ff ects of N and P fertilization on the greenhouse gas exchange in two nutrient-poor peatlands

Effects of N and P fertilization on the greenhouse gas exchange in two nutrient-poor peatlands M. Lund, T. R. Christensen, M. Mastepanov, A. Lindroth, and L. Ström Department of Physical Geography and Ecosystems Analysis, Lund University, Sölvegatan 12, 22362 Lund, Sweden Received: 2 April 2009 – Accepted: 29 April 2009 – Published: 6 May 2009 Correspondence to: M. Lund (magnus.lund@nateko.lu.se) Published by Copernicus Publications on behalf of the European Geosciences Union.


Introduction
Peatlands have over the last millennia accumulated vast amounts of carbon (C) in their soils, amounting to ca. one-third of the world total soil C pool (Gorham, 1991;Turunen et al., 2002).The average C accumulation rates in peatlands have been estimated to 15-30 g C m −2 yr −1 (Gorham, 1991;Tolonen and Turunen, 1996;Turunen et al., 2002).Peatland vegetation takes up atmospheric carbon dioxide (CO 2 ) through gross primary production (GPP).Carbon dioxide is subsequently released back to the atmosphere via Introduction

Conclusions References
Tables Figures

Back Close
Full Screen / Esc Printer-friendly Version Interactive Discussion autotrophic and heterotrophic respiration, collectively referred to as ecosystem respiration (R eco ).The net ecosystem exchange (NEE) constitutes the sum of these two opposing fluxes.Apart from the CO 2 exchange, peatlands are also a significant source of methane (CH 4 ).Methane is produced in the waterlogged, anaerobic subsurface zone by methanogenic bacteria, while part of it is consumed in the aerobic surface zone by methanotrophic bacteria (Whalen, 2005).The current C sink functioning of peatlands is primarily explained by limited decomposition rates due to the prevailing cool, anaerobic and nutrient-poor conditions (Clymo, 1984).Due to their low pH and nutrient status peatlands generally show a non-significant nitrous oxide (N 2 O) exchange (Martikainen et al., 1993).
During the last decades, increased nitrogen (N) deposition in many parts of the world has increased the N input to the ecosystems.In addition, global warming will lead to increased mineralization rates, which will release more nutrients for plant uptake (Rustad et al., 2001;Mack et al., 2004).Nutrient-poor peatlands (bogs) are generally dominated by Sphagnum mosses.They have the ability to intercept airborne nutrients, providing a competitive advantage over vascular plants that rely on nutrient uptake by roots (Malmer and Wall én, 2005).However, at high levels of N input the Sphagnum filter will become saturated, and additional N will leach down to the vascular plant root zone enhancing growth of vascular plants (Lamers et al., 2000;Rydin and Jeglum, 2006).An increased abundance of vascular plants may reduce C sequestration due to the shift towards more easily decomposable litter (Berendse et al., 2001;Malmer and Wall én, 2005).
Fungi and bacteria are the most important decomposers in peatlands (Coulson and Butterfield, 1978;Bragazza et al., 2006).Initial rates of microbial decomposition are generally correlated with substrate N and P concentrations; it can thus be expected that microbial breakdown increases in the short-term if the nutrient limitation is reduced, leading to increased rates of CO 2 and CH 4 effluxes (Aerts and de Caluwe, 1999).In an experiment carried out in a drained fen in France, N addition led to a steady increase in total microbial biomass (Gilbert et al., 1998).Bragazza et al. (2006)  in a natural gradient of N deposition from 2 to 20 kg N ha −1 yr −1 , and found enhanced decomposition rates under higher N deposition.This was explained by removal of N constraints on microbial metabolism and increased litter peat quality.The microbial community in peatlands is likely to respond more rapidly than plants to changes such as nutrient addition, because of their higher turnover rates.
Methane emissions from peatlands can be affected in several ways if subjected to increased nutrient availability.Some soils show inhibitory effects of N addition on CH 4 oxidation rates (Crill et al., 1994;Christensen et al., 1999;Kravchenko, 2002), while others show minor or no effect (Gulledge et al., 1997;Saarnio and Silvola, 1999).In a longer time perspective, CH 4 emissions from peatlands can be increased if the abundance of vascular plants increases, through their effects on the net CH 4 flux by providing gas conduits (Joabsson et al., 1999) and by releasing fresh, organic compounds to the rhizosphere serving as substrate for CH 4 formation (Str öm et al., 2003).However, one of the roles played by root exudates is to facilitate nutrient uptake (Walker et al., 2003), which could mean diminishing root exudation with increasing nutrient availability.The net effect of nutrients on the CH 4 exchange is likely dependent on peatland type and site-specific properties (Keller et al., 2006).Increasing the amount of N in an ecosystem also increases the potential for N 2 O emission, both through nitrification and denitrification, in accordance with the "hole-in-the-pipe" conceptual model by Firestone and Davidson (1989).
Previous studies have reported diverse fertilization effects on the peatland gas exchange.Bubier et al. (2007) found decreased GPP with the highest levels of fertilization, but no effects on R eco in an ombrotrophic bog.Saarnio et al. (2003) found minor increases in GPP in N fertilized plots in a boreal fen, but no effect on annual C balance.In contrast, Mack et al. (2004) report increases in net primary production but major decreases in soil C storage in Alaskan tundra after 20 years of fertilization.Keller et al. (2005) found no major effects on soil C cycling after six years of N and P fertilization in a temperate fen.Regarding CH 4 , Granberg et al. (2001)  emissions associated with increased sedge cover.
In this study, we investigate the effects of fertilization on the exchange of all greenhouse gases (CO 2 , CH 4 , N 2 O), in two ombrotrophic peatlands.To achieve this, we have added N and P to two contrasting bog ecosystems; a south-Swedish temperate bog with high rates of atmospheric N deposition (F äjemyr) and a north-Swedish subarctic bog (Storflaket) with low atmospheric N deposition, and performed greenhouse gas measurements in situ using the closed chamber technique.We hypothesize that we will observe: (1) increased CO 2 component fluxes (GPP, R eco ) in response to N addition in Storflaket and P addition in F äjemyr; (2) limited effects on CH 4 exchange; and (3) increased N 2 O emissions as a result of increased N availability.
The wet and dry N deposition in the area is estimated to ca. 15 kg N ha −1 yr −1 (Fig. 1).and Johansson, 2008).In this area, N deposition is ca. 2 kg N ha −1 yr −1 (Fig. 1), which indicates N limitation.The experimental area can be categorized as a dry to semidry ombrotrophic habitat.The vegetation is dominated by Sphagnum mosses (S. fuscum and S. balticum), dwarf shrubs (Empetrum nigrum, Andromeda polifolia), Rubus chamaemorus and E. vaginatum.

