Greenhouse gas fluxes in a drained peatland forest during spring frost-thaw event

Biogeosciences, 7, 1715–1727, 2010 www.biogeosciences.net/7/1715/2010/ doi:10.5194/bg-7-1715-2010 © Author(s) 2010. CC Attribution 3.0 License. Biogeosciences Greenhouse gas fluxes in a drained peatland forest during spring frost-thaw event M. K. Pihlatie1, R. Kiese2, N. Br¨ uggemann2, K. Butterbach-Bahl2, A.-J. Kieloaho1, T. Laurila3, A. Lohila3, I. Mammarella1, K. Minkkinen4, T. Penttil¨ a5, J. Sch¨ onborn2,6, and T. Vesala1 1Department of Physics, University of Helsinki, P. O. Box 48, University of Helsinki, 00014, Helsinki, Finland 2Institute of Meteorology and Climate Research, Atmospheric Environmental Research (IMK-IFU), Karlsruhe Institute of Technology, Garmisch-Partenkirchen, Germany 3Finnish Meteorological Institute, P. O. Box 503, 00101, Helsinki, Finland 4Department of Forest Ecology, University of Helsinki, P. O. Box 27, University of Helsinki, 00014, Helsinki, Finland 5Finnish Forest Research Institute, Vantaa Unit, Finland 6Meteorological Institute, Albert-Ludwigs-University Freiburg, Freiburg, Germany Received: 29 May 2009 – Published in Biogeosciences Discuss.: 23 June 2009 Revised: 8 April 2010 – Accepted: 28 April 2010 – Published: 25 May 2010 Abstract. Fluxes of greenhouse gases (GHG) carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) were measured during a two month campaign at a drained peatland forest in Finland by the eddy covariance (EC) technique (CO2and N2O), and automatic and manual chambers (CO2, CH4and N2O). In addition, GHG concentrations and soil parameters (mineral nitrogen, temperature, moisture content) in the peat profile were measured. The aim of the measurement campaign was to quantify the GHG fluxes during freezing and thawing of the top-soil, a time period with potentially high GHG fluxes, and to compare different flux measurement methods. The forest was a net CO2sink during the two months and the fluxes of CO2dominated the GHG exchange. The peat soil was a small sink of atmospheric CH4 and a small source of N2O. Both CH4oxidation and N2O production took place in the top-soil whereas CH4was produced in the deeper layers of the peat, which were unfrozen throughout the measurement period. During the frost-thaw events of the litter layer distinct peaks in CO2and N2O emissions were observed. The CO2peak followed tightly the increase in soil temperature, whereas the N2O peak occurred with a delay after the thawing of the litter layer. CH4fluxes Correspondence to: M. K. Pihlatie ([email protected]) did not respond to the thawing of the peat soil. The CO2 and N2O emission peaks were not captured by the manual chambers and hence we conclude that high time-resolution measurements with automatic chambers or EC are necessary to quantify fluxes during peak emission periods. Subcanopy EC measurements and chamber-based fluxes of CO2 and N2O were comparable, although the fluxes of N2O measured by EC were close to the detection limit of the system. We conclude that if fluxes are high enough, i.e. greater than 5–10µgNm−2h−1, the EC method is a good alternative to measure N2O and CO2fluxes at ecosystem scale, thereby minimizing problems with chamber enclosures and spatial representativeness of the measurements. 1 Introduction Drainage of peatlands for forestry has been a common practice in Fennoscandia during the past 100 years. In Finland, more than half of the original peatland area has been drained for forestry or agricultural use since the 1920s (Paavilainen and P¨ aiv¨ anen, 1995; Joosten and Claarke, 2002). Drainage lowers the groundwater table and improves the aeration of the peat, which increases the growth of trees. Thereby, drainage also changes greenhouse gas dynamics of the peatland, as a large part of the decomposition of the peat Published by Copernicus Publications on behalf of the European Geosciences Union. 1716 M. K. Pihlatie et al.: Greenhouse gas fluxes in a drained peatland forest during spring frost-thaw event switches from anaerobic to aerobic conditions with a shift from methane (CH4) to carbon dioxide (CO2) as the endproduct of decomposition (Moore and Dalva, 1993; Silvola et al., 1996; Minkkinen et al., 2002; Roulet et al., 1993, Martikainen et al., 1995; Nyk¨ anen et al., 1998). Stimulated aerobic decomposition of the peat releases nutrients, especially nitrogen, to the soil, which may lead to elevated emissions of nitrous oxide (N2O) (Martikainen et al., 1993; Silvola et al., 1996; Laine et al., 1996). However, the changes in N2O emissions after drainage seem to depend on the fertility of the original peatland, i.e. its nitrogen content or the C:N ratio of the peat, and the level of the water table after the drainage (von Arnold et al., 2005a, b). Drained peatlands which have been used for agriculture first and then planted with trees (afforested peat soils) are strong point sources of N2O. These N2O emissions are of the same order of magnitude as the emissions from drained peatlands which are still used for agriculture (Maljanen et al., 2001; Regina et al., 2004; M¨ akiranta et al., 2007). Drained