Temporal and seasonal changes in greenhouse gas emissions from a constructed wetland purifying peat...

Ecological Engineering 26 (2006) 241–251

Temporal and seasonal changes in greenhouse gas emissions froma constructed wetland purifying peat mining runoff waters

Anu Liikanena,∗, Jari T. Huttunena, Satu Maaria Karjalainenb, Kaisa Heikkinenb,Tero S. Vaisanenb, Hannu Nykanena, Pertti J. Martikainena

a Department of Environmental Sciences, Research and Development Unit of Environmental Health,University of Kuopio, P.O. Box 1627, FIN-70211 Kuopio, Finland

b North Ostrobothnia Regional Environment Centre, P.O. Box 124, FIN-90101 Oulu, Finland

Received 21 April 2005; received in revised form 22 August 2005; accepted 28 October 2005

Abstract

Constructed wetlands (CW), widely used to remove nutrients from runoff waters, transform some of the carbon and nitrogenthey receive into greenhouse gases, carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), and may therefore haveadverse atmospheric impacts. We studied seasonal and temporal changes in C degradation and emissions of CH4 and N2O of aboreal CW used to purify peat mining runoff waters 5 (in 1992) and 15 (in 2001–2002) years after construction. There was aremarkable change in the cycling of carbon in the wetland as the number of years in operation increased: the mean CH4 emissiontripled from 140 to 400 mg CH4 m−2 d−1 and the mean CO2 release (respiration) doubled from 7270 to 13 600 mg CO2 m−2 d−1

i ubled in 10y them©

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n the 10-year period. The reasons for the increased C gas production were the increased plant biomass, which doears, and a 3◦C higher average temperature in 2002 than in 1992. The N2O fluxes did not change during the study period:ean emissions were 340 and 450�g N2O m−2 d−1 in 1992 and 2002.2005 Elsevier B.V. All rights reserved.

eywords: Constructed wetland; Greenhouse gas; Methane; Nitrous oxide; Carbon dioxide; Peat mining

. Introduction

The construction of wetlands to reduce the amountf pollutants and nutrients from runoff and waste waters

rom different sources has become more common in

∗ Corresponding author. Tel.: +358 17 163591;ax: +358 17 163750.

E-mail address: [email protected] (A. Liikanen).

recent years, as a measure to prevent the pollutioeutrophication of the recipient waterbody. Construwetlands (CW) have been successfully used to rnutrients from treated wastewaters (Gale et al., 1994White et al., 2000), and recently to purify diffuse sourdischarge e.g. from peat mining (Heikkinen et al., 1995)and agriculture (Nairn and Mitsch, 2000; Woltemad2000). Peat mining, where virgin peatlands are draand dried to obtain peat for energy production, cawater pollution problems, as ditched wetland w

925-8574/$ – see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.ecoleng.2005.10.005

242 A. Liikanen et al. / Ecological Engineering 26 (2006) 241–251

contains high amounts of suspended solids and nutri-ents (Sallantaus, 1984). In wetlands, nutrients andorganic matter are removed permanently from terres-trial and aquatic ecosystems when they are convertedthrough microbial processes to gaseous compounds tobe released into the atmosphere. Some of these gases,such as carbon dioxide (CO2), methane (CH4), andnitrous oxide (N2O), can have adverse effects in theatmosphere because they act as greenhouse gases andcause global warming. Thus, there is a risk that waterpollution control can turn into an atmospheric pollutionproblem.

Natural wetlands are of importance in balancingthe atmospheric gas content. Wetland soils are knownto sequester C (Turunen, 1999). Carbon, taken up asCO2 from the atmosphere by wetland plants, endsup in the soil as plant detritus. Under saturated andoften anoxic conditions, wetland soil decompositionof detritus is slow and the CO2 uptake by plants gen-erally exceeds the CO2 release from decompositionprocesses, and C is accumulated (Alm, 1997). At thesame time as wetlands accumulate C, they can alsoemit C as CH4, which is produced in anoxic condi-tions in methanogenesis. Freshwater wetlands providea good environment for CH4 production: the supply ofsubstrates i.e. organic matter is high, and other carbondegradation processes are limited by the low availabil-ity of oxidizing agents such as O2, NO3

−, Fe3+ andSO4

2−. Natural water-saturated wetlands are generallyminor sources of N2O (Regina et al., 1996). Nitrouso tiono ofOl -l l.,1

ofr gasd nputo ys-t lyo itionp a-t useg biald beenfa e C

degradation, the release of CO2 (Paludan and Blicher-Mathiesen, 1996), and N2O production in constructed,agricultural wetlands (Paludan and Blicher-Mathiesen,1996; Freeman et al., 1997; Fey et al., 1999; Johanssonet al., 2003). Wetland construction and the area cov-ered by constructed wetlands are increasing globally(IWA, 2000), so there is an urgent need to clarify theatmospheric impact of such wetlands.

