Wetlands Ecol Manage (2018) 26:763–774 https://doi.org/10.1007/s11273-018-9607-x(0123456789().,-volV)(0123456789().,-volV) OR IGINAL PAPER Interacting effects of elevated atmospheric CO and hydrology on the growth and carbon sequestration of Sphagnum moss . . . Thomas R. Newman Neal Wright Barbara Wright Soﬁe Sjo¨gersten Received: 21 September 2017 / Accepted: 19 March 2018 / Published online: 1 June 2018 The Author(s) 2018 Abstract Peatlands are a critical carbon store com- Elevated CO levels increased Sphagnum height and prising 30% of the Earth’s terrestrial soil carbon. dry weight but the magnitude of the response differed Sphagnum mosses comprise up to 90% of peat in the among species. The most responsive species, S. fallax, northern hemisphere but impacts of climate change on yielded the most biomass compared to S. papillosum Sphagnum mosses are poorly understood, limiting and S. capillifolium. Water levels and the CO development of sustainable peatland management and treatment were found to interact, with the highest restoration. This study investigates the effects of water level (1 cm below the surface) seeing the largest elevated atmospheric CO (eCO ) (800 ppm) and increase in dry weight under eCO compared to 2 2 2 hydrology on the growth of Sphagnum fallax, Sphag- ambient (400 ppm) concentrations. Initially, CO ﬂux num capillifolium and Sphagnum papillosum and rates were similar between CO treatments. After greenhouse gas ﬂuxes from moss–peat mesocosms. week 9 there was a consistent three-fold increase of the CO sink strength under eCO . At the end of the 2 2 experiment, S. papillosum and S. fallax were greater Electronic supplementary material The online version of sinks of CO than S. capillifolium and the - 7cm this article (https://doi.org/10.1007/s11273-018-9607-x) con- water level treatment showed the strongest CO sink tains supplementary material, which is available to authorized strength. The mesocosms were net sources of CH but users. 4 the source strength varied with species, speciﬁcally S. T. R. Newman (&) fallax produced more CH than S. papillosum and S. School of Geography, Geology and the Environment, capillifolium. Our ﬁndings demonstrate the impor- University of Leicester, Leicester, UK tance of species selection on the outcomes of peatland e-mail: firstname.lastname@example.org restoration with regards to Sphagnum’s growth and S. Sjo ¨ gersten GHG exchange. School of Biosciences, The University of Nottingham, Nottingham NG7 2RD, UK Keywords C sequestration Elevated CO e-mail: Soﬁe.Sjogersten@nottingham.ac.uk Greenhouse gas ﬂuxes Peatland Sphagnum N. Wright B. Wright Climate change Micropropagation Services, Ley Springs, Loughborough Road, East Leake, Loughborough, Leicestershire LE12 6NZ, UK e-mail: Neal@microprop.co.uk B. Wright e-mail: Barbara@microprop.co.uk 123 764 Wetlands Ecol Manage (2018) 26:763–774 Introduction Atmospheric CO concentration has increased since pre-industrial times to ca. 408 ppm (ESRL Peatlands are important in the global carbon (C) cycle NOAA 2015) and is predicted to increase to 800 ppm due to their function as a long-term carbon store. This by the year 2100 (IPCC 2013). The effects of this store has built up through millennia due to very low elevation of atmospheric CO on Sphagnum moss are decomposition rates that are surpassed by productivity currently disputed, with studies indicating an increase rates (Clymo and Hayward 1982) making peatlands in growth rate (Jauhiainen and Silvde 1999; Heijmans net carbon sinks. It’s been estimated that today’s et al. 2001a; Saarnio et al. 2003), decreases in growth peatlands store ca 30% (* 550 Pg C) of the Earth’s rate (Grosvernier et al. 2001; Fenner et al. 2007) and terrestrial soil carbon while comprising only 3% of the no response (Van der Hejiden et al. 2000; Hoosbeek Earths land area (Gorham 1991;Yu 2012). et al. 2002; Toet et al. 2006). Contrasting responses Within peatlands in the northern hemisphere, between Sphagnum species are thought to be the cause Sphagnum mosses are often the dominant species, of some of the variation. Toet et al. (2006), found comprising up to 90% of peat (Clymo 1987). Sphag- increased abundance in S. recurve and no difference in num plays an important role in peatland formation and S. palustre when they were subjected to an elevation of ecological