forested peatlands cover 25% of forest area in Finland making these ecosystems potentially important sources of greenhouse gases. During the last two decades there has been debate whether the drainage of peatlands for forestry turns them from net sinks of carbon into net sources, and whether N2O makes up an important part of the total greenhouse gas balance. Intensive measurements of GHG emissions from drained peatland forests are scarce. Also, comparisons of different measurement techniques in these ecosystems are almost nonexistent. Mostof thestudies havebeen conductedwith chamber techniques using weekly to monthly measuring intervals. This measurement frequency may severely miss important emission events, so called “hot moments”, related especially to N2O emissions from soils (see e.g. Matzner and Borken, 2008; Papen and Butterbach-Bahl, 1999), such as frost-thaw periods which could be substantial in boreal environments (see e.g. Koponen et al., 2004, 2006). As a result, calculations of seasonal or annual budgets of greenhouse gases may be biased and potentially underestimated if the frequency of measurements or spatial coverage is not sufficient to cover variations. We report results of greenhouse gas emissions (CO2, CH4 and N2O) from a drained peatland forest in Kalevansuo, southern Finland. The measurement campaign lasted two months from the end of April until the end of June 2007, and was run under the NitroEurope IP EU-project. The main aim was to quantify the total GHG balance during a potentially high peak season in the spring, when the peat is melting and frost-thaw driven N2O fluxes are likely to occur. We measured net CO2exchange above and below the forest canopy and N2O exchange below the forest canopy by the eddy covariance (EC) method and compared these fluxes to soil CO2, CH4, and N2O fluxes measured simultaneously with automated and manual chamber techniques. Our aim was to estimate the net GHG exchange and the importance of different C and N flux components on the total GHG balance during the two-month measuring period. We hypothesise that N2O is an important component of the ecosystem greenhouse gas exchange due to “hot moment” emissions such as frost-thaw events. Our second aim was to evaluate the suitability of sub-canopy EC-based N2O measurements as a sophisticated alternative to traditionally used chamber methods in this environment. The quality control and flux error analysis of the EC N2O measurements at the site are presented in this issue in Mammarella et al. (2010). 2 Materials and methods 2.1 Site description The measurements were conducted at a Kalevansuo drained peatland forest classified as an ombrotrophic dwarf-shrub pine bog. The site is located in southern Finland (60◦390N, 24◦220E), where the mean annual precipitation is 606mm and the mean annual temperature is 4.3◦C. The bog was drained for forestry in 1971 by open, about 1m deep ditches dug with approximately 40m spacing between the parallel ditches. In 1973 the site was fertilised with phosphorus and potassium, following the guidelinepractises for drained peatlands. Drainage resulted in a lowered water table down to approx. 40cm from the peat surface, and a changed composition of ground vegetation from typical bog vegetation towards more of a forest understorey. However, some features such as the abundance of peatland dwarf shrubs and fairly high coverage of Sphagnum species still distinguish the site from upland forests. Currently the height of the tree stand is 15–18m, average basal area is 18m2ha−1, and average stem densities are 900, 750, and 40 stems per ha for the dominant Scots pine (Pinus sylvestris L.) trees and the smaller understorey downy birch (Betula pubescens) and Norway spruce (Picea abies L.) trees, respectively. The total LAI in the site is approximately 2m2m−2(Mammarella et al., 2010). Forest floor vegetation consisted mainly of hummock dwarf shrubs (Vaccinium vitis-idaea, Vaccinium myrtillus, Empetrum nigrum, Vaccinium uliginosum, Ledum palustre and Betula nana), sedges like Eriophorum vaginatum and mosses (Pleurozium schreberi,Dicranum polysetum, Sphagnum russowii, Spagnum capillifolium and Sphagnum angustifolium). The depth of the well decomposed Sphagnum peat at the site is approximately 2.5m with peat a pH of 5.0 and C/N ratio of 41 in the litter layer and 45 in the top 10cm of the peat soil. 2.2 Flux measurements Intensive GHG measurements were carried out from 25 April to 27 June 2007 within a homogenous and representative approx. 1ha plot of the forest (total area of approx. 