The interactive effects of elevated carbon dioxide and water table draw-down on carbon cycling in a Welsh ombrotrophic bog

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The interactive effects of elevated carbon dioxide and water table draw-down on carbon cycling in a Welsh ombrotrophic bog T. Ellis a,∗ , P.W. Hill b , N. Fenner a , G.G. Williams a , D. Godbold b , C. Freeman a a b

School of Biological Sciences, Bangor University, Deiniol Road, Bangor, Gwynedd LL57 2UW, UK School of the Environment and Natural Resources, Bangor University, Deiniol Road, Bangor, Gwynedd LL57 2UW, UK

a r t i c l e

i n f o

a b s t r a c t

Article history:

The effects of elevated atmospheric CO2 (eCO2 ) and water table draw-down on soil carbon

Received 14 January 2008

sequestration in an ombrotrophic bog ecosystem were examined. Peat monoliths (11 cm

Received in revised form

diameter, 25 cm deep) with intact bog vegetation were exposed to ambient or elevated (ambi-

6 October 2008

ent + 200 mg l−1 ) atmospheric CO2 , combined with a natural water table (level with the peat

Accepted 23 October 2008

surface) or a water table draw-down (−5 cm). Eight observations per treatment were included in the study, which was conducted over a 12 week period. Concentration of dissolved organic carbon (DOC), phenolic compounds and the fluxes of CO2 and CH4 were measured. The eCO2

Keywords:

treatment caused an increase in the CH4 and CO2 fluxes and a small decrease in both the

Wetland

DOC and phenolic concentrations. The water table draw-down invoked decreases in phe-

Peatland

nolic and DOC concentrations, a decrease in CH4 flux and a small increase in CO2 flux. The

Water table

combined (eCO2 + water table draw-down) treatment caused a larger than expected CH4 flux

Drought

decrease and CO2 flux increase and an increase in DOC concentration. Our results suggest

FACE

very different effects on the system dependent on the treatment applied. The draw-down

DOC

treatment principally increased oxidation of the rhizosphere resulting in increased decom-

Methane

position and as such a removal of material from the dissolved carbon pool. The data also

Priming

suggest labile carbon availability may be limiting the rate of decomposition and so slowing

Carbon quality

inorganic nutrient and carbon pool turn-over. The elevated CO2 addressed the labile-carbon

Elevated carbon dioxide

limitation. Under the environment of the combined treatment, these limitations were effectively removed, culminating in a destabilisation of the carbon-sequestering environment to a weaker sink (or even a source) of atmospheric carbon. © 2008 Elsevier B.V. All rights reserved.

1.

Introduction

The global atmospheric concentration of carbon dioxide has increased from pre-industrial levels of about 280–379 ppm in 2005 as a result of land-use change and fossil fuel burning (IPCC, 2007a), and is projected to increase between 540 and 970 ppm by 2100 (Hulme et al., 1999). This increase equates to

∗

Corresponding author. E-mail address: [email protected] (T. Ellis). 0925-8574/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2008.10.011

about 32% in the last 250 years of industrialisation (Nowak et al., 2004). More intense and longer droughts have been observed over wider areas since the 1970s and are expected to continue into the future, consistent with projected hydrological changes over continental landmasses (IPCC, 2007b). This rise in atmospheric CO2 concentrations and hydrological regime