resilience, through its low decomposition 180 ppm above ambient, while S. fuscum did not rate due to its recalcitrant and antibacterial nature respond to elevated CO (350, 700, 1000 and (Kroken et al. 1996) and by creating acidic, anoxic and 2000 ppm) at all (Jauhiainen et al. 1997). Further- nutrient-poor conditions within peatlands (Amesbury more, contrasting responses have been seen between 2013; Hajek and Vicherova 2014). Sphagnum’s studies conducted on the same species. For example, S. extraordinary physiology allows it to hold 16–26 magellanicum, was found to increase in growth rate by times its weight in water (Bold 1973; Amesbury Heijmans et al. (2001b) when subjected to 560 ppm 2013), in both live and dead non-decomposed Sphag- CO , while Heijmans et al. (2001a) reported a greatly num, depending on species. This buffers the impact of reduced growth rate. This suggests factors other than dry weather conditions on peat moisture levels, species are affecting Sphagnum’s response to elevated preventing the peat from drying out and decomposing. CO . Peatlands are classiﬁed as such once an organic soil Hydrology has been identiﬁed as one of the most layer of 30 cm or greater has formed; for northern important factors controlling Sphagnum growth and peatlands, the average peat depth is 1.3–2.3 m, but peatland carbon dynamics (Strack and Waddington peat depth can extend to 15–20 m in some peatlands 2007; Turetsky et al. 2012; Lanta and Kantorova 2015 (Clymo et al. 1998; Turunen et al. 2002). Accumula- etc.). Water availability impacts both C sequestration tion rates of Sphagnum peat are approximately and growth rate (Glime 2007), due to there being an -1 0.61–3.9 mm year (Royles et al. 2012; Amesbury optimum moisture content for Sphagnum photosyn- 2013), due to Sphagnums slow growth rate within the thesis (Rice and Giles 1996). Varying physiologies cold, nutrient poor peatlands (Strack 2008). The slow mean that different Sphagnum species vary in sensi- growth rate is important in the context of peatland tivity to changes in water level, each often growing management as once the peat is lost, the peatland will best within their own hydrological niche (Rydin and only recover slowly over centuries/millennia. Degra- Mcdonald 1985; Grosvernier et al. 1997; Taylor et al. dation of peatlands through drainage, mining and 2015). For example, mature S. fallax is observed to removal of Sphagnum allows stored C to be released struggle at low water levels (Carroll et al. 2009), while back into the atmosphere CO , through decomposition having higher growth rates than any other species in of the exposed peat. It is estimated that UK peatlands wet conditions (Buttler et al. 1998). Recent studies -1 are a net source of 3.72 Mtn CO year (Worrall 2eq have also highlighted the importance of precipitation et al. 2011), with only 1% of Englands deep peats on Sphagnum dominated peatlands carbon dynamics considered to be in an undamaged state (Natural and species distribution (Robroek et al. 2009; Radu England 2010). This makes restoring peatlands a and Dual 2018). Nijp et al. (2014) found precipitation priority, as restored peatlands not only protect the reduced the negative impacts of drought, while already sequestered carbon, but can reinstate their Robroek et al. (2007) found precipitation allowed carbon sink capacity (Strack 2008). species to survive outside of their hydrological niche. 123 Wetlands Ecol Manage (2018) 26:763–774 765 Materials and methods Precipitation also affects Sphagnum C uptake, with species responding differently to varying precipitation Experimental set up frequencies (Nijp et al. 2014). Water limitation has also been found to affect the impacts of other factors, Peat cores were collected from Cadishead and Little such as temperature and vegetation’s impact on CO and CH ﬂuxes (Silvola et al. 1996; Leppala et al. Woolden Mosses (North West England, UK M44 5LR of Grid ref: SJ697953). This site is typical of drained 2011). Currently large tracts of damaged peatlands are and degraded cut over peatlands in the UK. The site is being restored in the UK via application of Sphagnum currently under restoration and some areas have been containing beads and gel to encourage growth on re-ﬂooded. It is characterized by predominantly bare degraded peatland sites (Hinde et al. 