60ha). The Biogeosciences, 7, 1715–1727, 2010 www.biogeosciences.net/7/1715/2010/ M. K. Pihlatie et al.: Greenhouse gas fluxes in a drained peatland forest during spring frost-thaw event 1717 main measurements included micrometeorological eddy covariance (EC) measurements of CO2above and below the forest canopy and N2O fluxes below the canopy, automated as well as manual chamber-based measurements of CO2, CH4and N2O fluxes. The locations of the different measurement systems are shown in Fig. 1. The above canopy EC CO2flux measurement system (ECabove) included a METEK USA-1 ultra sonic anemometer (METEK GmbH, Elmshorn, Germany) mounted on the top of a 21.5m telescopic mast and a LI-7000 CO2/H2O analyzer (Li-Cor, Inc., Lincoln, NE, USA) mounted at 6m height in the tower. Air was drawn from the proximity of the sonic to the LI-7000 CO2/H2O analyzer using a Bev-ALine IV tubing (Thermoplastic processes, Stirling, NJ) with an inner diameter of 3.1mm. The storage flux of CO2was calculated from the concentration data measured at heights of 21.5m and 6m, the latter being measured with a LI-820 CO2 analyzer (Li-Cor Inc., Lincoln, NE, USA). The storage flux was added to the measured net ecosystem exchange (NEE), hereafter NEE referring to the sum of turbulent and storage fluxes. The mast was located in the centre of the measurement site (Fig. 1). The sub-canopy EC measurements (ECsub) were conducted at 4m height. The sub-canopy mast was located approximately 100m southwest of the tall mast, and approximately half way between the tall mast and the automatic soil chambers (see Fig. 1). The CO2fluxes were measured with a Li-7500 Open-Path Infrared CO2/H2O Gas Analyzer (LiCor, Inc., Lincoln, NE, USA) and a CSAT3 Sonic Anemometer (Campbell Scientific Inc., Logan, UT, USA). EC measurements of N2O fluxes were conducted at the same mast using the same CSAT3 anemometer and a tunable diode laser spectrometer (TGA-100A, Campbell Scientific Inc., Logan, UT, USA). Forest floor (soil and ground vegetation) fluxes of CO2, N2O and CH4were measured with the enclosure method using automatic (transparent) and manual (opaque) chambers. The automatic chamber system consisted of a valve-driven sampling system (custom-made by IMK-IFU) for nine soil chambers with dimensions of 50×50×15cm (length ×width ×height). The automatic chambers were located approx. 170m southwest of the tall EC mast, and approx.100msouthwestfromthesub-canopyEC mast (Fig. 1). The chambers were connected to a gas chromatograph (SRI Instruments, Torrance, CA, USA) equipped with an electron capture detector (ECD) for N2O and a flame ionization detector (FID) for CH4, and a GMD20D infrared CO2analyzer (Vaisala, Vantaa, Finland). The nine chambers were split into 3 sets of 3 chambers. One measurement cycle included closures of 3 chambers and a simultaneous calibration with a reference gas. Each chamber was closed for 48min, and the mean sampling intervals were 6, 18, 30, and 42min after the closure. The measurement system is described in more detail in Kiese and Butterbach-Bahl (2002) and Werner et al. (2007). The vegetation inside the automatic chambers was Fig. 1. Map of the measurement site showing the locations of above canopy eddy covariance (EC) mast (ECabove), sub-canopy EC mast (ECsub), manual chambers (MC, square) and automatic chambers (AC, circle). Dotted line next to one of the manual chamber groups show the place of soil gas concentration pits, and grey line around the sub-canopy EC mast show the footprint area from which 85% (at 30m) of the sub-canopy N2O fluxes originate (see Mammarella et al., 2010). similar than in the peatland generally, however, tall dwarf shrubs were not present. Detailed vegetation survey was not conducted for the automatic chambers. Manual chamber measurements were conducted once a week during April to June 2007, and fortnightly during July to September 2007. In total 16 circular metal collars were located in groups of four approx. 30–60m from the tall EC mast in the four main directions, and 10–150m north-east from the sub-canopy EC mast (Fig. 1). The collars were installed in 2004 at soil depth of 3–5cm, on top of the root layer. During chamber measurements, a 30cm high circular metal chamber was placed on the collar. Volume of the chamber was approx. 27L. Air inside the chamber was mixed with a fan, and the temperature inside the chamber was monitored with a thermometer in order to correct the fluxes. Gas samples (100ml) were collected with a syringe at 2, 15, 25 and 35min intervals and transferred immediately into 12-ml glass vials (Labco Exetainer®, Labco Limited, Buckinghamshire, UK). Ninety ml of the gas sample was used to flush the air in the vial with two needles. The rest 10ml of the gas sample was used to over-pressurize the vial after removing the flushing needle. Gas samples were analyzed within one week for N2O and CH4by a www.biogeosciences.net/7/1715/2010/ Biogeosciences, 7, 1715–1727, 2010 1718 M. K. Pihlatie et al.: Greenhouse gas fluxes in a drained peatland forest during spring frost-thaw event gas chromatograph (Agilent 6890 GC, Agilent Technologies Finland, Espoo, Finland) equipped with an ECD for N2O and an FID for CH4. 