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disturbance has led to considerable interest both in the potential of terrestrial biological systems to mitigate the effects of a changing climate through stimulated carbon sequestration and in the potential for destabilisation of existing carbon stores. Peatlands contain about 1/3 of the world’s organic carbon store, representing 455 Pg (Gorham, 1991), which accumulates as a result of an imbalance between photosynthetic production and decomposition. Peatlands are perceived to depend on water-logging for their stability and persistent accumulation of carbon (Freeman et al., 1998). These water logged environments result in a reduction in bimolecular oxygen available to the biologically active oxic surface layers, thus constraining the relatively efficient aerobic decomposition pathway where enzyme-inhibiting phenolic compounds do not accumulate, in favour of slower anaerobic mechanisms where the enzymic ‘latch’ (Freeman et al., 2001b) prevents decomposition. A shift to an aerobic environment as a result of water table draw-down or an increase in NPP as a result of increased photosynthesis due to greater CO2 availability may alter the balance of photosyntheticproduction/decomposition, and so cause changes in the rate of C-sequestration. The general impact of elevated CO2 (eCO2 ) on soil carbon cycling and the controlling processes of soil cycling are largely unknown due to the inconsistent results from investigative studies (Norby et al., 2001). Several papers have reported a decrease in soil carbon levels under eCO2 (Hoosbeek et al., 2004; Dijkstra et al., 2005), perhaps as a result of priming or stimulation of SOM decomposition due to co-metabolic decomposition following a rise in microbial activity from the addition of labile carbon into the system (Jenkinson et al., 1985; Cheng, 1999; Freeman et al., 2004c). Others have suggested no measurable change (e.g. Hill et al., 2007; Van Kessel et al., 2006), whereas some reports showed that eCO2 led to an increase in the carbon flux from plants to the soil (Diaz et al., 1993; Rice et al., 1994; Curtis et al., 1994a). The consensus for gaseous evolution of carbon under an eCO2 environment suggests an increase due to increased microbial respiration, measured by CO2 efflux (Rogers et al., 1992; Oneill, 1994; Runion et al., 1994; Dhillion et al., 1996; Williams et al., 2000). Many studies have also reported an increase in CH4 efflux under eCO2 (Dacey et al., 1994; Hutchin et al., 1995; Allen et al., 1994; Megonigal and Schlesinger, 1997; Wang and Adachi, 1999) possibly due to increased availability of methanogenic carbon substrate as a result of increased NPP (Whiting and Chanton, 1993; Whiting et al., 1991), and due to increased water-use efficiency of vascular species (Korner, 2000; Hui et al., 2001; Taiz and Zeiger, 2002). Water is considered the most important regulator of wetland biogeochemistry (Ponnamperuma, 1972). Constant and lower water table levels have been reported as a result of a simulated drought where water inflow onto a peat bog was constrained (Freeman et al., 1993b). Likewise, lower but stable summer water table levels have been reported in peatlands after drainage ditch digging and water diversion (Patterson and Cooper, 2007). Prior to restoration, similar continually lower levels have been reported as a result of peatland ditching (Cooper et al., 1998). The enzymes responsible for peat and litter decomposition are oxidases whose activity is greatly enhanced by the presence of oxygen (McLatchey and Reddy,

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1998; Freeman et al., 2001b, 2004b). Water table draw-down will effectively aerate the upper levels of the peat profile, resulting in a change in decomposition end products from anaerobically produced DOC and CH4 to aerobically produced CO2 , so reducing interstitial DOC concentrations (Freeman et al., 1993a; Fenner et al., 2005). Water table draw-down in peatlands significantly increased CO2 evolution in the field (e.g. Oechel et al., 1998; Heinsch et al., 2004; Martikainen et al., 1995; Silvola et al., 1996; Chimner and Cooper, 2003) and in mesocosm studies (e.g. Freeman et al., 1993b; Blodau et al., 2004; Blodau and Moore, 2003) in agreement with the linear relationship between water table and CO2 emissions observed under lab conditions by Moore and Dalva (1993). It is generally accepted that an increase in the size of the aerobic zone will have the potential to enhance CH4 oxidation (Moore and Knowles, 1989; Whalen and Reeburgh, 1990; Freeman et al., 1993b) and more importantly, methanogenesis may slow from a lowering of the anaerobic zone to depths containing less labile organic matter (Gorham, 1991; Freeman et al., 2002). Ecosystem responses to global change have mostly been addressed by experimental studies focusing on single parameter changes rather than interactive treatments (e.g. Rasse et al., 2005; Delucia et al., 1999; Hutchin et al., 1995). This trend is changing slowly (e.g. Shaw et al., 2002; Henry et al., 2005; Fenner et al., 2007), but there is still a lack of information considering the likelihood of more than one manifestation of global change occurring at any one time, and in particular the importance of peatlands in global carbon cycling. It was therefore the objective of this study to investigate the separate and interactive effects of elevated levels of carbon dioxide and a water table draw-down on the export of carbon from peatlands. It was hypothesised that the interactive treatment would instigate an additive-effect response from the ecosystem, measurable in the concentration and fluxes of the subsequently exported carbon.