Fertilization
Fertilization began in both sites in 2006.In F äjemyr, 16 plots (1×2 m) were randomly assigned one of the following four treatments (four replicates): high nitrogen (HN) addition, phosphorus (P) addition, nitrogen and phosphorus (HNP) addition, and control (CL).There was one additional treatment in Storflaket: low nitrogen (LN) addition (20 plots in total).Fertilization took place three times per year; in spring, summer and autumn.The amount of nutrients given to the plots starting in spring 2006 equalled 40 kg N ha −1 yr −1 (20 kg N ha −1 yr −1 in LN) and 0.5 kg P ha −1 yr −1 .In 2007, the amount of P was increased to 4 kg P ha −1 yr −1 since soil water analyses showed that the quotient between fertilization water concentration and soil water concentration was higher for N than for P. Nutrients were given as NH 4 NO 3 and NaH 2 PO 4 ×H 2 O dissolved in mire water, and spread evenly over the plots using a watering can.The CL plots received unfertilized mire water.The total amount of water used in the fertilization equalled 6 mm yr −1 .

Flux measurements
Greenhouse gas (GHG) flux measurements were performed weekly to biweekly in F äjemyr between March and November during 2007.In Storflaket, measurements were campaign-based and concentrated around the fertilization events; one week in May, two weeks in June-July and one week in September.Greenhouse gas flux measurements in connection to the fertilization events were performed on day 0, 1 (fertilization day), 2, 4, 7 and 14, in order to explore particularly short-term effects.In Introduction