2.3 Soil measurements Concentrations of N2O and CH4in the peat profile were measured at two pits located approximately 40m southwest of the above-canopy EC mast. The concentrations were measured in the peat at 5cm, 22 and 45cm below the litter layer. Gas collector cups were 100ml in volume and made of stainlesssteel. Thecupswereinstalledhorizontallyapprox.20cm apart from each other, upside down with an open end at the bottom in the soil and connected to the atmosphere via a 1/800 stainless steel tube. Gas samples were collected weekly during April to June from the depths 5 and 25cm and fortnightly during July to September from all depths (5, 22 and 45cm). At the time of gas sampling, the 5–10ml gas volume inside the tubing was discarded after which a 100ml gas sample was taken and transferred into 12-ml glass vials as described above. When a gas collector was below the groundwater table, a water sample of 50ml was taken with the syringe. Then the gas dissolved in the water was equilibrated with 50ml of ambient air by shaking the syringe rigorously for 10min. After shaking, 20ml of the gas sample was injected into a pre-evacuated 12-ml glass vial. Soil temperature and volumetric water contents were measured adjacent to the automatic chambers in the litter layer and at 5 and 10cm depths of the peat (Trime®TDR IMKO and Pt-100, IMKO GmbH, Ettlingen, Germany). In addition, soil temperatures in the litter layer, and at 5 and 30cm depths of the peat were measured close to the tall EC mast by FMI (Finnish Meteorogical Institute). The variation of the ground water level near the main EC mast was monitored by a PDCR 1830 level pressure sensor (Druck Inc., New Fairfield, CT, USA). Soil ammonium (NH4-N), nitrate (NO3-N) and total dissolved nitrogen contents were analysed from samples collected weekly during April to June 2007, and monthly during July to September 2007. Soil samples from the litter layer and peat (0–10cm) were collected in 5 replicates: four from close vincinity to the manual chambers (4 groups) and one from close vincinity of the automatic chambers. Fresh soil samples were stored at +4◦C and extracted with 1MKCl the next day after the sampling. The extracts were frozen at −18◦C until analysis by a flow injection analyzer (FIA 5012, Tecator) at the Finnish Forest Research Institute. Total carbon and nitrogen contents were analyzed from dried (40◦C) soil samples using a vario MAX CN elemental analyser. 2.4 Data analysis Flux rates of manual and automated chamber measurements were calculated with the following equation Fc=dC dt h, (1) where Fcis the flux of the target gas (gm−2s−1), Cis the gas concentration in the chamber air (gm−3) at standard pressure (101325Pa) and temperature measured in the headspace, tis closure time (s) and hthe height of the chamber (m). The development of the gas concentration inside the chambers was linear for the majority of the measurements. For the manual chamber data we compared fluxes calculated based on quadratic fit and linear regression. The use of a quadratic fit resulted in up to 30% higher fluxes of CH4and 20% smaller fluxes of N2O as compared to the linear regression. Due to only four data points and fluxes close to zero, we considered that the linear regression method was more reliable for this data and hence we calculated all the fluxes by a linear regression analysis (n=4). We filtered out bad quality data by removing data with R2-value 0.7 or less. EC fluxes were calculated as 30min average covariances between the scalars (CO2and N2O) concentration and the vertical wind velocity according to the commonly accepted procedures (Aubinet et al., 2000). The above canopy EC data acquisition was done with a modified version of a program by McMillen (1986). Coordinate rotation and data detrending by an autoregressive running-mean filter with a 200-s time constant were performed according to McMillen (1988). The lag between the time series resulting from the transport through the inlet tube was taken into account in the on-line calculation. An air density correction related to the sensible heat flux is not necessary, but the corresponding correction related to the latent heat flux was made (Webb et al., 1980). Corrections for the systematic high-frequency flux loss owing to the imperfect properties and setup of the sensors were carried out off-line using transfer functions with empiricallydetermined time constants. The data processing procedures have been presented in more detail by Lohila et al. (2007) and Aurela et al. (2009). The sub-canopy fluxes were calculated using software developed by the Micrometeorology group at the University of Helsinki, Department of Physics. The software is routinely used for post-processing EC data measured in several permanent sites and field campaigns. It contains all the update methods and corrections according to the Euroflux methodology (Aubinet et al., 2000; Lee et al., 2004). For the present study, the software was slightly modified in order to handle with the laser data, as reported by Mammarella et al. (2010). All signals were detrended for removing the average values and trends. A simple linear detrending procedure was used for calculating the CO2flux. The N2O signal measured by the TDL gas analyzer was characterized by stronger trends, caused mainly by instrumental drift, which can give an extra Biogeosciences, 7, 1715–1727, 2010 www.biogeosciences.net/7/1715/2010/ M. K. Pihlatie et al.: Greenhouse gas fluxes in a drained peatland forest during spring frost-thaw event 1719 contribution to the estimated flux in the case that the fluctuations