Cold-season nitrous oxide dynamics in a drained boreal peatland ...

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Cold-season nitrous oxide dynamics in a drained boreal peatland differ depending on land-use practice Marja Maljanen, Jyrki Hyto¨nen, and Pertti J. Martikainen

Abstract: Drained peat soils are important sources of greenhouse gases such as nitrous oxide (N2O), methane (CH4), and carbon dioxide (CO2). These gases are produced in soil and they can be emitted year-round. We measured N2O and CH4 flux rates and total respiration (RTOT) over a year from a drained peatland with one subsite as a grass field and another forested. The field acted annually as a small source (0.36 ± 0.73 kg Cha–1) and the forest as a sink (–1.93 ± 0.50 kg Cha–1) for CH4. Mean annual RTOT rates were 660 and 297 mgm–2h–1 in the field and in the forest, respectively. Annual N2O emission rates were 34.8 ± 2.4 kg Nha–1 from the field and 25.5 ± 5.5 kg Nha–1 from the forest. More than 80% of the annual N2O emissions took place during winter. In the field, high emissions were detected during thawing in April when N2O accumulated in soil during the winter was released. In the forest, N2O emissions peaked when the top soil was freezing in January and accumulation of N2O in soil was less than in the field. The timing of the episodic high N2O emissions thus differed depending on the land use. Re´sume´ : Les sols tourbeux draine´s sont une importante source de gaz a` effet de serre tels que l’oxyde nitreux (N2O), le me´thane (CH4) et le dioxyde de carbone (CO2). Ces gaz sont produits dans le sol et peuvent eˆtre e´mis pendant toute l’anneé. Nous avons mesure´ les taux d’e´mision de N2O et de CH4 ainsi que la respiration totale (RTOT) pendant un an dans une tourbie`re draineé occupeé en partie par un terrain herbeux et en partie par une foreˆt. Sur une base annuelle, le terrain herbeux se comportait comme une faible source (0,36 ± 0,73 kg Cha–1) et la foreˆt comme un puits (–1,93 ± 0,50 kg Cha–1) de CH4. Le taux annuel moyen de RTOT e´tait respectivement de 660 et 297 mgm–2h–1 dans le terrain herbeux et la foreˆt. Le taux annuel d’e´mission de N2O e´tait de 34,8 ± 2,4 kg Nha–1 dans le terrain herbeux et de 25,5 ± 5,5 kg Nha–1 dans la foreˆt. Plus de 80 % des e´missions annuelles de N2O sont survenues pendant l’hiver. Dans le terrain herbeux, de fortes e´missions ont e´te´ de´tecteés durant le de´gel en avril lorsque le N2O accumule´ dans le sol durant l’hiver e´tait relaˆche´. Dans la foreˆt, les e´missions de N2O ont culmine´ lorsque la couche superficielle du sol e´tait geleé en janvier et l’accumulation de N2O dans le sol e´tait plus faible que dans le terrain herbeux. Le moment ou` surviennent les fortes e´missions e´pisodiques de N2O e´tait donc diffe´rent selon l’occupation des sols. [Traduit par la Re´daction]

Introduction Of Finland’s original peatland area of 10 million ha, 5.5 million ha have been drained for forestry, and at present, approximately 0.3 million ha of drained peatlands are used for agriculture (Myllys and Sinkkonen 2004; Turunen 2008). Peat soils used for agriculture differ from peatland forests, especially with respect to their vegetation cover and soil chemical and physical properties. Agricultural practices, for example cultivation, fertilization, liming, ploughing, and other soil-improvement measures, change the chemical and physical properties of the top layer (0–20 cm) of the peat (Hyto¨nen and Wall 1997). These soils have therefore a higher bulk density than forested peat soils (Paavilainen and Païva¨nen 1995). Nitrous oxide (N2O) and methane (CH4) are greenhouse gases with 298 and 25 times higher global warming poten-