2.

Methods

Peat was collected from an upland, ombrotrophic bog dominated by Sphagnum sp. and Juncus sp. in the Migneint Valley, North Wales, UK (NGR SH816440). The water table of the site at the time of core collection was level with the surface of the litter layer at the base of the Sphagnum stems. Cores were collected from sites with similar characteristics, principally the vegetation type and water table height. Thirty-two intact peat monoliths (0.11 m diameter × 0.25 m depth) were collected from the field site in plastic core-liners. Each liner was sharpened at one end to facilitate its insertion into the peat profile. The liner was used as a template, around which a serrated blade was used to cut into the peat. The core-liner was then lowered onto the peat-core gently in order to avoid compaction of the peat and thus preventing any changes to its physical structure. Cores were allowed to acclimatise at the experimental facility for 10 weeks prior to the initiation of the CO2 fumigation. The elevated CO2 environment was maintained through the use of Free Air Carbon dioxide Enrichment (FACE) technology, using 8.5 m diameter FACE rings at the Bangor-FACE facility in North Wales. The experiment site was divided

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between eight plots; four maintained at elevated CO2 and four at ambient CO2 . Atmospheric CO2 was elevated to an average value of 558 mg l−1 in accordance with IPCC projections for the date 2050, continuously monitored by on-line infra-red gas analysers. Peat monoliths were contained within pipes sealed to the ground but open to the atmosphere. The pipes served two purposes, isolating the cores from hydrological interaction with the surrounding soil and acting as a reservoir for the distilled water used in maintaining the water table and stabilising diurnal temperature fluctuations. Eight peat cores were allocated to each treatment and distributed evenly across the treatment plots. Water table levels were stringently maintained, equal with the vegetative surface for the control and 5 cm below the surface for the draw-down treatment. The treatment water table was gradually lowered over a 6-week period prior to CO2 fumigation to simulate natural drought conditions after Freeman et al. (1993a). The water table was lowered relative to the vegetative surface by raising the peat monoliths above the water-surface by supporting each core from below with piping of the same material used for the core-liners. Water table was then maintained at the maximum extent of the draw-down for the remainder of the experiment. Sampling commenced at the end of May 2005 at the experimental facility. 15 ml of pore-water samples were collected using low-tension lysimetry to extract a sample from within the rhizosphere at 15 cm depth from the peat surface. Samples were filtered at 0.45 ␮m through cellulose acetate filters immediately after sampling and then stored at <4 ◦ C until analysis. Dissolved organic carbon concentrations were measured by proxy to absorbance values at the UV wavelength of 254 nm (Korshin et al., 1997). Values of absorbance at 254 nm correlated with direct measurements of DOC using a Total Organic Carbon analyser (Shimadzu 5000, Kyoto, Japan) with an r value of 0.92 and were calibrated accordingly with values reported as concentration of DOC in mg l−1 (Fig. 1). Phenolic compound concentrations were assayed using a modification of the method of Box (1983). To 1 ml of sample, 150 ␮l of 200 mg l−1 Na2 CO3 solution was added and 50 ␮l of Folin–Ciocalteau

Fig. 1 – Correlation of UV absorbance at 254 nm cm−1 with DOC calculated from the difference between total pore-water carbon and inorganic carbon. R2 = 0.85.