of the concentration are correlated with the fluctuations of the vertical wind velocity. In order to remove the instrumental drift effect and to reduce the random flux variability, a running mean filter (McMillen, 1988) was performed prior to calculation of the N2O flux. A more detailed description of the data processing of N2O EC signal is given in Mammarella et al. (2010). Alag-timeof2.3s was obtained for the above-canopy CO2 signal, maximizing the cross-covariance function between the CO2concentration and the vertical wind velocity. The same procedure was applied to the sub-canopy N2O signal, but because the N2O emissions were very close to detection limit of the system, it was not possible to clearly determine experimentally the N2O lag time. Then using a procedure similar to Pihlatie et al. (2005), we used a fix lag time of 1s. The same value was obtained by using the sample flow and volumes of the inlet tubing and the sample cell, for estimating the theoretical N2O lag time. The CO2flux was corrected for density fluctuations effect (WPL correction; Webb et al., 1980), while such correction was unnecessary for N2O fluxes, because of the presence of high flow sample dryer in the system (PD1000 Nafion®dryer, Campbell Scientific, Inc., Logan, UT, USA). Temperature fluctuations do not need to be corrected because they can be assumed to be damped in the sampling tube (Rannik et al., 1997). No Burba correction was used for the eddy covariance data from open path CO2 analyzer even though the correction may slightly increase the flux levels (Burba et al., 2008). The EC fluxes were corrected for the high frequency flux underestimation according to Mammarella et al. (2010). For typical mean wind velocity in the sub-canopy layer, the flux loss was about 5% and less than 10% for CO2and N2O, respectively. Statistical tests (paired t-test) for the flux and soil measurement data was done with SPSS statistical program (SPSS Inc., Chicago, IL, USA). 3 Results 3.1 Environmental conditions At the start of the measurement campaign part of the peat was still frozen. The air temperatures varied from below 0◦C in the end of April to a maximum of 27◦C in the beginning of June (Fig. 2). Prior to the start of the measurement campaign the soil had melted and frozen several times. The first pronounced freeze-thaw cycle was recorded in the end of March, one month prior to the measurement campaign (data not shown). However, as indicated by temperature measurements of air and litter layer, the peat surface layer was still freezing and thawing during the measuring campaign in the end of April (Fig. 2). During the intensive measurement period (25 April–27 June) the soil temperature increased from around 0◦C up to approx. 16◦C in the upper part (5cm depth) of the peat soil. Rainfall during April–June was low with low intensities except for two events in mid April and in the end of May, resulting in short increases in the soil water content (Max. 22vol%) and water table (see Fig. 2). Despite these short increases, the water table and soil moisture decreased (−25cm to −40cm; 16 to <10vol%) during the intensive measurement period. 3.2 Concentration of soil ammonium, nitrate and total dissolved nitrogen Soil nitrate (NO− 3-N) concentrations were close to zero throughout the whole measuring period, whereas soil ammonium (NH+ 4-N) and total nitrogen (tot-N) concentrations were elevated at the beginning of the measurement period with a maximum during the frost-thaw event in May, and decreased towards the end of the measuring campaign (Fig. 6). The concentrations of NO− 3-N, NH+ 4-N and tot-N were always higher in the litter layer than in the peat at 0–10cm depth (data not shown). Total dissolved nitrogen concentrations in the soil varied between 50–230mgNkg−1dry soil, and were approximately one order of magnitude higher than the concentrations of NH+ 4-N in the soil. 3.3 CO2fluxes EC measurements above the forest canopy revealed that the site was on average a net sink for CO2during the measuring campaign, from late April to late June 2007 (see Fig. 3). The daily net ecosystem exchange (NEE) of CO2 increased from approximately −0.014mgCm−2s−1during April to maximum of −0.064mgCm−2s−1in the middle of June. The drained peatland forest was a weak source of carbon (0.02mgCm−2s−1) on few rainy days during the measurement period. Overall, the CO2exchange followed the changes in air and soil temperatures being higher (uptake) in warm and lower (up to emission) in cold days (see Figs. 2 and 3). In contrast to the net CO2uptake of the whole forest ecosystem, soil and ground vegetation together turned out to be a source of CO2to the atmosphere. Both, CO2fluxes below the forest canopy measured by the EC and by automatic chambers on the soil surface showed an increasing emission trend from April to June (Fig. 3). Forest floor CO2fluxes (automatic chambers) and sub-canopy fluxes (sub-canopy EC) increased from a minimum of 0.001mgCm−2s−1in the end of April to a maximum of 0.013mgCm−2s−1and 0.03mgCm−2s−1, respectively, in the end of May when also soil and air temperatures reached their