tial, respectively, than carbon dioxide (CO2) with a time horizon of 100 years (Solomon et al. 2007). Concentration of CH4 has increased by a factor of 2.6 since the industrial era (World Meteorological Organization 2007). Drained peat soils act as small sinks or sources for CH4 depending on soil aeration status (e.g., Ullah et al. 2009). N2O in the atmosphere is also increasing annually at a rate of 0.3% (World Meteorological Organization 2007), and agriculture is globally its most important anthropogenic source. In Finland, drained peat soils used for agriculture are responsible for 25% of the anthropogenic N2O emissions (KasimirKlemedtsson et al. 1997). N2O emissions from peat soils drained for forestry are assumed to be lower than those from agricultural soils (Martikainen et al. 1993; Maljanen et al. 2003a; Minkkinen et al. 2007) and some forested sites have even shown N2O uptake (von Arnold et al. 2005a). However, the wintertime N2O emissions from boreal for-

Received 25 August 2009. Accepted 14 January 2010. Published on the NRC Research Press Web site at cjfr.nrc.ca on 11 March 2010. M. Maljanen1 and P.J. Martikainen. Department of Environmental Science, University of Kuopio, University of Eastern Finland, Kuopio Campus, P.O. Box 1627, Kuopio FI-70211, Finland. J. Hyto¨nen. Finnish Forest Research Institute, Kannus Research Unit, P.O. Box 44, Kannus FI-69101, Finland. 1Corresponding

author (e-mail: [email protected]).

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doi:10.1139/X10-004

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ested peatlands are not well documented, even though these soils cover larger areas than agricultural soils. Recent studies have shown that emissions of N2O in the boreal soils may account for more than half of the annual N2O emissions to the atmosphere (e.g., Regina et al. 2004; Maljanen et al. 2009). These periods of high emissions at low temperatures have been linked to soil freezing and thawing events (e.g., Christensen and Tiedje 1990; Ro¨ver et al. 1998; Koponen et al. 2004; Matzner and Borken 2008). However, there can be large differences in the N2O fluxes between years depending on, for example, the timing of soil freezing (Maljanen et al. 2009). Teepe et al. (2001) suggested that N2O production in frozen soil occurs in unfrozen water films covering soil particles. This unfrozen water, where soluble microbial subtrates are concentrated as a result of ice formation in the soil, is surrounded by ice that limits the oxygen supply to the water film, thus supporting denitrification there. There is a rapid change in the temperature response of the N2O production rate close to 0 8C, and N2O production rates just below 0 8C can be higher than those above 0 8C (Holtan-Hartwig et al. 2002; Koponen et ¨ quist et al. 2004; Maljanen et al. 2009). The al. 2004; O overall reasons for this shift are poorly known. Sharma et al. (2006) suggested that the freezing–thawing stress could change the microbial community but this has not been noticed in other studies (Koponen et al. 2006b). We hypothesized that N2O dynamics in drained boreal peatland soils would depend on the land use as a result of different soil characteristics. Therefore, we measured N2O fluxes and soil physical and chemical properties over a 1year time period from a drained peatland having subsites for agriculture and forestry. The N2O dynamics were compared with those of CH4 and total soil respiration (Table 1).

Can. J. For. Res. Vol. 40, 2010 Table 1. Soil characteristics (sampled from depths of 0–10 cm) with annual emissions (±SE) (n = 6) of N2O and CH4 and modelled mean total respiration, RTOT.

Organic matter (%) N (%) C (%) C:N pH (H2O) Bulk density (gcm–3) NO–3-N (mgg–1) NH+4 -N (mgg–1) Annual N2O (kg N2O-Nha–1) Annual CH4 (kg CH4-Cha–1) Modelled RTOT (g CO2m–2h–1)

Field Forest 89 91 3.0 2.5 55 53 18.2 21.4 5.0 3.8 0.22 0.15 75.6±18.7 3.6±0.4 30.4±8.5 23.2±5.4 34.8±2.4 25.5±5.5 0.36±0.73 –1.93±0.50 660 297

Fig. 1. Schematic of the study site. Field and forest sites were separated by a main ditch (width approximately 2 m and depth 1.5 m). Squares indicate locations of the collars for gas flux measurements, ‘‘M’’ shows the locations of soil moisture and temperature probes, and ‘‘C’’ shows the locations of gas collectors.