Wetland gas dynamics are also greatly affected byclimatic and weather conditions, especially by temper-ature and moisture (Martikainen et al., 1993; Mooreand Dalva, 1993; Silvola et al., 1996; MacDonald etal., 1998). Both the rate of photosynthesis (the sourceof energy and carbon in ecosystems) and the het-erotrophic microbial activities producing greenhousegases increase with increasing temperature. Moistureconditions, i.e. the height of the water table, in wetlandsdetermine the location and extent of oxic and anoxicmicrobial processes in the peat profile. Production ofCH4, a strictly anaerobic process, is highest in wet-lands with high water tables (Moore and Dalva, 1993),whereas CO2 production increases as the water tablefalls, due to higher decomposition rates of organic mat-ter in oxic conditions (Silvola et al., 1996). Emissionsof N2O have been found to be higher with lowering ofwetland water table resulting from the increase in nitri-fication activity and the increased availability of NO3for denitrification (Martikainen et al., 1993; Reginaet al., 1996). Wetland gas fluxes have a strong sea-sonal and temporal variability resulting from variationi bialp s andy1 H( ta d too et-l

onlyo -h , thes ng-t on-s wer ithin1a ralp

xide is produced as an intermediate in denitrificar as a byproduct in nitrification. In wetlands, lack2 restricts nitrification and the low supply of NO3

−imits denitrification, hence N2O production in wetands is low (Martikainen et al., 1993; Regina et a996).

When a wetland is created for the purificationunoff or waste waters, microbial processes andynamics are likely to be altered. The increased if nutrients will increase the productivity of the ecos

em (Tanner and Sukias, 1995) and the increased suppf organic matter can then increase the decomposrocesses (Nykanen et al., 1998). Enhanced degrad

ion is likely to increase the production of greenhoases, which are by- or end-products of microecomposition processes. Increased C loading has

ound to enhance CH4 emissions (Tanner et al., 1997)nd the input of NO3− has been reported to increas

n the environmental factors regulating the microrocesses behind the gas fluxes. In some seasonears a wetland can act as a source or sink for C (Alm,997) and there can be great differences in the C4Nykanen et al., 1995) and N2O fluxes (Huttunen el., 2002). Therefore, long-term studies are needebtain a holistic picture of the gas dynamics of a w

and.Previous studies have generally focused on

ne of the three (CO2, CH4, N2O) important greenouse gases. Furthermore, after loading the CWhort-term effects can differ enormously from the loerm ones, which are the most important when cidering the environmental impacts of CWs. Hereeport both seasonal and long-term changes (w0 years) as the release of CO2 (respiration), CH4,nd N2O in a boreal wetland constructed on natueatland.

A. Liikanen et al. / Ecological Engineering 26 (2006) 241–251 243

2. Methods

2.1. Study site

The study area was a constructed surface and subsur-face flow wetland on a natural peatland at Kompsasuo(Fig. 1) intended to purify draining waters from the

adjacent peat mining area (total area 180 ha, 50 hadrained to the CW). The minerotrophic Kompsasuopeatland (a fen) is situated in the southern aapa mirezone in northern Finland (65◦44′43′′N, 25◦57′80′′E).The CW (area 2.4 ha, 4.3% of the catchment area)is surrounded by coniferous forests of the mid-borealtype. The Kompsasuo peatland was ditched and dried

Fl

ig. 1. Schematic figure of the Kompsasuo peat mining area and consocated 30, 60, and 90 m from the inlet ditch. Water sampling points o

tructed wetland. Three sub-sites for gas flux measurements (black squares)f inlet and outlet ditches are indicated as black circles.


for peat mining in 1986–1989, and the CW was putinto use in 1987. Draining waters from the peat miningarea flow first into a sedimentation basin, and thereafterthrough an inlet ditch to a distribution ditch located inthe upper part of the CW. Waters flowing across theCW are conducted by a cross ditch to an outlet ditchdischarging to the River Hamarinjoki (Fig. 1). A moredetailed description of the wetland can be found inIhmeet al. (1991)andMikkonen (2003).