maximum. In June a decrease in temperature was followed by a decrease in CO2fluxes, however, this was more pronounced in the sub-canopy EC fluxes. In the end of June forest floor and sub-canopy fluxes leveled around 0.01mgCm−2s−1, however still following changes in the air and soil temperatures (Figs. 2 and 3). Mean forest floor (0.008mgCm−2s−1) www.biogeosciences.net/7/1715/2010/ Biogeosciences, 7, 1715–1727, 2010 1720 M. K. Pihlatie et al.: Greenhouse gas fluxes in a drained peatland forest during spring frost-thaw event Fig. 2. (a) Air temperature, (b) soil temperatures in litter layer (hummock and hollow) and in peat, and (c) soil moisture (vol/vol), ground water table depth (WT) and precipitation at the drained peatland pine forest during April–September 2007 (intensive measurements 25 April– 27 June). and sub-canopy CO2exchange (0.009mgCm−2s−1) over the measuring period were almost identical and a paired ttest analysis did not reveal any statistical differences (Table 1). Forest floor CO2and sub-canopy exchange correlated positively with air and soil temperatures. The soil temperature at 5cm depth explained most of the variability in forest floor CO2flux rates (r=0.96, p<0.01). The correlation was less pronounced for sub-canopy EC based fluxes due to a more scattered temporal emission pattern also reflected in higher values of CV% (Table 1, Fig. 3). Furthermore, we found a negative correlation of forest floor CO2fluxes with soil moisture (−0.60, p<0.01) and water table depth (−0.76, p<0.01). These correlations were not significant for the ECbased sub-canopy measurements. The measurement campaign can be divided into two distinct periods: a cold and a warm period. During the cold period (30 April–10 May) the net forest floor CO2fluxes, the sum of soil respiration and CO2photosynthesis of ground vegetation, and the CO2net ecosystem exchange (NEE) above the forest canopyweresmall(Fig.4). Duringthewarm period (5 June–15 June) both the net CO2emissions of the forest floor (Fig. 4c) and the net CO2uptake of the forest canopy (Fig. 4d) increased. During both cold and warm periods, the sub-canopy CO2fluxes followed a small but clear diurnal trend when the net CO2emission decreased during day-time and increased during night-time (Fig. 4a and c). The comparison of the mean and median GHG exchange measured by above canopy EC and sub-canopy EC and by automatic forest floor chambers during the entire two-months measurement period is shown in Table 1, and the cumulative fluxes are shown in Table 2. During the period of 25 April– 21 June the cumulative CO2fluxes measured by sub-canopy EC (42.5gCm−2) and forest floor chambers (37.7gCm−2) did not statistically differ from each other, and accounted for Biogeosciences, 7, 1715–1727, 2010 www.biogeosciences.net/7/1715/2010/ M. K. Pihlatie et al.: Greenhouse gas fluxes in a drained peatland forest during spring frost-thaw event 1721 Table 1. Mean and median fluxes of CO2, CH4and N2O and coefficient of variation (CV%1) measured by eddy covariance, and automatic and manual chambers in Kalevansuo peatland forest during 25 April–27 June 2007. ECaand ECsstand for eddy covariance above and below the canopy, respectively, and AC and MC stand for automatic and manual chambers, respectively. mgCO2-Cm−2s−1µgCH4-Cm−2h−1µgN2O-Nm−2h−1 CO2ECaCO2ECsCO2AC CH4AC CH4MC2N2O ECsN2O AC N2O MC2 Mean3−0.031a0.009b0.008b−37.1a−18.5b3.2a4.5b6.8c Median −0.026 0.008 0.008 −35.6 −15.2 2.5 3.9 6.8 CV% 180 75.7 45.3 40.2 144 123 62.3 42.8 1Coefficient of Variation was calculated as CV% = stdev of the flux/mean flux×100. 2Measurement period 25 April–18 June 2007. 3Different superscripts indicate significant differences between flux rates of one component measured with different methods. Fig. 3. (a) Daily mean CO2exchange measured with eddy covariance above the forest canopy (EC above) and inside the canopy (EC sub) and automatic chambers (AC) at the drained peatland pine forest. Error bars stand for standard deviations. 42 and 37% of the total NEE (−102gCm−2), respectively (Table 2). 3.4 CH4fluxes Kalevansuo peatland forest was a small sink for CH4during the measurement campaign (Fig. 5a). The CH4uptake measured with the automatic chambers increased from around −30µgCm−2h−1to a approximately of −60µgCm−2h−1 in June. The CH4fluxes measured with manual chambers were constantly by at least a factor of two smaller than the CH4fluxes measured with the automatic chambers (Fig. 5). The fluxes of CH4were not affected by thawing of the soil but followed more closely the groundwater table and soil moisture content in the peat. CH4uptake correlated positively with soil water content (r=0.38, p<0.01) and water table depth (r=0.44, p<0.01), and negatively with soil temperatures at 5cm and at 30cm depth (r=−0.50, p<0.01; r=−0.62, p<0.01), respectively, and CO2fluxes measured by the automatic chambers (r=−0.50, p<0.01). Fig. 4. Daily time course of CO2fluxes at the drained peatland pine forest measured with automatic chambers (AC) and subcanopy eddy covariance (EC sub) (a), (c) and above canopy eddy covariance (EC above) (b), (d) during a cold period in 30 April– 10 May 2007 (a), (b) and a warm period in 5–15 June 2007 (c), (d). Dots represent median values for each hour (AC, EC sub) or half hour (EC above) over the 10-day period. Error bars represent standard deviations. 