2010; Wittram peat with cotton grass and birch growing around the et al. 2015). However, information is limited with edges with Sphagnum growing only in areas where regards to which species are most effective in quickly reintroduction has been attempted. To account for variation within the peat substrate, establishing and recreating peatland functions such as C sequestration, particularly in the context of global all 72 peat cores (approximately 18 cm 9 16 cm) were collected from random locations within the non- change, e.g., elevated atmospheric CO concentra- tions. Such information is urgently needed to ensure ﬂooded part of the peatbog. Collection and transporta- tion of cores took place within 8 h, after collection that restoration efforts are as effective as possible under future elevated atmospheric CO levels. For cores were refrigerated at ? 4 C. example, some studies indicate that elevated CO Three weeks after collection, peat cores were then levels may result in increased CH ﬂuxes peatlands cut into 17.4 cm 9 11 cm cylinders and transplanted dominated by Sphagnum species (Saarnio et al. into 17.4 cm 9 15.6 cm clear plastic, cylindrical polypropylene containers. The treatments were 2000, 2003; Ellis et al. 2009) but effects differ among studies made across European peatland sites (Silvola applied in a randomised block design (n = 4) with species (S. fallax, S. papillosum and S. capillifolium) et al. 2003). To date, a hand full of studies have considered a possible interaction between hydrology and water level (1, 4 and 7 cm below the peat surface) as the treatments. Each combination of species and and raised atmospheric CO on the growth of Sphag- num moss (e.g., Jauhiainen et al. 1997; Toet et al. water level was repeated four times within each 2006) despite that interactions with hydrology may growth room. The randomized block design was used control how different Sphagnum species respond to to account for any possible variations of conditions elevated CO . Indeed, interactions between hydrology within each growth room. The two CO treatments 2 2 and elevated atmospheric CO concentrations may used were 400 and 800 ppm CO (ambient and cause the contrasting responses of Sphagnum species elevated), 800 ppm is the predicted 2100 concentra- tion (IPCC 2013), due to the slow growth rates of to elevated CO found in previous studies. Such interaction would have important implications for Sphagnum this was chose as a good analogue for future restoration attempts. To achieve the two CO peatland restoration, suggesting that selecting the correct species for the environment is critical for treatments cores were randomly allocated to one of two growth rooms in which the atmosphere was restoration success. The aim of this study was to determine whether adjusted to the target CO concentration. Apart from the CO concentration the settings in the growth room there is an interaction between hydrology and elevated atmospheric CO on the growth and greenhouse gas were identical: Day length consisted of a 16 h -2 -1 (GHG) exchange of three Sphagnum moss species photoperiod (300 lmol m s ), including a 1 h -2 -1 grown on peat-monoliths. To achieve this aim two dawn/dusk and 8 h night (0 lmol m s ). Daytime speciﬁc hypotheses were tested: (i) elevated atmo- conditions were set to a temperature of 21 C and a relative humidity of 50%, while night-time conditions spheric CO will have a greater positive effect on S. fallax growth and GHG ﬂuxes than other species; (ii) were set to 15 C and 75% respectively. low water table will inhibit the effect of elevated CO on Sphagnum growth. 123 766 Wetlands Ecol Manage (2018) 26:763–774 Water levels within the growth rooms where consistent throughout the course of the experiment: Temperature was Once the peat cores were placed in the growth rooms, measured at 27.5 ± 0.1 C in direct light with an air the three water level treatments were applied to the temperature of 21.2 ± 0.1 C, relative humidity, cores, which were then allowed to equilibrate for 52 ± 1% and ﬁnally light was 234 ± 0.4 lmol -2 -1 1 week. All water applied to the cores initially and m s . throughout the experiment was rainwater that was Sphagnum height was measured using an adaption ﬁrstly collected as runoff from glasshouses located in of the cranked wire method detailed by Clymo (1970). East