Materials and methods Study site The peatland studied is located in Kannus in northwestern Finland (63854’N, 23856’E) close to the coastline of Bothnian Bay. The peat depth was more than 1 m. One subsite of this peatland was drained for agriculture in the 1930s. It has been used for hay production and was not grazed or fertilized during the study period. The vegetation in the hay field was a mixture of timothy (Phleum pratense L.) and meadow fescue (Festuca pratensis Huds.) sown in 2004. Hay is the most common crop on agricultural peat soils in Finland, and similar peat soils as here, with high organic matter content, cover 85 000 ha of the land area in Finland (Myllys and Sinkkonen 2004). Measurements on the field were made on a study plot of 15 m 10 m separated from the larger grass sward. The other subsite, drained and forested peatland, is located opposite the field plot behind a main ditch separating the field and the forest (Fig. 1). The forested subsite was drained in the 1960s to improve forest growth. The dominant tree species were Scots pine (Pinus sylvestris L.) (85%) and downy birch (Betula pubescens Ehrh.) (15%). The age of the trees varied from 50 to 55 years, the mean height was 19 m, and the mean stemwood volume was 230 m3ha–1. The forested peatland site is classified according to Laine and Vasander (2005) as Vaccinium vitis-idaea L. type II (PtkgII), which is the most com-

mon drained peatland forest type in Finland covering more than 600 000 ha (J. Laine, personal communication). The long-term average annual temperature (1971–2001) in Kannus is 2.8 8C and the average annual precipitation is 561 mm (Drebs et al. 2002). Of the total precipitation, approximately 50% falls as snow. Snow cover typically appears in mid-November and melts in late April. The coldest month is February (long-term average annual temperature – 9.2 8C) and the warmest is July (long-term average annual temperature 15.8 8C) (Drebs et al. 2002). The long-term average maximum snow depth is 44 cm (in the middle of March). The water table depth in the field was on average 0.45 m in the autumn 2007, 0.4 m from April to June 2008, and deeper than 0.7 m from July to October 2008. In the forest, the average water table depth was 0.5 m in autumn 2007, 0.45 m from April to July 2008, and deeper than 0.7 m from August to October 2008. Water table depth was not recorded during the winter months when the top soil was frozen. Environmental variables Soil frost was measured using frost-depth gauges filled with methylene blue (Gandahl 1957) and soil temperature Published by NRC Research Press