The biological state of the CW has changed in thecourse of time, from 1992 to 2002. The most remark-able change is the increase in above-ground plantbiomass (Table 1). The dominant vegetation species in1992 wereSphagnum angustifolium, Sphagnum papil-losum, Menyanthes trifoliata (Huttunen et al., 1996),whereas in 2001–2002 the dominant specieswereMenyanthes trifoliata, Carex lasio carpa and Poten-tilla palustris (Kilpelainen, personal communication).Between 1992 and 2002, the prevailing peat type inthe CW changed from Menyanthes–Carex–Spaghnum

to Spaghnum–Carex in the surface layer, and fromSphagnum–Carex to Carex–Sphagnum peat deeper inthe profile. Properties of runoff waters and retention ofnutrients in the CW in 1992 and 2002 are presented inTable 1. The hydraulic loading rate and loading of Ndiminished, and chemical O2 demand of the inflowingwater decreased in this time period.

2.2. Gas flux measurements

The gas fluxes of the CW were measured at threesub-sites located 30, 60, and 90 m from the distribu-tion ditch. To study long-term variations in the CW gasdynamics, the fluxes of CH4, N2O, and CO2 were deter-mined in the wetland in the summers (June–August) of1992 (5 years in operation) and 2002 (15 years in oper-ation). In 1992, the gas fluxes were determined fourtimes, on the 1 and 29 June, 27 July, and 17 August(one to two plots at each sub-site), and six times in2002, on the 3 and 17 June; 1, 15, 29 July; and 12

Table 1Weather and properties of the Kompsasuo CW in 1992 and 2002

Wetland of Kompsasuo 1992 2002

WeatherAnnual rainfall (mm) 820 510Rainfall in June–August (mm) 340 190Average air temperature in June–August (◦C) 13 (0.4) 16 (0.3)

W

6)

L

R

A el were een out andi was de .

etlandHydraulic loading rate (l s−1 km−2)Plant biomass (g m−2)Water table level (cm)

oading rate (mg m−2 d−1)Total nitrogenTotal organic carbon(NO2

− + NO3−) N

NH4+ N

Total phosphorusDissolved reactive phosphorus PO4

2− PTotal suspended solidsChemical oxygen demand (CODMn)

etention (mg m−2 d−1)Retention of PRetention of NRetention of org CRetention of suspended solidsRetention of chemical oxygen demand

verages of air temperature, loading rates, and water table levnflows is a total retention during June–August. Plant biomass

250 (32) 210 (12)780 (97) 1310 (230)

2.2 (0.24) 1.7 (0.5

73 (34) 25 (10)ND 360 (100)

22 (13) 5 (3)32 (14) 5 (3)

1 (0.3) 1 (0.2)0.3 (0.1) 1 (0.2)

89 (26) 68 (6)1080 (420) 490 (140)

0 122 13ND 12

2 220.1 0.05

measured in June–August. Retention i.e. the difference betwtermined in August. Standard errors are given in parentheses


August (four plots at each sub-site). The seasonal varia-tion in gas fluxes was studied in 2001–2002. The fluxeswere determined 15 times between August 2001 andAugust 2002 (four plots at each sub-site), two to threetimes a month during the snow-free period and twice inwinter.

The gas fluxes were measured using a static cham-ber technique (Crill et al., 1991). An aluminium frame(60 cm× 60 cm) was inserted into the soil beforetaking the flux measurements. In 1992, the frameswere placed 1 h before measurement started and wereremoved afterwards. In 2001–2002, the 12 frames(four at each sub-site), were left in the soil dur-ing the whole sampling period. Measurement startedafter an aluminium dark chamber had been placedover the frame. In winter, the chambers were placeddirectly into the soil and packed into place with snow.Gas samples (50 ml) were taken with polypropylenesyringes (Terumo, Europe, Leuven, Belgium) equippedwith three-way stopcocks (Codan Steritex, Hoejvan-gen, Denmark) from the chamber at 4–7 min intervalswithin a 24–60 min measurement period. The gas sam-ples were analyzed within 24 h in a laboratory using twogas chromatographs (Hewlett Packard P 5890 Series II,Palo Alto, CA, USA) equipped with a flame ionizationdetector for CH4, an electron capture detector for N2Oand a thermal conductivity detector for CO2 (Nykanenet al., 1995). The gas flux (mg or�g m−1 d−1) was cal-culated from the increase/decrease in the chamber gasconcentration over time using the frame surface area.I wasn ted.H lcu-l ere<