Leake, Loughborough, Leicestershire. The rain- Initial height of the moss (peat level) was marked on water was then ﬁltered through a sand gravity ﬁlter, the side of the clear plastic container; from this the leaving the rainwater free of particulate contaminants height of the tallest Sphagnum plant in each individual such as dirt washed from the glasshouse surface. container was measured to the nearest mm. Rainwater was used to simulate ombrotrophic peat- As Sphagnum was grown from micro propagated land conditions. Water level treatments were moni- plants, the establishment rates of the different Sphag- tored daily for the ﬁrst month and then once every num species could be quantiﬁed. This was inferred 2 days thereafter via visual inspection. If needed water through the Normalized Difference Vegetation Index was applied to the cores using a pressurised water (NDVI), a numerical indicator that uses the visible and sprayer to reduce disturbance to the peat surface. This near-infrared bands of the electromagnetic spectrum kept variation in water level to within 1 cm throughout to assess crop healthiness (Govaerts and Verhulst the course of the experiment. 2010). This was done using a hand held device (GreenSeeker , Trimble Navigation Ltd., Sunnyvale Sphagnum application California, USA), which gave a reading between 0 and 1 where 0 indicates bare soil and 1, established green After 1 week, each species of Sphagnum was applied vegetation (FSNAU 2016). The Green Seeker was to the relevant core using liquid gels, currently used held approximately 10 cm above the Sphagnum for application to peatlands as part of restoration surface, with a reading for each core taken over 5 s programmes, containing micro propagated material of while the device was continually moved over the each species. The Sphagnum suspended within the gel whole surface of the core, giving an average reading was micropropagated from source material originating for each core. from the Peak Districk, UK, by MicroPropagation The total Sphagnum biomass production from each Services (MicroPropagation Services (EM) Ltd 2015). container was quantiﬁed at the end of the experiment. The micro-propagated Sphagnum was suspended For this all Sphagnum growth in each container was within a gel containing basic nutrients, at an approx- removed, weighed (wet weight), dried at 70 C and imate concentration of 150,000 individual plants per weighed again (dry weight). L, which ranged from 1 to 5 mm in length (for CO ﬂux and evaporation rate from each core was example gel see SI. 1). The gel was applied at a rate of measured using an EGM-4 Infrared Gas Analyser 3L per m (0.072L per core) using a large pipette (IRGA) connected to a CPY-3 using an open dynamic before being spread evenly over the core surface. The system (PP systems, Hitchin, UK). CO ﬂux rates were application rate is the same as trials conducted using recorded every minute over a 10 min period. IRGA the same gel in the restoration of the Cadishead time wishing was every 8 s, from which a linear peatland (Wright et al. 2012). regression is calculated to determine CO ﬂux over each 1 min period. To ensure ﬂuxes were not affected Measurements of Sphagnum performance by disturbance following the installation of the CPY-3 and GHG ﬂuxes chamber over the mesocosms, we used only the ﬁnal three readings, these were averaged and used for Measurements of Sphagnum performance and CO subsequent data analysis. Positive ﬂuxes represent a ﬂuxes were made bi-weekly for each parameter apart net release of CO from the mesocosms. from CH gas samples and dry weight, which were For CH ﬂux determination, gas samples were determined at the end of the experiment. Conditions collected from each container using a modiﬁed version 123 Wetlands Ecol Manage (2018) 26:763–774 767 of the closed static chamber method (Collier et al. 2014). Each container was carefully sealed using a lid with a septum in its centre giving a headspace of 0.11 dm . Then, using a 30 ml syringe with a 25 gauge hypodermic needle, gas samples were then taken after 1, 30 and 60 min. Samples were then transferred into 12 ml pre-evacuated exetainers before each sample was analysed via gas chromatography (GC-2014; Shimadzu, UK) using a Flame Ionization Detector (FID), with hydrogen as the carrier gas. The oven temperature