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was recorded with iButton temperature loggers (Dallas Semiconductor Corp., USA) with a resolution of 0.5 8C. Air temperature was recorded at a weather station about 20 km from the study site. Soil moisture was recorded using the time-domain reflectometry method with soil moisture probes (CS625) and dataloggers (CR200) (Campbell Scientific, UK). Soil samples for analysis of NO–3 and NH+4 were collected at a depth of 0–10 cm (five replicate samples) in October 2007 and October 2008. NO–3 was extracted with distilled water and NH+4 with 1 molL–1 KCl solution. The amount of NO–3 was analyzed by an ion chromatograph (DX 120; Dionex Corporation, USA) and NH+4 with a spectrophotometer (Ultrospec 3000 Pro; Biochrom, UK) using the method of Fawcett and Scott (1960). Gas flux measurements N2O and CH4 flux rates and soil respiration from the snow-free soil were measured between September 2007 and October 2008 with a static chamber method using aluminum chambers equipped with a fan (60 cm 60 cm, height 30 cm) and aluminum collars (60 cm 60 cm, height 30 cm) preinstalled in the soil (14 sampling occasions, six replicates; see Figs. 1 and 2). After closing the chamber, gas samples of 30 mL were taken with a 50 mL polypropylene syringe (Terumo) at 5, 10, 20, and 25 min intervals from the headspace of the chamber. Samples were injected within 24 h into preevacuated 12 mL vials (Labco Excetainer) and were analyzed for N2O, CH4, and CO2 with a gas chromatograph (Agilent 6890N; Agilent Technologies, USA) equipped with an autosampler (Gilson, USA) and electron capture and flame ionization detectors. Compressed air containing 0.389, 3.0, or 50.1 mL N2OL–1, 386 mL CO2L–1, and 1.98 mL CH4L–1 was used for daily calibration. The flux rates were calculated from the linear increase or decrease in the gas concentrations in the headspace of the chamber. If there were any indications of failures in the gas sampling or gas analysis, the results were excluded. Gas fluxes from the snow-covered plots, when the snow depth exceeded 20 cm (three sampling occasions, six replicates; see Fig. 2), were determined by measuring gas concentration gradients from the snow 2 cm above the soil surface and from the ambient air and by calculating associated diffusion rates in the snow from the snowpack density (Sommerfeld et al. 1993; Maljanen et al. 2003b). Gas samples (40 mL) were drawn with a stainless steel probe (diameter 3 mm, length 100 cm) from the snow inside the collars used for chamber sampling. Simultaneously, snow samples were collected with a PVC tube (diameter 10.2 cm) for porosity measurements. The intact samples were weighed for calculation of the average porosity of snow using the density of pure ice (0.9168 gcm–3). Total respiration (RTOT) was estimated using a linear regression model with the instantaneous CO2 flux data and soil temperature at a depth of 5 cm as an independent variable, which was here the best fit for RTOT. The equation was in the form ½1

RTOT ðmg CO2 m2 h1 Þ ¼ b0 þ b1 T 5 cm

where b0 and b1 are the parameters estimated by linear re-

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gression (SPSS 14.0, SPSS Inc., Chicago, Illinois). The values of the parameters b0 and b1 were 171.08 and 30.17, respectively (R2 = 0.39), for the forest and 142.46 and 107.37, respectively (R2 = 0.88), for the field. The hourly RTOT was then calculated with continuous soil temperature data (T5 cm) for forest and field separately. Gas concentrations in soil N2O concentrations in soil were measured simultaneously with the gas flux measurements. Samples of 30 mL were taken with syringes from the preinstalled silicon tubes (diameter 1.0 cm, wall thickness 0.3 cm, length 110 cm, volume 86 cm3) inserted horizontally in sets (n = 5 in the field and n = 4 in the forest) at depths of 5 and 20 cm in the soil (Maljanen et al. 2007b) beside the collars for the gas flux measurements (see Fig. 1). Samples were treated and analyzed for N2O with a gas chromatograph as described above. Statistical analysis The differences in the flux rates between the field and forest sites were analyzed with a linear mixed model (SPSS 14.0, SPSS Inc.) where the collar number is set as a random factor. Correlation between gas flux rates and environmental factors was studied with Spearman rank correlation. Annual emissions of the gases were calculated from time-weighted average flux values.