3.5 N2O fluxes Kalevansuo drained peatland forest was a small source of N2O during the measurement period from April to June 2007. Mean emission rates varied between 3.2µgNm−2h−1measured by the sub-canopy EC technique, 4.5µgNm−2h−1by the automatic chambers, and 6.8µgNm−2h−1by the manual chamber techniques (Fig. 6, Table 1). Independent of the measuring technique N2O emissions hardly exceeded 10µgNm−2h−1except for a short period at the beginning of the measuring campaign when elevated N2O emissions could be detected at least with the temporally highly resolved EC and automatic chamber measurements (see Fig. 6). The elevated N2O emissions coinside www.biogeosciences.net/7/1715/2010/ Biogeosciences, 7, 1715–1727, 2010 1722 M. K. Pihlatie et al.: Greenhouse gas fluxes in a drained peatland forest during spring frost-thaw event Table 2. Cumulative greenhouse gas CO2, CH4and N2O fluxes at the Kalevansuo drained peatland forest measured by eddy covariance and automatic chambers during the intensive measurement period 25 April–26 June 2007. Component cumulative flux, cumulative flux, GWR100 CO2eqv. Cm−2gGHGm−2 CO2ECa(NEE) −102 −373 −373 CO2ECs42.5 156 156 CO2AC 37.7 138 138 CH4AC −0.046 −0.062 −1.30 N2O AC 0.006 0.009 2.77 1Measurement period 25 April–21 June 2007. 2GWP100 refers to Global Warming Potential with a 100-year time horizon. Fig. 5. (a) Daily mean fluxes of CH4measured with automatic (AC, n=9) and manual (MC, n=16) chambers, (b) soil concentrations of CH4at three depths and in the ambient air measured at the drained peatland. First column of the figures represent the period of intensive measurements, the second shows the data outside the measurement campaign. Error bars stand for standard errors of the mean. with the coldest period (air temp <0◦C) within the measuring period and a rapid increase in air temperatures up to 15◦C (Fig. 6). A significant uptake of atmospheric N2O was never detected. In general, N2O fluxes measured with the EC technique were more variable than chamber based N2O fluxes which is indicated by a much higher CV% of 123 as compared to values of CV% of 62.3 and 42.8 by the automatic and manual chambers, respectiveley (Table 1). N2O emissions measured by the automatic chambers correlated negatively with air temperature (r=−0.50, p<0.01) and soil temperatures in the litter layer, at 5cm and at 30cm depths (r=−0.48, p<0.01; r=−0.47, p<0.01; r=−0.46, p<0.01), respectively, soil moisture content (r=−0.46, p<0.01), and Fig. 6. (a) Mean soil N2O fluxes measured with eddy covariance, automatic and manual chambers, (b) soil concentrations of N2O at three depths and in the ambient air, and (c) mineral nitrogen and totalnitrogen concentrations in thelitter layerofthe soil during April– September 2007 at the drained peatland pine forest. First column of the figures represent the period of intensive measurements, the second shows the data outside the measurement campaign. Error bars stand for standard errors of the mean. CO2fluxes (r=−0.48, p<0.01). Positive correlations were found with water table depth (r=0.40, p<0.01) and CH4uptake (r=0.30, p<0.05). 3.6 CH4and N2O concentration in peat profile During the intensive measuring campaign from April to June 2007 CH4and N2O concentrations in the peat profile were close to ambient air concentrations of ∼1.8ppmv and ∼0.35ppmv, respectively (Figs. 5b and 6b). In general, during the intensive measurement campaign the CH4concentrations decreased (i.e. consumption) and N2O concentration slightly increased (i.e. production) with peat depth in the topsoil. From July to September the concentrations of CH4in deeper peat layers (22 and 45cm depth) increased markedly. The highest concentration of 1400ppmv was measured at 45cm depth in September. At the same time the CH4concentrations in the litter layer were close to the ambient air concentrations and the net fluxes measured by manual chambers showed that the soil was still a sink of CH4(Fig. 5a and b). Biogeosciences, 7, 1715–1727, 2010 www.biogeosciences.net/7/1715/2010/ M. K. Pihlatie et al.: Greenhouse gas fluxes in a drained peatland forest during spring frost-thaw event 1723 Nitrous oxide concentrations at 22cm depth were most of the time higher than the concentration just below the litter layer at 5cm (Fig. 6b). Concentrations at 45cm depth measured during July to September varied between 0.210– 0.240ppmv and were much lower than at 5 or 22cm depths and well below the atmospheric concentration. 