a respi-r nts( doesn f netC tmo-s

uresw ure-m inedw intot em-p 5,10,

15, 20 and 30 cm below the surface were measured witha Fluke 52 K/J thermometer.

2.3. Determination of dissolved gases andnutrients in water

Water samples for dissolved gas (CH4, N2O andCO2) and nutrient NO2− + NO3

− N, NH4+ N, total

N, SO42−, dissolved organic C, total organic C) analy-

ses were taken from the inlet and outlet ditches (10 cmdepth) of the CW and from the chamber frames (2 cmdepth) in 2001 and 2002 at the same time as the gasflux measurements were made. The concentration ofdissolved gases was determined using a headspaceequilibration technique (McAuliffe, 1971). Water sam-ples of 30 ml for gas analyses were taken with 50 mlpolypropylene syringes equipped with three-way stop-cocks, and were preserved with sulphuric acid (1 ml20% H2SO4) in the field. In the laboratory, the waterwas equilibrated in syringes with 30 ml of added N2gas by shaking for 3 min. The concentrations of CH4,N2O and CO2 were determined from the headspacegas, and the dissolved gas concentrations were calcu-lated according to Henry’s law using the values fromLide and Fredrikse (1995). The gas analyses were donewithin 24 h after sampling. Total N, NO2− + NO3

− N,NH4

+ N, SO42−, dissolved organic C, and total

organic C were analyzed according to SFS standards(SFS-ENISO 11905-1, SFSStandardization, 1998;SFS-EN ISO 13395, SFSStandardization, 1996; SFS3 -1 SS

2

sta-t effi-c gasfl ls ofw . Dif-f 2 and2 sb withah eeni nt’st

f the increase/decrease in the gas concentrationon-linear (r2 < 0.9), the measurement was rejecowever, gas fluxes that were negligible were ca

ated even if the coefficients of determination w0.9.

The measured CO2 flux is the total CO2 release fromerobic and anaerobic decomposition processes,ation of soil animals and dark respiration of plashoots and roots). This flux measured in the darkot include photosynthesis and is not a measure oO2 exchange between the ecosystem and the aphere.

The depth of the water table and soil temperatere determined while taking the gas flux measents. The depth of the water table was determith a piezometer from perforated pipes inserted

he soil close to the frames. Air temperature and teratures at the soil surface and at the depths of 3,

032, SFSStandardization, 1976; SFS-EN ISO 10304, SFS Standardization, 1992; SFS-EN 1484, SFtandardization, 1997).

.4. Statistics

The SPSS statistical package was used in theistical analyses of the data. Pearson correlation coients (two-tailed significance) were determined foruxes, gas concentrations in wetland water, leveater table, and air and surface soil temperatures

erences in gas fluxes between the summers of 199002 were tested with Student’st-test, and variationetween the sub-sites in 2001–2002 were testednalysis of variance (ANOVA) using Tukey’sb postoc test. Differences in gas concentrations betw

nlet and outlet ditches were tested with Stude-test.


3. Results

3.1. Temporal variation in gas fluxes in thesummers of 1992 and 2002

Fluxes of CH4 were higher in summer 2002 (fromJune to August), on average 400 mg CH4 m−2 d−1,than in summer 1992, when the average emissionwas 140 mg CH4 m−2 d−1 (Table 2) (P < 0.001). Theincrease in the CH4 emissions was most evident atthe sub-sites located 60 and 90 m from the inletditch. CO2 release was also higher in 2002, on aver-age 13 600 mg CO2 m−2 d−1, than in 1992, when theaverage release was 7270 mg CO2 m−2 d−1 (Table 2)(P < 0.001). However, fluxes of N2O were similar in1992 and 2002 (Table 2), with averages of 340 and450�g m−2 d−1, respectively. These N2O N fluxeswere 0.3% (in 1992) and 1.1% (in 2002) of the totalN load, and 1.0% (in 1992) and 5.7% (in 2002) ofthe NO3

− N load that the CW received. Emission ofN2O N was 1.0 and 2.2% from the N-retention in 1992and 2002, respectively.