was 40 C, the injector 80 C and the FID -1 Fig. 1 The effect of elevated CO and species on NDVI 250 C. The ﬂow rate was 30 ml min ; the column (mean ± SE) over the 13 week experimental period. Black was a HayeSEp N packed column, 60/80 mesh. lines represent atmospheric CO of 400 ppm; light grey lines Measurements for each container were analysed using represent atmospheric CO of 800 ppm. Diamonds and solid lines represent S. fallax; squares and dashes lines represent S. a linear regression model and the ideal gas law to papillosum; triangles and dotted lines represent S. capillifolium calculate gas ﬂux rate for each container, which was -2 -1 then used to calculate CH ﬂux in mg m h where capillifolium established slower than under the ambi- positive gas ﬂuxes represent net CH emissions. ent treatment (F = 2.96, P = 0.061). (2,51) Sphagnum grew taller under eCO (F = 16.34, 2 (1,51) Statistical analysis P\ 0.001) but this effect varied among species (F = 5.31, P= 0.008) with S. fallax responding (2,270) The effects of time, hydrology, species and CO most signiﬁcantly to eCO (F = 48.78, P\ 0.001) 2 (2,51) treatment and their interactions on height, evapora- (Fig. 2a). S. papillosum height was also found to tion, CO ﬂux and NDVI were tested using repeated increase growth under eCO (2.9 mm taller by week 2, measures analysis of variance (ANOVA) after box- 13) the effect was not as large as S. fallax (7.4 mm cox transformations had been performed where nec- taller by week 13), while S. capillifolium indicated no essary, to normalize the data. Time was treated as a difference in height between the treatments (Fig. 2a). categorical variable, with each measurement week The lowest water level (7 cm below peat surface) had treated as a separate category. Treatment effects on shorter plants (near signiﬁcant water level effect: dry weight and CH ﬂux were tested using ANOVA F = 2.49, P = 0.093) than both the medium (4 cm (2,51) with hydrology, species, and CO treatment as ﬁxed below peat surface) and high (1 cm below peat effects, where CO ﬂux was normalised to the CO 2 2 surface) water levels (Fig. 2b). ﬂux from the ﬁrst set of measurements (week 3). This The eCO treatment yielded 90.3% increase in removed the impact of variation in the initial CO ﬂux biomass (294.2 ± 47.2 g m ) compared to the ambient rates from the cores. All calculations were performed treatment (154.6 ± 23.7 g m )(F = 30.94, P \ (1,51) using the data analysis program GenStat 17th Edition 0.001). However, the effect size varied among species for windows 10.1 (VSN International 2011). (F = 4.17, P = 0.021), with S. fallax showing a (2,51) 129.7% increase in biomass production under eCO -2 (eCO 142.4 ± 17.1 g m , ambient Results -2 62.0 ± 9.8 g m ), followed by a 101.0% increase -2 for S. capillifolium (eCO 66.8 ± 16.1 g m , ambi- Growth -2 ent 33.2 ± 6.4 g m ) and ﬁnally a 43.2% increase -2 for S. papillosum (eCO 85.068 ± 14.0 g m , ambi- Different establishment rates were found between -2 ent 59.4 ± 7.6 g m ) (Fig. 3a). species (F = 7.20, P = 0.002), with S. fallax (2,51) Sphagnum grown at higher water level treatments establishing fastest with NDVI [ 0.8 in week 7, which produced more biomass (F = 7.64, P = 0.001) (2,51) was 2 weeks earlier than S. papillosum and 3 weeks (Fig. 3a). The eCO and water level treatments earlier than S. capillifolium (Fig. 1). Under eCO S. interacted (F = 4.13, P = 0.022) with the high (2,51) fallax and S. papillosum established faster, while S. 123 768 Wetlands Ecol Manage (2018) 26:763–774 Fig. 2 a The effect of elevated CO and species on plant height b The effect of water level on plant height (mean ± SE) over the (mean ± SE) over the 13 week experimental period. Black lines 13 week experimental period. Diamonds and black line represent atmospheric CO of 400 ppm; light grey lines represents a water level 1 cm below peat surface; squares and represent atmospheric CO of 800 ppm. Diamonds and solid dark grey line represent a water level 4 cm below peat surface; lines represent S. fallax; squares and dashes lines represent S. triangles and light grey line represent a water level 7 cm below papillosum; triangles and dotted lines represent S. capillifolium. peat surface Fig. 3 a The effect of species on dry weight (mean ± SE) (mean ± SE) recorded at the end of the 13 week experimental