Results Weather conditions and physical properties of soil Air temperature during the study period from October 2007 to October 2008 varied from –21.6 to 26.4 8C, being on average 3.5 8C, 0.7 8C higher than the long-term average value (Drebs et al. 2002). The sum of precipitation during the period was high, 928 mm, 370 mm more than the longterm average (Drebs et al. 2002). The mean annual soil temperature at a depth of 5 cm was higher in the field (5.0 8C) than in the forest (2.9 8C), but during the winter, it remained rather constant, around –0.5 8C. There were two cold periods when soil temperature dropped below that, the first one at the end of January 2008 and the second at the end of March 2008 (Fig. 2). During the latter, only the forest soil temperature decreased, down to –2.8 8C. However, soil frost was deeper and longer lasting in the field than in the forest. The period with snow cover lasted about 5 months at both sites. The maximum snow cover was thinner in the forest (20 cm) than in the field (36 cm) (Fig. 2), but the mean snow density was similar at both sites, 0.21 gcm–3. CH4 fluxes In the field, CH4-C flux rates varied from an uptake of 0.09 mgm–2h–1 to an emission of 0.57 mgm–2h–1 and did not correlate with soil temperature or with soil frost depth (Fig. 2). The highest emissions from the field were measured in June. In the forest, the CH4 flux rates varied from an uptake of 0.09 mgm–2h–1 to a emission of 0.09 mgm–2h–1. The highest uptake rates were measured in June (Fig. 2). In the forest, the CH4 flux rates correlated negatively with soil temperature; in other words, CH4 uptake rates increased with increasing temperature (R = –0.640, p = 0.008) and with decreasing depth of soil frost (R = –0.647, Published by NRC Research Press

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Fig. 2. (A) N2O, (B) CH4 dynamics in the field and in the forest with error bars (SE). (C) Field- and forest-modelled daily total respiration (CO2). Circles without bars (SE) show the measured instantaneous CO2 flux rates. Horizontal lines indicates the period when the gas fluxes were determined with a gradient method; for all other occasions, they were measured with chambers. (D) N2O concentration in soil air at depths of 5 and 20 cm. (E) Soil frost depths from field and forest are shown together with soil volumetric moisture in field (solid line) and in forest (broken line). (F) Snow depth from field and forest. (G) Soil temperatures at a depth of 5 cm. Air temperature and daily precipitation are shown at the bottom.


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p = 0.007). The mean daily CH4 flux rates did not correlate with CH4 concentration but they correlated negatively with soil moisture both in the forest (R = –0.665, p = 0.05) and in the field (R = –0.729, p = 0.001); in other words, CH4 uptake rate increased with increasing soil moisture. There was a significant difference in the flux rates between the sites (F = 4.27, p = 0.040). On an annual basis, the field was a small source (0.36 ± 0.73 kg Cha–1) and the forest site a small sink (–1.93 ± 0.50 kg Cha–1) for CH4.

ing depth of soil frost (R = 0.807, p < 0.001) and depth of snow cover (R = 0.775, p < 0.001) and decreased with increasing air temperature (R = –0.515, p < 0.001). The N2O emissions from the field also correlated with the soil N2O concentration (R = 0.314, p = 0.007). N2O concentration in forest soil correlated with the depth of soil frost (R = –0.695, p < 0.001), air temperature (R = –0.345, p = 0.012), depth of snow cover (R = 0.548, p = 0.007), and N2O emissions (R = 0.369, p = 0.007).

Total respiration RTOT There was a clear seasonal variation in the flux rates, the highest RTOT occurring in the summer and the lowest during the winter. However, in January 2008 after a cold period, slightly enhanced RTOT rates were measured from the forest but not from the field. Excluding this date, the wintertime RTOT rates were rather similar for both sites, but during the snow-free period, RTOT was higher in the field than in the forest (F = 25.6, p < 0.001) (Fig. 2). The mean daily RTOT increased with increasing soil moisture and soil temperature both in the field (R = 0.586, p = 0.017 and R = 0.973, p < 0.001) and in the forest (R = 0.518, p = 0.040 and R = 0.847, p < 0.001). The modelled mean annual RTOT values were 297 mg CO2m–2h–1 in the forest and 660 mg CO2m–2h–1 in the field (Fig. 2) and the annual cumulative RTOT values were 2600 and 5800 g CO2m–2 in the forest and in the field, respectively.