3.2. Seasonal and spatial variation of gasdynamics in 2001–2002

Emissions of CH4 varied from 0.4 to4320 mg CH4 m−2 d−1 (380± 46 mg CH4 m−2 d−1,average± standard error of the mean): fluxes weresW tot( er

Fig. 2. Emissions of (A) CH4, (B) N2O, and (C) CO2 from the Komp-sasuo CW at sites 30, 60, and 90 m from the distribution ditch in2001–2002. Means of four plots and standard error of the means arepresented.

TA August) in the sub-sites located 30, 60, and 90 m from the distribution ditch

G 2002

ll sites 30 m 60 m 90 m All sites

m140 550 470 180 40043 45 76 23 35

m7270 13500 16600 10900 136001000 1190 1290 620 670

�

340 420 540 390 450100 110 200 170 90

mallest in winter and highest in autumn (Fig. 2A).ater CH4 concentrations in the wetland close

he chambers varied from 0.05 to 102�M CH4Fig. 3A), ranging from 14 to 33 000 times high

able 2verage gas emissions from the CW in 1992 and 2002 (June–

as flux 1992

30 m 60 m 90 m A

g CH4 m−2 d−1

Mean 305 34 67S.E. 80 5.9 18

g CO2 m−2 d−1

Mean 9050 5020 7290S.E. 1520 1350 1970

g N2O m−2 d−1

Mean 170 380 440S.E. 125 89 240


Fig. 3. Concentrations of (A) CH4 in the wetland, (B) CH4 in the ditches, (C) N2O in the wetland, (D) N2O in the ditches, (E) free CO2 in thewetland and (F) free CO2 in the ditches in 2001–2002.

than the CH4 concentration in equilibrium with theatmosphere. The water CH4 concentrations correlatedwith the CH4 fluxes (r = 0.4, P = 0.004). The CH4emissions had only weak correlations with surface soil(r = 0.218, P = 0.012) or air temperatures (r = 0.199,P = 0.014). The level of water table, which was almostconstant in the wetland during the measurementperiod (Fig. 4A), did not correlate with the CH4 flux.

The CH4 fluxes and concentrations were higher atthe sites (30 and 60 m) closer to the inlet ditch ascompared to those at the site 90 m from the ditch(Fig. 2A) (P < 0.001). Water CH4 concentrations ofthe inlet and outlet ditches, from 1.7 to 20�M CH4(Fig. 3B), were lower as compared to those of the soilwater in the wetland (Fig. 3A) (P < 0.001). During thesnow-free period, CH4 concentrations were higher


Fig. 4. (A) Level of the water table and (B) soil surface temperaturein the Kompsasuo CW at sites 30, 60, and 90 m from the distributionditch in 2001–2002. Means of four plots and standard error of themeans are presented.

in the outlet ditch than in the inlet ditch (Fig. 3B)(P = 0.026).

Fluxes of N2O varied from uptake of−1110 torelease of 2720�g N2O m−2 d−1 (280± 44�g N2Om−2 d−1 on average) (Fig. 2B). The N2O fluxes werehigh in spring and summer, but negligible in autumnand winter. When emissions were high, the variationin the flux rates was also greatest; both the minimumand maximum N2O fluxes were measured in summer.Thus, temperatures in soil (r = 0.273,P = 0.002) andair (r = 0.237,P = 0.004) correlated only poorly withthe N2O flux. Water N2O concentrations in the wetlandvaried from 6.3 to 99 nM N2O (Fig. 3C), which wasfrom 0.5 to 10 times the atmospheric equilibrium.The water N2O concentrations correlated with theN2O fluxes (r = 0.357, P = 0.017). The dynamicsof N2O were related to N availability in water:the N2O emission correlated positively with watertotal N concentration (r = 0.346,P = 0.022), and thewater N2O concentration correlated positively withthe NO3

− N concentrations (r = 0.330, P = 0.015)but negatively with the NH4+ N concentrations(r =−0.319,P = 0.019). There were no differences in

the N2O fluxes or concentrations between the sub-siteswithin the wetland. In the ditches (Fig. 3D), waterN2O concentrations were similar to those measured inthe soil water in the wetland (Fig. 3C).