reagent (Sigma–Aldrich). The mixture was incubated for 1 h at room temperature in 1.5 ml centrifuge tubes. A standard curve was prepared by applying the same chemicals to 0–10 mg l−1 phenol solution. Samples and standards were centrifuged at 10,000 × rpm for 4 min to remove any precipitate and so reduce potential for quench. The change in colour of the reactants was measured spectrophotometrically at 750 nm. When sample concentrations were out of the range of the standard curve, samples were diluted with ultra-pure water and the assay was repeated. Trace gas samples were collected 2 h either side of midday GMT using a closed chamber technique. The increase in greenhouse gas concentrations within transparent chambers (1.7 l capacity), during the 1 h period over which accumulation is approximately linear (Freeman et al., 1993b), was related to an initial background concentration measured within each chamber to give an estimate of flux. Fluxes were calculated as mg of CO2 or CH4 per m2 peat per hour. Samples were extracted from the chambers through a Suba-seal septum into evacuated Vacutainers for analysis using an Ai Cambridge model 92 gas chromatograph using a Porapak QS column at 35 ◦ C with an N2 carrier gas flow of 30 ml min−1 and a flame ionisation detector. Sampling occurred once biweekly and then once monthly throughout the 12-week fumigation period. Two-tailed model-1 regression statistical analyses were performed by comparison of the regression coefficient of each treatment-set with its associated control. Significance was accepted at the probability level of 95% or higher. Results of the treatments are expressed as deviation from the control and represent the treatment effect. Repeated measures ANOVA was employed to analyse for treatment effects in CO2 efflux and simple ANOVA for the difference between treatments of the CH4 flux. All statistical procedures were conducted using SPSS statistical software (version 12.0, SPSS Inc., Chicago, IL). The environment manipulation system used in this study gave us great control over both the atmospheric concentration of CO2 and also the constancy of the extent of the water table draw-down. The method used of elevating the core height relative to the water table rather than lowering the water table to the peat surface greatly reduced the uncertainty of water level due to precipitation, and a capped water reservoir associated with each core reduced water-level changes due to evaporation. When combined with the precision of the CO2 monitoring and control system, we can be confident of the effectiveness of our treatments and as such the validity of our results.

3.

Results

3.1.

DOC

The combined treatment (eCO2 + draw-down) induced the most surprising response in the peat pore-water DOC concentration, which increased significantly (P < 0.05) relative to the control (Fig. 2). The absolute DOC concentrations increased in all treatments over time. However, the DOC concentration significantly declined (P < 0.05) relative to the control in response to the individual draw down and eCO2 treatments (Fig. 2).

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Fig. 2 – DOC deviation from the control. Values are mean ± S.E.M.; n = 8.

The effect of all treatments became more pronounced with time (Fig. 2). By the end of the treatment period, the combined treatment (eCO2 + draw-down) caused the greatest difference in concentration relative to the control (16.0%, 10.2 mg l−1 ), followed by the draw-down (−12.6%, 33.8 mg l−1 ) and then the eCO2 (−8.5%, 26.0 mg l−1 ) treatment, where negative values denote a decrease in concentration and positive values, an increase.

3.2.

CO2

Throughout the experiment, relative to the control the treatments all induced a further increase in CO2 evolution (Fig. 3). At week 4, the time of maximum CO2 emission, the combined treatment (eCO2 + draw-down) invoked a large (19 times, 86 mg m−1 h−1 ) and significant (P < 0.02) response. The eCO2 treatment produced a +527% (33 mg m−2 h−1 ) change in CO2 evolution and the draw-down treatment a change of +42% (4 mg m−2 h−1 ). Although these increases were large, the inherent natural variability of the ecosystem rendered the differences statistically non-significant. Only the combined

Fig. 4 – (a) CH4 flux data displayed for week 12. Differences between all treatments are significant (P < 0.005). Values are mean ± S.E.M.; n = 8. (b) CH4 flux deviation from the control. Values are mean ± S.E.M.; n = 8.

treatment induced a significant response (P < 0.05) revealed by repeated measures ANOVA over the time-span of the experiment.

3.3.

Fig. 3 – CO2 flux deviation from the control. Values are mean ± S.E.M.; n = 8.

CH4

A generally decreasing trend of CH4 evolution over the course of the experiment was observed within all treatments and also the control. Highest values were measured for all treatments at week 4 after an initial increase in efflux (Fig. 4a). The draw-down manipulation altered CH4 evolution relative to the control by a maximum of −29% (−0.80 mg m−2 h−1 ), a negative deviation from the control treatment (Fig. 4b). This deviation decreased towards the end of the experiment. The eCO2 treatment changed the CH4 efflux trend relative to the control by +58% (1.56 mg m−2 h−1 ). eCO2 data show there was generally less CH4 evolved over the experimental time-frame, but increasingly a more positive deviation from the control. Statistically, eCO2 did not cause a significant change in the CH4 flux. The combined treatment induced a negative deviation, suggesting that over time this treatment effect is likely

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Fig. 5 – Phenolic compound concentration deviation from the control. Values are mean ± S.E.M.; n = 8.

to reduce CH4 efflux. Over the course of the experiment, net CH4 emission reduced by 80% (3.8 mg m−2 h−1 ) compared to the control treatment. Statistically, there was no effect of any treatment until the end of the experiment when between treatments, all sample populations differed (P < 0.005).