recorded at the end of the 13 week experimental period. For period. For each water level, black bars represent growth under each species, black bars represent growth under 400 ppm 400 ppm atmospheric CO ; light grey bars represent 800 ppm atmospheric CO ; light grey bars represent 800 ppm atmo- atmospheric CO 2 2 spheric CO b The effect of water level on dry weight water level resulting in the largest increase of 142.0% ﬂux rates over the ﬁrst 7 weeks period. After 7 weeks -2 (79.9 ± 9.2 g m ) in biomass under eCO compared there was a large increase in the CO uptake (increas- 2 2 -2 to a 66.0% increase (35.6 ± 8.1 g m ) at the 4 cm ingly negative CO ﬂux) under eCO (F = 14.59, 2 2 (1,51) -2 and 19.2% (8.5 ± 5.4 g m ) increase at the 7 cm P \ 0.001). Speciﬁcally, CO uptake was more than water level treatments (Fig. 3b). 5-times greater in week 7 and consistently 3-times greater than the ambient treatment from week 9 to the Gas ﬂux end of the experiment (Fig. 4a; Time 9 eCO inter- action: F = 27.28, P \ 0.001). Increasingly (5,270) In parallel with the NDVI and height data, elevated lower water levels were found to have a signiﬁcantly and ambient CO treatments had almost identical CO larger CO uptake (F = 5.00, P = 0.010) (Fig. 4b). 2 2 2 (2,51) 123 Wetlands Ecol Manage (2018) 26:763–774 769 Fig. 4 a The effect of elevated CO on CO ﬂux (mean ± SE) represent 4 cm below the surface; triangles and light grey line 2 2 over the 13 week experimental period. Diamonds and black line represent 7 cm below the surface. c The effect of species on CO represent 400 ppm atmospheric CO ; squares and grey line ﬂux (mean ± SE) over the 13 week experimental period. represent 800 ppm atmospheric CO . b The effect of water level Diamonds and solid line represent S. fallax; squares and dashed (cm below the peat surface) on CO ﬂux (mean ± SE) over the line represent S. papillosum; triangles and dotted line represent 13 week experimental period. Diamonds and black line S. capillifolium. d The effect of species on CH ﬂux represent 1 cm below peat surface; squares and dark grey line (mean ± SE) -2 -1 CO ﬂuxes were found to differ between species m h ) (Fig. 4d); the species and eCO treatments 2 2 (F = 4.29, P = 0.019), with S. capillifolium did not interact with the water level treatments. (2,51) having the smallest uptake of CO -2 -1 (- 69.12 ± 14.74 mg m h ) compared to S. -2 -1 papillosum (- 101.30 ± 15.47 mg m h ) and S. Discussion -2 -1 fallax (- 91.56 ± 16.43 mg m h ) by week 13 (Fig. 4c). Over the course of the experiment, CO Growth responses uptake increased more for S. fallax and S. papillosum over the course of the experiment than S. capillifolium Our ﬁnding that eCO increased Sphagnum growth are (F = 6.24, P \ 0.001) (Fig. 4c). in agreement with multiple studies (Jauhiainen and (10,270) Only the species treatment was found to have an Silvola 1999; Heijmans et al. 2001a; Saarnio et al. effect on CH ﬂux (F = 3.66, P = 0.029), 2003), however as this area of research is contested, 4 (2,123) with S. fallax producing the most CH the results are also in disagreement with several -2 -1 (5.28 ± 2.22 mg m h ), twice as much as S. studies (Berendse et al. 2001; Grosvernier et al. 2001; -2 -1 papillosum (2.09 ± 1.09 mg m h ) and 10 times Fenner et al. 2007). as much as S. capillifolium (0.48 ± 0.18 mg 123 770 Wetlands Ecol Manage (2018) 26:763–774 The greater effect of elevated CO on S. fallax S. capillifolium (Carroll et al. 2009). One reason for growth and establishment than either of the other this could be the impact of precipitation rewetting on species supports the ﬁrst hypothesis which predicted reducing the impact of drought by precipitation that fast growing S. fallax would respond most increasing the water contents of the top layer (Nijp positively to the eCO treatment. This is in agreement et al. 2014), negating the impacts of a lower water with previous studies (Berendse et al. 2001; Mitchell table. However, differences between species utiliza- et al. 2002; Granath et al. 2012) which suggested that tion of precipitation was seen by Nijp et al. (2014) increased CO will increase the growth of certain between S. majus, S. balticum and S. fuscum, which species more than others. Variation in species was not seen within our ﬁndings. This suggests that responses may be caused by differences in the our experiment did not differentiate water level to a effectiveness of species to utilize the increased high enough degree to examine differences between atmospheric CO concentration. Under elevated CO the species. 