Release of CO2 varied from a wintertime mini-mum of 90 to summertime maximum of 33 660 mgCO2 m−2 d−1 (9730± 530 mg CO2 m−2 d−1 on aver-age) (Fig. 2C), and had a strong correlation with soil(the best correlation,r = 0.757,P < 0.001, was with thesurface soil temperatures) and air (r = 0.6, P < 0.001)temperatures (Table 2). The concentration of free CO2in the wetland water varied from 60 to 890�M CO2(Fig. 3E), which was from 1.3 to 47 times higher thanthat in equilibrium with the atmosphere. The concentra-tion of free CO2 correlated with CO2 release (r = 0.341,P = 0.012). The CO2 release rates and free CO2 concen-trations in water were similar at the three sub-sites in thewetland. However, CO2 concentrations in the ditcheswere lower than those in the wetland (P < 0.001), butwere similar to those in the inlet and outlet ditches(Fig. 3F–E).

4. Discussion

4.1. Carbon gas dynamics

After 15 years in operation, the CW of Komp-sasuo had high CO2 and CH4 release rates. The aver-age CH4 release (380 mg CH4 m−2 d−1) from the CWi gee lm ta stCw romn ;H t-l gwr romw ;S

ants ismsti thep t

s only slightly higher than the range of averamissions (8–345 mg CH4 m−2 d−1) found in naturainerotrophic wetlands in the same region (Nykanen el., 1998; Huttunen et al., 2003). However, the higheH4 emissions (up to 4320 mg m−2 d−1) in our studyere clearly higher than those previously reported fatural sites (up to 790 m−2 d−1) (Nykanen et al., 1998uttunen et al., 2003). Agricultural constructed we

ands have also shown high CH4 release upon loadinith wastewaters (Tanner et al., 1997). The CO2 release

ates were similar to those previously reported fetlands in the boreal zone (Nykanen et al., 1998ilvola et al., 1996).High primary production and changes in pl

pecies composition were most likely the mechanhat led to the increased C cycling and CH4 emissionsn the CW. During the 10 years from 1992 to 2002,lant biomass in the CW doubled (Table 1). The highes


CH4 emissions were measured in 2002 from the sites(30 and 60 m) closest to the inlet, where the biomasswas 25–53% higher than at the farthest sub-site (90 m)(Kilpelainen, personal communication). Primary pro-duction, providing fresh organic material for degrada-tion, is known to be one of the most important variablescontrolling CH4 emissions from wetlands (Whitingand Chanton, 1993). The vegetation in the CW hadchanged, the coverage of plants with aerenchyma (vas-cular plants), herbs, sedges, and graminoides hav-ing increased, and coverage of bryophytes havingdecreased from 1992 to 2002 (Kilpelainen, personalcommunication). Vascular plants act as conduits forCH4, exhausting CH4 in their aerenchyma directlyfrom anoxic peat into the atmosphere (Bubier andMoore, 1994). Thus, the increased coverage of vascu-lar plants was the obvious reason for the increased CH4emissions.Nykanen et al. (2002)found that N fertiliza-tion in ombrotrophic peatland increased the coverage ofcotton grass, also a vascular plant, and CH4 emissionswere increased for that reason. The enhanced cyclingof C was probably not directly related to the C load:for example, release of CH4 C was 84% of from the Cload and was 25 times higher than the C retention (in2002). Thus, most of the C released as CH4 originatedfrom the C produced within the wetland.

The three degrees higher average temperature in2002 (16◦C) compared with 1992 (13◦C) might alsohave induced higher CO2 production rates and CH4emissions in 2002 than in 1992. The production ofb er-a 6;M tt ithh per-a oC edf era-tT aina .Ne atert thet 2w rt thed he

flux data collected in 2001–2002 show that tempera-ture did not correlate strongly with the CH4 fluxes inthe Kompsasuo CW. Thus, temperature might not bethe key factor controlling CH4 fluxes in constructedsystems. In the CW, the highest CH4 release was mea-sured in the autumn, when the plant biomass producedduring summer was decomposing.