3.4.

Phenolics

Relative to the control, the individual treatments induced a change in phenolic concentration (draw-down −49%; eCO2 −15.4%). This was most pronounced by the end of the experiment. However, the combined treatment displayed a much lower level of deviation (−4.2%; Fig. 5), where it is reasonable to assume there was no change in phenolic concentration as a result of a draw-down under an eCO2 environment. Regression analysis revealed no significant differences between treatments.

4.

Discussion

4.1.

Hydrology change

The water table draw-down in this study stimulated a reduction in DOC and phenolic concentration, and a decreasing trend for the CH4 flux but little change in the flux of CO2 . The oxygenation of the upper levels of the peat as a result of water table draw-down has important implications for carbon cycling in peatlands. Enhanced aeration as a result of a draw-down tends to reduce DOC including the recalcitrant phenolic materials (Freeman et al., 1993a, 1994, 2001a,b; Pind et al., 1994; Hughes et al., 1998; Evans et al., 2002; Worrall et al., 2002; Fenner et al., 2005). When water is less abundant, DOC and in particular phenolicrich humic compounds tend to adsorb onto precipitated particles (Tipping and Woof, 1990). Of potentially greater importance, however, is the release of the ‘latch’ mechanism restricting the activities of carbon degrading enzymes (Freeman et al., 2001a). This latch release removes the inhibit-

ing factors slowing the activities of the hydrolase enzymes (Freeman et al., 2004c), thus encouraging the decomposition of DOC. The lower CH4 emissions during the draw-down induced in this investigation largely agree with other studies (e.g. Moore and Knowles, 1989; Freeman et al., 1993b,2002; Hughes et al., 1999). Increasing the size of the aerobic zone of the upper layer of the peat profile via water table draw-down effectively increases the depth of soil in which methane can be oxidised (Whalen and Reeburgh, 1990; Gorham, 1991). This will also have the effect of lowering the methanogenic zone to a depth containing less labile organic matter (Gorham, 1991). The reduced concentration of DOC measured may account for some of the CH4 efflux reduction since it effectively removes the energy source of labile low molecular weight carbon used in methanogenesis (Hughes et al., 1999; Roura-Carol and Freeman, 1999; Best and Jacobs, 1997). Other water table manipulation studies have also found oxidation of sulphides (e.g. Lucassen et al., 2002) which produces the methane-suppressing sulphate (Oremland, 1988). Although the reducing trend of CH4 efflux was not unexpected, the small amount of the reduction (−29%) was. When evaluated in the light of these studies, the data presented may suggest that methanotrophy (methane oxidation) may play a less important role in the regulation of methane flux than methanogenesis in bogs. The net CO2 efflux displayed little response to draw-down in this study. This finding agrees with the findings of Bridgham et al. (1991), and Updegraff et al. (2001) who found no effect of water table manipulation on ecosystem respiration in North Carolina peatlands and peatlands in Minnesota, respectively. This may have been due to a lack of easily oxidisable carbon (Chimner and Cooper, 2003), though this reasoning contradicts the concept of reducing DOC concentrations due to a shift to CO2 -end-products in aerobic environments (Fenner et al., 2005). Since high molecular weight phenolic compounds did not increase in concentration, it seems likely that the change was due to a limitation in production of low molecular weight material. This could be attributed to either water stress leading to reduced levels of root exudates or a removal of carbon from the system to insoluble compounds (Tipping and Woof, 1990). Some studies measured a pH drop due to water table draw-down over much longer time periods than those considered here (e.g. Lukkala, 1929; Laiho and Laine, 1990; Laine et al., 1995) which suppresses phenol oxidase activity (Pind et al., 1994) and so has a retarding effect on decomposition (Ivarson, 1977; Bergman et al., 1999), regardless of the tolerance of peatland microbes to acidic conditions (Williams and Crawford, 1983).