4 Discussion 4.1 CO2fluxes Eddy covariance (EC) measurements above the forestcanopy revealed that the Kalevansuo drained peatland pine forest was a net sink of CO2during the measuring period from the end of April to the end of June. The measurements below the forest canopy by sub-canopy EC and automatic chambers showed that the forest floor was a net source of CO2, however, only a small part of the net CO2uptake of the whole forest ecosystem. During few rainy days in the campaign (in total 5 days) the Kalevansuo peatland forest turned from a net sink of carbon to a net source. This finding is in line with the study by Lohila et al. (2007) where they found that an afforested boreal peatland turned from a net sink to a source of carbon during rainy days in the summer. Total NEE at the Kalevansuo drained peatland forest from spring to early summer (25 April–21 June, −102gCm−2) is comparable to NEE values reported from boreal forests growing on mineral or peat soils (Suni et al., 2003; Lohila et al., 2007). In this study the diurnal variation in the CO2exchange of the soil and forest floor vegetation was very small measured by the sub-canopy EC and non-existent measured by the automatic soil chambers. Similarly small diurnal variation in the forest floor CO2exchange of a boreal forest ecosystem has been measured earlier by Launiainen et al. (2005) and Kulmala et al. (2008). However, much stronger diurnal variation in the CO2exchange of soil and forest floor vegetation has been measured in a temperate forest ecosystem on mineral soil (Subke and Tenhunen, 2004). In our study the lack of diurnal variation in the CO2exchange of the forest floor may result from (1) a small photosynthetic activity of the forest floor vegetation as compared to the soil and forest floor respiration, or (2) the possibility of high photosynthetic activity during day-time and a simultaneous increase in the soil respiration due to temperature dependency, which then compensates for the photosynthesis. The net forest floor CO2 fluxes measured by sub-canopy EC during April–June period compare well with sub-canopy EC measurements carried out in a boreal pine forest (Launiainen et al., 2005), and chamber based measurements in other drained peatland forests (Martikainen et al., 1995; Alm et al., 1999). Correlation of forest floor CO2fluxes was highest with soil temperatures in 5cm depth. This shows that rather the top-soil, getting fresh litter input from vegetation, is the major source of CO2as compared to the peat body itself, thus, stimulated decomposition of the peat due to aeration by drainage has already diminished. In contrast to N2O emissions no increases in CO2emissions following thawing of the litter layer could be detected. The intermittent increase of CO2emissions in the end of April can be related to a significant increase in soil and air temperatures, however, in a period when temperatures were never below 0◦C. As the measurements started after the first freeze-thaw cycles, it is unclear whether such freeze-thaw induced CO2peaks occurred at the site although the absence or less pronounced effect of frost-thaw cycles on in situ CO2 emissions in forest ecosystems is also reported in the review of Matzner and Borken (2008). 4.2 CH4fluxes Automatic and manual chamber based measurements revealed that the peatland forest was a sink for atmospheric CH4during the whole measuring period from end of April to end of June 2007. This means that the drainage was deep enough to change the aeration status and, thus, the conditions favourable for methanogenes to those favourable for methanotrophs. The high influence of the water table depth on the CH4exchange of peatlands has been observed in other studies (Martikainen et al., 1993, 1995) and is further reflected by the significant positive correlation of CH4uptake rates with changes in water table depth during the observation period. Maximum uptake rates of >60µgCH4-Cm−2h−1were significantly higher than observed by Martikainen et al. (1995) for a drainded fen with comparable water table depths. In a large study combining data from drained and undrained peatland forests in Finland Minkkinen et al. (2007) found that in general, undrained sites functioned as CH4sources whereas drained sites functioned either as CH4sinks or still as small sources for CH4. In their study the mean CH4uptake rates varied from 1 up to 90µgCH4-Cm−2h−1. For the Kalevansuo site Minkkinen et al. (2007) reported an annual CH4uptake of 0.2gCm−2. A simple linear extrapolation from the cumulative flux to a full year resulted in an uptake of 0.09gCm−2yr−1for the automatic chambers and 0.06gCm−2yr−1for the manual chambers. This indicates that this drained peatland forest is a significant, but slitghtly smaller CH4sink as compared to boreal forests in general (−0.15gCm−2yr−1) (Dutaur and Verchot, 2007). We found that CH4was produced throughout spring and summerat22and25cm depth in the peatprofile. Atthesame time the net flux of CH4was negative, showing CH4uptake. This implies that the Kalevansuo site was well drained and the oxic top-layer of the peat was sufficient not only to oxidize the CH4produced in deeper layers, but also to oxidize additional atmospheric CH4. This observation is in-line with observations at other sites, where also CH4concentrations well above atmospheric concentrations were detected in deeper soil layers, while soil was still fucntioning as a net sink for armospheric CH4(Butterbach-Bahl and Papen, www.biogeosciences.net/7/1715/2010/ Biogeosciences, 7, 1715–1727, 2010