Conclusions References
Tables Figures

Back Close
Full summer 2006, aluminium collars (depth: 20 cm) were inserted in each plot.The collars had grooves that were filled with water during GHG flux measurements to avoid gas leakage.Transparent chambers (40×40 cm, height: 37 cm) equipped with a fan and a pressure vent were fitted on top of the collars during GHG flux measurements.The flux measurements were carried out using a closed, flow-through chamber system.The system was equipped with a pump that continuously drew air from the chamber headspace through high density polyethylene tubing to the gas analysers at a rate of ca. 1 L min −1 .Concentrations of CO 2 were measured using an infrared gas analyser (LI-820, LiCor Inc, USA).Methane and N 2 O concentrations in F äjemyr were measured using a photoacoustic multigas infrared gas analyser (Innova 1312, Innova AirTech Instruments A/S, Denmark).We used soda lime to remove CO 2 and a Nafion tubing (Perma Pure LLC, USA) to minimize H 2 O fluctuations before sample air entered the Innova.To check the reliability and comparability of the CH 4 flux measurements, we sampled chamber headspace air in syringes (grab samples) at the same time as the Innova analyses at two occasions (1 June and 12 September), and analysed them using a gas chromatograph (GC) at Lund University (Shimadzu GC 17A).We found that the agreement for significant fluxes was high (r 2 =0.89, n=11) with a close to 1:1 relationship between GC and Innova (regression equation: GC=0.94Innova+0.088).In Storflaket, CH 4 concentrations were measured using grab samples, which were subsequently analysed on a GC at Abisko Scientific Research Station (Shimadzu GC 14B) within one day.For each plot, three separate gas flux measurements were performed on each measurement occasion: (1) transparent chamber measurements of NEE during two minutes; (2) darkened chamber (covered by beaver nylon to inhibit photosynthesis) measurements of R eco during two minutes; and (3) darkened chambers measurements of CH 4 (and N 2 O in F äjemyr) during ca. 25 min.

Ancillary measurements
In connection to the GHG flux measurements, soil temperature at 5 cm depth (T s ) using Tiny-loggers (T-0063, Amestec Oy, Finland) and water table depth (WTD) using perfo-Introduction

Conclusions References
Tables Figures

Back Close
Full Screen / Esc Printer-friendly Version Interactive Discussion rated plastic tubes were measured from just outside the collars.Vegetation inventories (point-intercept method) were performed in July 2006 and July 2007 using a 50-point frame installed over each collar.Approximately 180 m from the fertilization experiment area in F äjemyr, additional parameters such as air temperature, photosynthetic photon flux density (PPFD) and WTD were continuously recorded in connection to an eddy covariance system (Lund et al., 2007).

Data handling and statistical analyses
The GHG flux rates were calculated from the change in gas concentration as a function of time during chamber closure.Gross primary production was calculated as the difference between in time adjacent measurements with transparent and darkened chamber, respectively.For CH 4 and N 2 O fluxes, the coefficient of determination (r 2 ) values for a linear regression of concentration against time had to be higher than 0.7 for the fluxes to be considered reliable.We have adopted the micrometeorological sign convention where negative flux values indicate gas uptake by the ecosystem, while positive values represent emission to the atmosphere.
All data was tested for normality using the one-sample Kolmogorov-Smirnov test before running any parametrical statistical tests.The evaluation of the fertilization effect on the GHG flux exchange was performed in the following ways; -Repeated measures analysis of variance (RM-ANOVA) was used to test for treatment effects on GPP, R eco , NEE and CH 4 exchange.GHG flux measurements from all plots were temporally averaged for spring (March-May), summer (June-August) and autumn (September-November). Soil temperature, WTD and coverage of vascular plants, shrubs, sedges and Sphagnum mosses in each chamber were treated as covariates if significant (p<0.05).RM-ANOVA was performed in SPSS 12.0.1 (SPSS Inc., 2003) using type III sums of squares (Bubier et al., 2007).
-To take diurnal and seasonal dynamics in the CO 2 exchange into account, the Introduction

Conclusions References
Tables Figures

Back Close
Full annual time series of GPP and R eco in F äjemyr were reconstructed for each plot separately using the following simple nonlinear models (Saarnio et al., 2003); where b 0 , b 1 and b 2 are regression parameters.Stepwise regression was used to test whether T s and WTD were significant variables in explaining respiration fluxes.Half-hourly readings of PPFD, T s and WTD were derived from continuous measurements on the bog close to the measurement site (Lund et al., 2007).Measurements on day 0, 1 and 2 were averaged when computing the R eco function to minimize temporal autocorrelation.The time series of GPP and R eco were reconstructed between March and November for each plot.Subsequently, NEE was calculated as the sum of GPP and R eco .Treatment effects on the integrated CO 2 flux components and NEE was tested for using one-way analysis of variance (ANOVA), and difference from control was tested with two-sided Dunnett post hoc test.