4.2.

Elevated atmospheric CO2

The elevated CO2 environment evoked some unexpected results, principally a decrease in DOC concentration. This is likely to be due either to a decrease in the quantity of carbon entering the DOC pool, or to a shorter residence time of this carbon. The often-reported fertilisation effect (Zak et al., 1993; Rouhier et al., 1994; Cheng, 1999; Curtis et al., 1994b; Diaz et al., 1993; Vanveen et al., 1991) suggests that plant inputs to the DOC pool would be likely to increase rather than decrease in

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an eCO2 environment, although it is not certain whether this was the case here. Wetlands may respond via a positive feedback to climate change by stimulating the decomposition of a larger pool of dead organic matter (Kang et al., 2001). These decreases in soil carbon content may be the result of priming, which may increase under eCO2 as a result of co-metabolic decomposition following a rise in microbial activity (Cheng, 1999; Fontaine et al., 2007). This is particularly an issue in labile carbon limited environments such as ombrotrophic bogs (Saarnio et al., 2003; Freeman et al., 2001b). Plant material grown at eCO2 frequently has a higher proportion of relatively labile non-structural carbohydrate (Sadowsky and Schortemeyer, 1997; Ball, 1997). If enhanced labile-carbon availability resulted in increased microbial biomass, this could initiate a chain of consumption: Enhanced rates of microbial decomposition, combined with increased microbial biomass stimulate microbial grazing, thereby increasing the proportion of nutrients undergoing rapid cycling (Zak et al., 1993). This might lead to an increase in the decomposition rate of DOC, and thus a lower concentration. However, we did not find the increase in gaseous carbon losses that might be expected if this were the case. Consequently we are unable to explain the reduction in DOC under eCO2 . The 58% increase relative to the control in CH4 efflux by the end of the experimental eCO2 treatment was within expected limits (e.g. Saarnio and Silvola, 1999). This increase was relatively weak compared with some studies (e.g. Hutchin et al., 1995; Megonigal and Schlesinger, 1997), but some authors have reported a lack of significant difference between the treatment and the control (e.g. Kang et al., 2001). Three possible mechanisms could explain the increase measured. Firstly, stimulated carbon assimilation by plants provides the organic substrates necessary for methanogenesis (Oremland, 1988), by increasing the availability of labile carbon to the biologically active rhizosphere (Yavitt et al., 1987; Megonigal and Schlesinger, 1997). Secondly, vascular tissue can act as a conduit for CH4 from soils to the atmosphere (Bellisario et al., 1999) which may increase as a response to eCO2 (Megonigal and Schlesinger, 1997). Thirdly, perhaps as an amalgamation of these processes, an increase in photosynthetic rates under eCO2 and the consequential increase in primary productivity (Whiting and Chanton, 1993) may increase CH4 production (Dacey et al., 1994; Megonigal and Schlesinger, 1997). However, if any of these mechanisms were active in this system, they were not sufficiently active to provoke a significant response. The simultaneous decrease in DOC concentration and increase in CH4 flux leads to speculation that the efflux of CH4 may be responsible for the DOC decrease as DOC is used as a methanotrophic substrate. The large natural variation between replicate samples somewhat masks the response of CO2 efflux to the elevated CO2 treatment, suggesting the experimental design may have benefited from an increase in the n value. The positive 527% trend in efflux measured at the peak deviation was not significant but is noteworthy. Kang et al. (2001) suggest increased DOC production occurs under eCO2 , subsequently providing the substrate for microbial respiration.

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In this study, a decrease in DOC concentration may have resulted from this suggested increase in microbial cycling in which CO2 is produced as an end-product. Little evidence of increased decomposition was found as a result of the eCO2 treatment. An increase in CO2 production may therefore indicate an increase in root respiration and microbial consumption of more easily available labile carbon pools rather than decomposition of more recalcitrant material (Rouhier et al., 1994).

4.3.