2 2 it’s possible that water became the limiting factor for growth, even under the highest water level treatments. CO and CH ﬂux responses 2 4 Silvola (1991) showed that the differences in water needed for photosynthesis vary between species. Greater CO assimilation under eCO was found in S. 2 2 Additionally, Nijp et al. (2014), found that Sphagnum fallax and S. papillosum than S. capillifolium, this species vary in their ability to exploit transient water lends additional support to our ﬁrst hypothesis which resources. This could account for the differences seen predicted that elevated CO would increase C seques- in growth rates under increased CO between the tration due to increased growth rates but that responses species, with some species better able to exploit the would differ among species. Our ﬁnding of increased elevated CO under the hydrological conditions. CO uptake under eCO agrees in part with ﬁndings by 2 2 2 Sphagnum also have higher water level limits due to Van der Heijden et al. (2000), who found that blocking of CO diffusion to the leaf via the layer of photosynthesis in mature plants of S. fallax was water surrounding the leaf, with species differing on initially stimulated by elevated CO (700 ppm). the extent of water level found limiting (Glime 2007). However, in contrast to our ﬁndings, acclimation For example, optimum water content for photosyn- was observed with stimulated S. fallax returning to the thesis for S. fuscum are lower than those required for S. levels of the controls after 3 days. This could have angustifolium. This could have impacted Sphagnum been due to differences between establishing and growth within our experiment due to the high mature S. fallax. frequency of precipitation and surface water this Our ﬁndings somewhat contrasted with Harpensla- provided to the Sphagnum. ger et al. (2015), who grew four Sphagnum species on We found that higher water levels coupled with peat monoliths incubated between 18 and 23 C over a eCO caused a greater increase in dry weight support- 12 week period. Harpenslager et al. (2015), found that ing our second hypothesis, which predicted that low S. fallax and S. palustre had positive net C efﬂuxes water tables would inhibit the effect of elevated CO similar to bare peat in spite of increased biomass. on Sphagnum growth. Similar results were found for S. Harpenslager et al. (2015) suggested that these results fuscum by Jauhiainen et al. (1997), who suggested that were due to the release of CO derived from this was due to water limitation of photosynthesis of bicarbonate due to lowering of the monoliths pH. Sphagnum at low water levels. We did not ﬁnd an While this is dissimilar to our ﬁndings, these releases interaction effect of hydrology on species, in contrary were species dependant, highlighting how different to previous studies, which have identiﬁed that each species can impact total CO ﬂux beyond sequestra- species of Sphagnum grows best within its own tion. These differences in growth and C ﬁxation hydrological niche (Grosvernier et al. 1997). It’s also between the species were in congruence with our been found that each species responds to variation in results. water tables differently (Jauhiainen et al. 1997; Carroll Overall our ﬁndings suggest that in the context of et al. 2009) due to differences in their physiologies. In future elevated CO , all three species of Sphagnum particular, S. fallax has been found to be highly will increase in CO assimilation due to increased dependent on water level, holding 30% less water than growth, however, it is suggested that not all species 123 Wetlands Ecol Manage (2018) 26:763–774 771 will react the same, with certain species seeing a much while the plots in the other two studies encompassed greater increase in CO assimilation (in this case S. both Sphagnum mosses, sedges and dwarf shrubs fallax and S. papillosum) compared to others. Whilst making it difﬁcult to isolate Sphagnum species our study did not ﬁnd acclimation (such as Van der responses in the two ﬁeld studies. Heijden et al. (2000), this could be due to our focus on The