Short-term fertilization effect
An immediate short-term effect was generally seen in the R eco measurements during the first days after nutrients were added.In Storflaket, N addition caused an instantaneous increase (Fig. 2).Respiration fluxes were significantly higher than CL (two-sided Dunnett test: p<0.05) on day 1 (fertilization day) for HNP plots in spring, for LN and HN plots in summer, and for LN and HNP plots in autumn.Respiration fluxes were also significantly higher for HN plots on day 2 in autumn.In F äjemyr, R eco was significantly higher for HN, HNP and P plots on day 1 in autumn.No such effect was seen in the GPP or CH 4 fluxes in neither of the two sites.

Exchange of N 2 O
The N 2 O exchange in F äjemyr was generally close to zero, with both negative and positive fluxes.Interestingly, N 2 O emission peaks of ca. 150 µg N 2 O m −2 h −1 were detected.In total six fluxes exceeded 100 µg N 2 O m −2 h −1 , where five out of six occurred in N plots.Average N 2 O fluxes during the measurement period were 24.4,10.9, −16.1 and −6.1 µg N 2 O m −2 h −1 for HN, HNP, P and C plots, respectively.There were no significant differences between these averages (ANOVA: p=0.256).Introduction

Conclusions References
Tables Figures

Back Close
Full Screen / Esc Printer-friendly Version Interactive Discussion

Repeated measures ANOVA
Statistics from the repeated measures analysis (RM-ANOVA) are shown in Table 1, and estimated marginal means (average fluxes adjusted for covariates, if any) in Table 2.In F äjemyr, there was a significant treatment effect on GPP (p=0.026).Phosphorus fertilized plots constantly showed higher GPP rates than plots that did not receive P (Table 2).In summer, all nutrient addition treatments had higher CO 2 uptake rates on average compared to control, while in spring and autumn the rates in HN plots were similar to CL plots.There was no significant treatment effect for R eco (p=0.272), even though average values were always higher for fertilized plots than CL plots.A similar picture appeared for NEE (p=0.150),where P plots tended to have a continuously higher net CO 2 uptake.There was a close to significant treatment effect for CH 4 fluxes (p=0.091),where HN and P plots had on average higher rates than CL plots, while the HNP plots had counter-intuitively lower rates.
In Storflaket, both GPP (p=0.001) and R eco (p=0.018) had statistically significant treatment effects.Nitrogen plots had the highest rates of both GPP and R eco , while average flux rates in P plots were similar to CL plots (Table 2).There was a strong tendency that fertilization decreased NEE in Storflaket (p=0.056), because GPP increased more than R eco .The CH 4 fluxes were relatively low in this site and spatial variation was high, which led to no significant treatment effect (p=0.690).

Regression modelling
The nonlinear regression models showed good performance in explaining the variation in R eco and GPP in each plot in F äjemyr; r 2 values range between 0.72-0.95and 0.65-0.96for R eco and GPP, respectively ( tween March and November correlated significantly with shrubs (p equalled 0.002 and 0.003 for GPP and R eco , respectively).Since this effect is not related to the fertilization treatments, the sums from each of the 16 plots were detrended using the residuals from the regression analysis with shrubs as independent variable, and then adding the average flux.This procedure removes the trend associated with shrubs, but does not affect the average fluxes.
The average sums of R eco , GPP and NEE in all plots were 634, -509 and 125 g C m −2 , respectively (Fig. 3).As was the case for the RM-ANOVA analysis, there was a significant treatment effect for GPP (p=0.009), and Dunnett test showed significantly different GPP sums for P and HNP plots compared to CL plots (p equals 0.003 and 0.035, respectively), while GPP sum in HN plots was not significantly different (p=0.102).For R eco , no significant treatment effect was found (p=0.251), even though fertilized plots on average respired more CO 2 than CL plots.Nor was any significant treatment effect found for NEE (p=0.292).