Discussion

N2O fluxes No net N2O uptake was measured in any of the collars. The N2O emission dynamics were different between the sites, even though there were no significant differences between the mean emission rates. N2O emissions from the forest peaked in January 2008, in contrast with the field, showing the peak in May 2008 during soil thawing (Fig. 2). In the forest, the N2O emissions peaked during the snowfree period when the soil temperature at a depth of 5 cm dropped to –1.5 8C and when depth of soil frost reached its maximum (20 cm). The maximum N2O emission rates measured from one individual collar were 8300 mgm–2h–1 in the field (May 2008) and 9900 mgm–2h–1 in the forest (January 2008). The mean daily N2O emissions did not correlate with soil moisture, soil temperature, or soil frost depth. Annual N2O emissions (±SE) from both sites were high, 34.8 ± 2.4 kg N2O-Nha–1 in the field and 25.5 ± 5.5 kg N2O-Nha–1 in the forest. A major part of the annual emission originated during winter, from 81% in the field to 84% in the forest. N2O concentration in soil In the field, N2O started to accumulate after the top soil was frozen in January 2008. The ice in the top soil limited diffusion of gases, especially in the agricultural soil (van Bochove et al. 2001). The accumulated N2O was then released mainly during soil thawing in April 2008 (Fig. 2). Soil N2O concentrations were higher in the field than in the forest (F = 19.0, R < 0.001). The maximum concentrations in the field were 960 mLL–1 at 5 cm and 1280 mLL–1 at 20 cm (Fig. 2). In the forest soil, the maximum N2O concentrations were 26.5 mLL–1 at 5 cm and 37.3 mLL–1 at 20 cm. N2O concentration in the field increased with the increas-

Soil temperature and moisture were different in the forest and in the field. Under the forest canopy, the snow cover was thinner than on the open field, resulting in lower soil temperatures during winter, as also shown by Kubin and Poikolainen (1982). However, the depth of soil frost remained shallower in the forest than in the field. This could be a result of higher bulk density and higher soil moisture in the field. The forest soil acted as a sink and the field as a small source for CH4. The annual CH4 flux rates were in the range reported previously for cultivated or forested peat soils in Finland (Maljanen et al. 2003c, 2007b; Pihlatie et al. 2009). CH4 fluxes were determined mainly by soil moisture and temperature. The highest CH4 emissions from the field occurred in the early summer after a dry period, when the soil moisture rapidly decreased. At the same time, no changes were seen in forest soil moisture, and forest soil acted as a sink for CH4. The CH4 emissions during a dry period were surprising; however, similar findings have been reported from afforested agricultural peat soil in Finland (Maljanen et al. 2001a). The reason for these CH4 emissions during a dry and warm period could be enhanced respiration that consumed O2 in the soil so that the low O2 concentration then probably favoured anaerobic CH4 production over CH4 oxidation. The RTOT rate was higher in the field than in the forest, indicating a higher decomposition rate in the ploughed and fertilized agricultural soil than in the drained forest site. The average annual and winter respiration rates from the grass field, about 600 and 250 mg CO2m–2h–1, respectively, were similar to those measured previously from a grassland on peat soil (Maljanen et al. 2001b). The annual mean respiration rate for the forest soil was slightly higher than that (from 100 to 200 mg CO2m–2h–1) reported by von Arnold et al. (2005a, 2005b) for drained boreal peatland forests. The annual N2O emission from the unfertilized grass field (34.8 kg N2O-Nha–1) was sixfold higher than the average emission of 5.7 kg N2O-Nha–1 reported for N-fertilized peat soils used for grass production in Finland (Maljanen et al. 2007a). Similar high N2O emissions have been measured in Finland only from fallow soils (without vegetation) in southern Finland (Regina et al. 2004) or from some afforested agricultural peat soils (Ma¨kiranta et al. 2007). One reason for the high N2O emissions could be the very rainy growing season in 2008. High soil moisture in the top soil may limit O2 availability and thus favour N2O production in denitrification (e.g., Liu et al. 2007). During a very rainy period in July 2008, the N2O concentration in the field soil increased about 10-fold from the June value, indicating Published by NRC Research Press

