlands, but close to the winter value obtained with an eddy correlation method for a subarctic mire site 75 km south of our site [Aurela et al., 2001a]. The proportion of winter respiration (14–20%) to the total annual respiration is quite similar to that reported for other boreal peatlands [Alm et al., 1999b].

[52] According to Melloh and Crill [1996], the CH4 fluxes during winter accounted for 2.0–9.2% of the total annual fluxes on a temperate mire. Alm et al. [1999b] reported that winter CH4 fluxes could range from 0.3 to 1.7 g C m−2 on bogs and 2.0–6.0 g C m−2 from fens, whereas fluxes from drained peat surfaces were near zero. Our winter CH4 flux of the annual emissions (5–12%) from wet mire surfaces were similar to those measured from minerotrophic fens in a boreal region [Alm et al., 1999b], and from ombrotrophic bogs in West Siberia [Panikov and Dedysh, 2000].

4.5. Palsas Affect Carbon Cycling on Peatlands

[53] Palsas have great importance to the biogeochemistry of peatlands. They have lower CH4 emissions than the wet peatland surfaces. The carbon balance on palsas is variable, and only a surface with shrub vegetation can fix carbon. Palsas also considerably affect their surroundings. The palsa margin maintained high CH4 emissions despite the fact that the palsa nearby is the only part of the mire having permafrost, a phenomenon found also by Bubier et al. [1995]. Some palsas have steep sides, causing an accumulation of drifting snow on the sides, which inhibits freezing of the adjacent ground and the establishment of permafrost [Seppälä, 1994]. During 1992, the temperature recordings at this same mire showed that the minimum temperature at the margin of palsa was −8.9°C before settling of permanent snowfall in December. Later in the winter, with the presence of snow, the temperature remained at zero at the margin, even though air temperature declined to −34.9°C [Seppälä, 1994]. It is important that the palsas are sources for nutrients (phosphorus, Table 2) and also carbon to the surrounding mire. Nutrients would affect the CH4 and CO2 fluxes partly via the development of the palsa margin vegetation, which would affect carbon sequestering and the CH4 emissions. In fact, at transect 2, there was a small palsa without degrading peat, and near it the gas fluxes were lower than at the margin of the large degrading palsa (results not shown).

[54] There were only low fluxes at the palsa surface during summer, as also found by Bubier et al. [1995]. The palsa is a dry habitat, lacking waterlogged conditions and promoting high CH4 fluxes, while still having CH4 oxidation ability, as confirmed by laboratory experiments (results not shown). The high bulk density of palsa peat (Table 2) probably prevents gas diffusion into the peat, thus allowing only a low uptake of CH4. The mesic structure of palsa peat leads also to CH4 release during wet periods in summer and late winter when water remains in the peat. Furthermore, methane can also be released from deeper CH4 stores in the palsa. However, the reason for variability of CH4 fluxes from autumn to spring remains unknown. The measured low CH4 flux from the collars and from the snow profile on the palsa would represent a release of accumulated CH4 in palsa peat, not the actual production, since the air temperature at the palsa remained above zero only during daytime in April (when these fluxes were measured), but it was below zero during the night. During midwinter, the upper parts of palsa tops are totally frozen, due to the thin snow cover. Our CH4 fluxes in winter from the palsa were substantial, but lower than those measured from the wet transects.

5. Conclusions

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

[55] The wet surfaces of the palsa mire were sinks for carbon during two climatically different years, due to similar hydrology maintained by the floating of the peat surface, which prevented drying of the surface peat, when the water table sunk during the summer. Palsas containing permafrost are important in regulating the fluxes of CH4 and CO2 of palsa mire due to the dry surfaces with low CH4 fluxes and carbon accumulation capacity, but also due to the release of organic matter and nutrients to the surrounding mire. In addition, snow accumulation on the sides of the palsa leads to the formation of the warmest soil of the whole mire and consequently also to the highest CH4 fluxes. On wet surfaces subject to frost, the summer CH4 fluxes first originate from newly bound carbon above the frost layer. After the disappearance of frost, CH4 is also released from the CH4 stores in the peat profile, thus keeping the flux rates high until late summer, even though the peat temperature decreases. The CH4 emissions from the wet surfaces were close to those reported for boreal fens.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

[56] This work was funded by European Union contract (ENV4-CT97-0583) as part of CONGAS project and partly by TUNDRA (ENV4-CT97-0522), and Arctica project funded by Academy of Finland. The authors wish to express their gratitude to the staff of Kevo Subarctic Research Institute at Utsjoki for logistic support during the field work. We also thank the LAPP project for the additional weather data. Carrie Turunen revised the English text.

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  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information
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Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Methods
  5. 3. Results
  6. 4. Discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References
  10. Supporting Information

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