Discussion
The vegetation inventories showed no major shifts in vegetation distribution during the two year fertilization period, with the exception of shrub increase in HNP plots in Storflaket.The increased nutrient availability may have caused changes in the competitive pattern in the ecosystems, but such changes are not yet detectable.Accordingly, this study reports effects on the greenhouse gas exchange dynamics that is not derived from vegetation shifts, but instead from biochemical and microbial responses to fertilization.Gross primary production and R eco in both sites correlated significantly with the amount of vascular plants (also amount of shrubs in F äjemyr).Higher abundance of vascular plants, at the cost of mosses, leads to higher CO 2 flux components.However, these indices may also act as a proxy for biomass or LAI, which is known to correlate with GPP and R eco (Lindroth et al., 2007).For CH 4 , there was a significant correlation Introduction

Conclusions References
Tables Figures

Back Close
Full with sedges in F äjemyr, and close to significant for Storflaket.Sedges (E.vaginatum) add an additional transport pathway through their aerenchymateous tissue (Joabsson et al., 1999) for CH 4 to escape from the anaerobic zone directly to the atmosphere, and hence avoid being oxidized to CO 2 in the aerobic zone.
A short-term (hours to days) response in R eco to nutrient addition was generally seen in both sites (Fig. 2).Although the addition of water during dry conditions may have stimulated microbial activity, the effect was prominent only in plots receiving nutrients.Due to the short-term nature of the response, we believe that nutrient-limited soil microorganisms were responsible for the increased CO 2 effluxes, which demonstrates the potential for higher decomposition rates in conditions of alleviated nutrient limitation.This interpretation is also supported by the lack of effect on GPP.Berg and McClaugherty (2003) discuss that early stage decomposition of easily decomposable material is stimulated by high levels of major nutrients (N, P, S), while in a later stage when degradation of lignin controls litter decomposition, N may even have a suppressing effect on degradation.Thus, when nutrients were added to the plots, there was an increase in decomposition of easily decomposable material.After some time, in combination with decreased nutrient availability due to plant uptake etc., the fresh C substrate pool became exhausted and the stimulating effect of increased nutrient availability ceased.Whether decomposition becomes suppressed in a longer term (Berg and McClaugherty, 2003) can not be seen in our data.
Both in the RM-ANOVA analysis and the regression modelling it was found that P was significantly stimulating photosynthetic CO 2 uptake in F äjemyr.The increase was ca.36% as compared to the CL plots as calculated from the GPP time series sum.Nitrogen addition also seemed to stimulate growth, but mainly during summer (Table 2).This may be because Sphagnum mosses represent the main part of the ecosystem CO 2 uptake during early and late part of the growing season.During summer when the vascular plants are active, the relative importance of Sphagnum mosses is decreased.Earlier studies have found that Sphagnum does no longer capitalise on increased N input at high N deposition levels and that additional N will leach through the Sphagnum

Conclusions References
Tables Figures

Back Close
Full Screen / Esc Printer-friendly Version Interactive Discussion layer and become available for vascular plant uptake (Lamers et al., 2000;Rydin and Jeglum, 2006).Our GPP data from F äjemyr suggests that Sphagnum mosses are not N limited, due to the similarity between N and CL plots during spring and autumn, but rather P limited.It also suggests that the Sphagnum filter fails (Lamers et al., 2000) and that N leaches down to vascular plant root zone causing increased growth of vascular plants during summer.It can not be excluded that this already occurs in F äjemyr at the present N deposition of ca. 15 kg N ha −1 yr −1 .In a longer time perspective, this may act to decrease the net C sink in this ecosystem (Berendse et al., 2001;Malmer and Wall én, 2005).
In Storflaket, GPP was significantly stimulated by N addition, while P addition had low or no effect.In addition, also R eco showed a significant treatment effect with higher rates in N plots.In this area where N deposition is low, both plants and microorganisms are limited by N, while addition of P does not seem to have any major effect.Our results, based on a whole-ecosystem approach using in situ GHG flux measurements, support the findings of Aerts et al. (1992), who performed a comparable fertilization experiment also in Swedish mires that focused on Sphagnum productivity.Their results indicated that growth was mainly N limited in an area with low N deposition, while it was primarily P limited in a high N deposition area.However, in a four-year fertilization experiment (Aerts et al., 2001) no effects on productivity were found.They argue that nutrient addition may initially ease the nutrient limitation to plant growth, while in a longer term other environmental factors become increasingly important.The modelled CO 2 components in F äjemyr (Fig. 3) in this study can be compared to flux data from a nearby eddy covariance measurement site (Lund et al., 2007).Sum of NEE, GPP and R eco during the same period as the chamber measurements, calculated according to Lindroth et al. (2007), are −55.7,−548 and 492 g C m −2 , respectively.
There may be several reasons for higher R eco and similar GPP sums found in the chamber measurements.Firstly, the fertilization area is drier than the footprint area of the eddy covariance tower; higher respiration rates can therefore be expected.Secondly, photosynthesis may be underestimated in the chamber measurements since climate Introduction