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enhanced N2O production, but N2O emissions increased only slightly. High soil moisture probably then limited N2O diffusion from the soil and N2O was consumed by denitrifiers in the top soil (Chapuis-Lardy et al. 2007; Wagner-Riddle et al. 2008) before it could escape to the atmosphere. The measured annual N2O emission from the forest was even 15 times higher than the average emission measured with similar chamber methods for minerotrophic peatlands drained for forestry in Finland (Martikainen et al. 1993; Huttunen et al. 2003; Maljanen et al. 2003a). Similar high emissions have been reported from some afforested agricultural peat soils (Ma¨kiranta et al. 2007) but not from drained forest soils without any cultivation history. Also, the emissions during the growing season were higher (average 32 mg N2O-Nm–2h–1) than those measured previously in Finland (Martikainen et al. 1993; Huttunen et al. 2003; Maljanen et al. 2003a) and in boreal peatland forests in Canada (Ullah et al. 2009). A major part of the annual emissions resulted from the emission peak after soil freezing in January. The reason for this high N2O burst is unknown. However, similar freezing-related peaks have been measured from agricultural fields in situ (Maljanen et al. 2007b, 2009) and in laboratory incubations with forest soil cores (e.g., Koponen et al. 2006a). It could be a result of physical processes and decreases in the soil porosity, as Mastepanov et al. (2008) reported for CH4 fluxes in tundra soil, or it could be of biological origin (e.g., Ro¨ver et al. 1998). However, this very high emission peak was measured only during one sampling occasion in January and calculating annual emissions using time-weighted average values could overestimate the annual emission. But even if this high emission peak is ignored, the annual emission from forest was 7 kg N2ONha–1, which is still higher than the mean emission from peatland forests in Finland without any cultivation history. This N2O emission from the forest using the GWP approach with a 100-year time horizon (Solomon et al. 2007) corresponds to 1192 g CO2m–2year–1. CH4 uptake as CO2 equivalents is –6.4 g CO2m–2year–1. We did not measure net CO2 emission, which also includes the photosynthesis and respiration of the trees. The net CO2 exchange from forested peatlands in Finland has been reported only from one minerotrophic pine forest site, giving an annual CO2 uptake of 900 g CO2m–2 (Laurila et al. 2007). If we compare the N2O emission as CO2 equivalents with C gas uptake (CH4 uptake from our site together with CO2 uptake from Laurila et al. (2007)), the N2O emission will compensate for the C gas sink (CO2 plus CH4 uptake) and turn forested peatland into a net source of greenhouse gases (286 g CO2 equiv. m–2year–1). Similarly, the grass field is a greenhouse gas source. Using the average net CO2 emission from Finnish peat soils with grass (Maljanen et al. 2007a) and the measured N2O and CH4 emissions, the grass field is a much stronger net greenhouse gas source, 3130 g CO2m–2year–1, than the forest. However, the overall net CO2 emissions from the northern forested peatlands are still poorly described, as are the wintertime N2O emissions. According to these results, it is possible that some peatland forests can be net emitters rather than sinks of greenhouse gases as a result of high wintertime N2O emissions. Furthermore, in peatland forest soils, the most critical period for high N2O emission peaks can be at the time of soil freezing

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rather than, as it is with agricultural peat soils, at the time of soil thawing. We may have missed some emission peaks because the sampling interval was from 2 to 3 weeks. This points out the need for continuous measurements of N2O fluxes in northern soils also during cold seasons, for instance with the eddy correlation method (Pihlatie et al. 2009). In our study, the N2O emissions during the unfrozen season were less than 20% of the annual emissions; thus, the main part of the N2O emissions originated from the 6-month winter period. The results here show that the timing of episodic high N2O emissions can depend on land use and the ‘‘critical periods’’ may lead to strongly biased annual emissions if the measurement frequency is low.

Acknowledgements This study was funded by the Academy of Finland project 108409 and FiDiPro programme 127456. Seppo Vihanta is thanked for field assistance and Hanne Sa¨ppi for help in the laboratory.

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