In the Kompsasuo CW, the C gas dynamics were notcontrolled by rainfall and the water table as happens innatural wetlands. The level of the water table is one ofthe key parameters controlling CH4 and CO2 dynamicsof wetlands, since it determines whether degradationproceeds aerobically (above the water table) or anaer-obically (below the water table) (Moore and Dalva,1993; Silvola et al., 1996; MacDonald et al., 1998). Inthe CW, the water table was kept at a constant level,close to the soil surface, so water conditions were sim-ilar throughout the year, and mostly anaerobic degrada-tion was possible in the peat. No clear seasonal patternin CH4 emissions was observed byTanner et al. (1997)in the CWs they studied, either. A constant adjustedwater table in the constructed systems does not controlgas fluxes.

4.2. Nitrous oxide dynamics

The fluxes of N2O from the Kompsasuo CW weregenerally higher than the average fluxes (−30 to230�g N2O m−2 d−1) from natural fens in the sameregion (Regina et al., 1996; Huttunen et al., 2002). Invd ta ul-t ighera( -s offw theN therC maneo oC Ws( -eh dsa get this

oth CO2 and CH4 increases with increasing tempture (Moore and Dalva, 1993; Silvola et al., 199acDonald et al., 1998).Silvola et al. (1996)report tha

heQ10 for CO2 production in minerogenous fens wigh water table was 3.2. Assuming a similar temture response for CO2 production in the KompsasuW, the mean CO2 production in 1992 when calculat

or the temperature conditions in 2002 (mean tempure 16◦C) would have been 10 000 mg CO2 m−2 d−1.he 3◦C higher temperature in 2002 could explbout half of the higher CO2 emissions in 2002ykanen et al. (1998)reported that theQ10 for CH4missions from a minerogenous fen with high w

able was 3.7. With this temperature response,emperature corrected average CH4 emission in 199ould have been 210 mg CH4 m−2 d−1. The highe

emperature in 2002 could explain only 26% ofifference in CH4 fluxes between 1992 and 2002. T

irgin wetlands, the low availability of NO3− restrictsenitrification and N2O production (Martikainen el., 1993). Constructed wetlands receiving agric

ural wastewaters or sewage waters have shown hverage N2O emissions (3120–6750�g N2O m−2 d−1)Fey et al., 1999; Johansson et al., 2003) than the Kompasuo CW, which is loaded with peatmining runaters. Obviously, the N load to the CW increased2O emissions, as has been reported earlier for oWs (Paludan and Blicher-Mathiesen, 1996; Freet al., 1997; Johansson et al., 2003). The proportionf loaded N converted to N2O N in the KompsasuW (0.3–1.1%) was higher than that for other C

0.02–0.53%) (Johansson et al., 2003). The Intergovrnmental Panel on Climate Change (IPCC, 1996)as suggested that N2O N emissions from watersheffected by agricultural activity or municipal sewa

reatment might be 0.75% of the loaded N, and


figure corresponds well with the N2O emissions fromthe Kompsasuo CW. The emission of N2O representedonly a negligible proportion (less than 2.2%) of the Nremoved in the wetland. Thus, N removal in this CW isunlikely to lead to serious atmospheric pollution prob-lems.

4.3. Atmospheric impact of the CW

The use of the wetland for purifying runoff watersincreased the CH4 emissions in the long run. Unfor-tunately, the net ecosystem C exchange (whether theCW was a sink or source of atmospheric CO2) cannotbe determined from the measured CO2 flux, since itdoes not include the CO2 taken up in photosynthesis.The annual release of CH4 and N2O from KompsasuoCW were 3300 kg CH4 and 2.5 kg N2O (calculatedfrom the average emissions), respectively. In Finland,about 1000 ha of wetland have been constructed totreat peat mining runoff water (personal communica-tion, P. Tahtinen, Vapo Oy Energia). These CWs annu-ally contribute 1.4 Gg of CH4 and 1000 kg of N2O.Previously, peat mining fields have been estimated toproduce annually 0.4 Gg of CH4 and 0.1 Gg of N2O(Crill et al., 2000), thus CWs that purify peat miningrunoff water make a significant contribution to totalCH4 emissions in the peat mining industry. For compar-ison, natural wetlands in Finland are estimated to emitannually 540 Gg of CH4 (Crill et al., 2000) and agri-cultural soils 11 Gg of N2O (Kuusisto and Hamekoski,2 fromC aren realc es ofa

A

o-c d andR tera 0-0 ankt th-n then ela,J or

the sampling. Ella Kilpelainen is acknowledged for thevegetation data.

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