Combined treatment environment

The combined treatment produced some surprising interactive results. A 19 times increase in carbon dioxide suggests the conditions for respiration were optimised. High water tables and water-use efficiency in an eCO2 environment (Tubiello et al., 1999; Megonigal and Schlesinger, 1997) can constrain oxygen availability in the rhizosphere, and subsequently root respiration and carbon decomposition. A reduction in labile DOC availability under drought conditions will similarly constrain the amount of CO2 produced by a wetland ecosystem. We propose the combined treatment environment effectively removed these constraints through the supply of both the necessary bi-molecular oxygen and labile carbon, accounting for the very significant difference measured. High output of CO2 would also be expected when the change in DOC concentration is considered. The significant increase in DOC concentration would provide an abundance of substrate for microbial cycling processes under optimised conditions, as supplied by the combined treatment. We hypothesise that the increase in DOC was a result of plant-exuded carbon and not as a result of directly stimulated decomposition. The phenolic compound concentration did not change significantly as a result of treatment (−5.4%), suggesting limited stimulation of decomposition potentially from increased water-use efficiency through lower transpiration rates, thus moderating the effect of the draw-down treatment–incidentally supported by the reduced production of CH4 observed to be stimulated by the eCO2 environment treatment. Inputs to the soil solution from root exudation would likely be dominated by relatively labile compounds with low phenolic content (e.g. sugars, amino acids and organic acids; Farrar et al., 2003; Jones et al., 2004). Increases of labile soil carbon in response to elevated CO2 are well documented (Diaz et al., 1993; Curtis et al., 1994b; Jones et al., 1998; Cheng, 1999) and will increase microbial metabolism (Zak et al., 1993) leading to microbial priming of the peatland ecosystem (Fontaine et al., 2007) which is often labile-carbon limited (Saarnio et al., 2003; Freeman et al., 2004a). These priming compounds are traditionally found in very low concentrations as a result of rapid microbial cycling (Jones et al., 2004), which would ultimately lead to the removal of carbon from the system as CO2 . However, since the observed increase in DOC concentration occurred regardless of the export of carbon as CO2 it is likely microbial cycling to CO2 was not the only stimulated process. The molecular weight of the dissolved carbon compounds decreased with the combined treatment, ostensibly due to a change in the litter quality (Hirschel et al., 1997; Franck et al., 1997; Norby et al., 2001) to material containing lower levels of phenolic compounds. Lower

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concentrations of phenolic material and an optimised environment for litter decomposition combined with increased DOC exudation culminated in a continually positively deviating response to treatment when compared with the control environment. The results of this experiment should be considered principally within the context in the short term, as no details of the mechanisms of adaptation of the vegetation to the persistence of the new conditions are available.

5.

Conclusions

Our results suggest very different effects on the system dependent on the treatment applied. The draw-down treatment principally increased oxidation of the rhizosphere resulting in increased decomposition of carbon and as such a removal of material from the dissolved carbon pool. The data also suggest labile carbon availability may be limiting the rate of decomposition of the peat and so slowing inorganic nutrient and carbon pool turn-over. The elevated CO2 environment effectively removed the labile-carbon limitation. The carbon dioxide and methane fluxes both showed a minor increase. We hypothesise a change in the quality of the plant-exuded carbon to less recalcitrant forms which are more easily used and so turned-over more rapidly. The elevated CO2 treatment effectively primed co-metabolic decomposition of the biomass by increasing microbial activity. The combined treatment, simulating a water table draw-down event in a future elevated carbon dioxide atmosphere, provided an environment where the previously identified limitations to decomposition and nutrient cycling were removed. The hypothesised litter and exudate carbon quality improvement, stimulated microbial metabolism and enhanced availability of oxygen manifested themselves as a positive feedback of carbon from the peat to the atmosphere in gaseous form and via an intermediate dissolved organic form. Ultimately, our results suggest a destabilisation of peat bog ecosystems which could lead to them becoming weaker sinks (or even sources) of carbon, the extent of which is not apparent when individual effects are studied in isolation from other environmental forces.

Acknowledgements The authors acknowledge funding from the European Social Fund and The North Wales Wildlife Trust.

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