species-speciﬁc effects on CH ﬂuxes of the establishing Sphagnum and not mature plants. three Sphagnum species used in this study is important Greenhouse gas budgets for peatlands are often as it suggests that the Sphagnum species composition complex, with most peatlands being sources of CH per se, and not just the mix of functional groups (Strom (Moore and Roulet 1995; Saarnio et al. 2007; Leppala et al. 2005; Leppala et al. 2011) is an important control et al. 2011). As CH has a Global Warming Potential of CH . Although the mechanisms behind the species- 4 4 (GWP) of 28 over 100 years (IPCC 2013), the speciﬁc effects on CH ﬂuxes are unclear. They may increased CO sink strength in response to eCO relate to differences in production of labile carbon and/ 2 2 shown here could be offset if CH emissions increase. or phenolic compounds among Sphagnum species In our 13-week study CH emissions remained low, (Ellis et al. 2009). Transportation of C to the although they increased slightly at the end of the rhizosphere has been shown to vary with species experiment. (Reich et al. 2001). It’s possible that similar variation The GWP of the CH4 emissions over 100 years may account for the differences seen in emissions (CO equivalents (CO )) of the three species was between the Sphagnum species. 2 2eq 147.84 ± 62.12, 58.52 ± 23.98 and Furthermore, relationships between species and -2 -1 13,44 ± 5.04 mg m h for S. fallax, S. papillosum speciﬁc methanogenic and methanotrophic communi- and S. capillifolium respectively. If emissions of CO ties could have also had an impact. This has been and CH from each species are taken into account, S. shown for plant functional types within peatlands by fallax would have a positive net emission Robroek et al. (2015), which showed removal of plant -2 -1 (56.28 mg m h CO ), while S.papillosum and functional types (gramminoids and ericoids) signiﬁ- 2eq S.capillifolium would have a negative net emissions cantly alter microbial community structure. This ﬁt’s -2 -1 (- 42.78 and -55.68 mg m h CO respec- with Opelt et al. (2007) who found that diversity of 2eq tively). This suggests that while S. fallax was shown bacterial communities was higher in S. fallax than S. to have the greatest C assimilation over the course of magellanicum. the experiment, S. papillosum and S. capillifolium had lower net emissions. This highlights the importance of CH release within these systems which can offset Conclusions carbon uptake to a substantial amount and even become a net source of emissions. Our study highlights the importance of Sphagnum The increase in CH emissions towards the end of species selection when undertaking restoration of a the experiment was likely due to recovery of peatland. The clear differences in species establish- methanogenic populations following the increased ment and growth under increased CO and CH ﬂux 2 4 moisture content of the peat (Bellisario et al. 1999; between the species indicates that selecting the correct Leppala et al. 2011) and increased C availability due to Sphagnum species or combination of species is critical increased photosynthesis (Megonigal and Schlesinger to the success and GHG balance of recolonization 1997) and C transportation to the rhizosphere (Orem- attempts. While water level was not shown to interact land 1988; Williams et al. 2000). The lack of response with species, it’s interaction with CO suggests that CH ﬂuxes to eCO in our study contrasts with environmental factors may also play an important role 4 2 ﬁndings from FACE studies on UK and Finnish in establishment success. ombrotrophic peatlands which found modest increases Furthermore, differences between the species CO in peatland CH ﬂuxes in response to eCO (Saar- and CH ﬂuxes within the initial establishment period, 4 2 4 nio and Silvola 1999; Ellis et al. 2009). An important suggests that using different species will impact the difference between our study and those of Saarnio and overall GHG balance the peatland once restored. Silvola (1999) and Ellis et al. (2009) are the fact that Within the establishment phase, the increase in CO our mesocosms contained a sole Sphagnum species uptake found in response to eCO suggests that the 123 772 Wetlands Ecol Manage (2018) 26:763–774 for the restoration of cut-over bogs. 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