Conclusions References
Tables Figures

Back Close
Full Screen / Esc Printer-friendly Version Interactive Discussion controlled chambers were not used.When measuring NEE with transparent chambers photosynthesis may be reduced due to (1) humidity increases forming droplets on plant leaves preventing CO 2 uptake through stomata; (2) temperature increase inside chamber during measurements will increase respiration rates to a higher extent than photosynthetic rates; and (3) the CO 2 concentration reduction inside chambers during measurements will affect photosynthetic uptake (Hutchinson and Livingstone, 2001;Welles et al., 2001;Kutzbach et al., 2007).Thirdly, there are also potential errors associated with eddy covariance measurements such as underestimation of respiratory fluxes due to vertical and horizontal advection (Baldocchi, 2003).Consequently, we believe that GPP is slightly underestimated in the chamber measurements while R eco is accurate.This propagates into the estimation of the growing season NEE in which 14 out of 16 plots acted as a CO 2 -C source to the atmosphere.
No treatment effect was found for CH 4 emissions, but the positive correlation with sedges indicates a potential for increased CH 4 flux in the future, as sedges are thought to become more abundant if the nutrient limitation is reduced (Rydin and Jeglum, 2006).In F äjemyr, P and HN plots showed on average (insignificantly) higher CH 4 emissions than the CL plots, while HNP plots did not show further increases in emissions as could be expected.However, the combined effect of single factor responses is often found to be non-additive (Shaw et al., 2002).For HN plots, increased CH 4 fluxes may be associated with inhibition of CH 4 oxidation by methanotrophic bacteria.In HNP plots decreased flux can be due to decreased plant root exudation rates, because of alleviated nutrient limitation, leading to decreased substrate availability for methanogenic bacteria.
In addition to the gaseous C exchange, our data set indicates a strong possibility for increased N 2 O emissions with increased N availability.Even though the flux measurements were performed at a low temporal resolution, several N 2 O peaks were detected.This signifies the importance of taking N 2 O exchange into account when considering N fertilization as a means of increasing C sequestration in ecosystems, since N 2 O is a strong greenhouse gas that may offset potential increases in CO 2 uptake.Introduction

Conclusions References
Tables Figures

Back Close
Full -In accordance with our first hypothesis, GPP in the high N deposition site F äjemyr was significantly stimulated by P addition, while in Storflaket, both GPP and R eco were significantly increased in plots receiving additional N.
-Complex and nonlinear responses to nutrient addition were seen for the CH 4 exchange, while N 2 O peaks were detected in N fertilized plots.
-Future GHG flux exchange in nutrient-poor peatlands is dependent on changes in ecosystem structure such as plant composition due to increased nutrient availability, along with climatic changes associated with global warming.Full
investigated bogs Introduction found that N addition slightly decreased emissions, while Nyk änen et al. (2002) found increased Introduction

The water table is generally below the surface, which causes the topographical pattern of F äjemyr to be dominated by hummocks, lawns and carpets. Hollows and open
pools are scarce.These rather dry conditions allow the existence of dwarf shrubs in the study area, mainly Calluna vulgaris and Erica tetralix.Moss layer is dominated by Sphagnum magellanicum and S. rubellum.Sedges, mainly Eriophorum vaginatum, • 58 E, alt: 380 m) close to Abisko Scientific Research Station.Long-term mean annual temperature and precipitation are −0.8 • C and 304 mm, respectively.The mire is underlain by permafrost with an active layer of 60-70 cm in the study area in late summer( Åkerman  Introduction of), while in Storflaket, there was a significant increase of shrubs in HNP plots (t-test: p=0.04).

Table 3
). Soil temperature was always a significant variable in explaining R eco .Water table depth was significant in 13 out of 16 plots, constantly showing a negative correlation with R eco , indicating that lowered water table leads to increased respiration rates.The resulting sums of the time series modelling of GPP and R eco in each plot be-

Table 1 .
Statistics (F and p values)from the repeated measures analysis (RM-ANOVA) for treatment (between-subject) effects on CO 2 component fluxes (NEE, R eco , GPP) and CH 4 fluxes in F äjemyr and Storflaket.Shown are also F and p values for any significant covariates used in the analyses.