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Experimental Burial Inhibits Methanogenesis and Anaerobic Decomposition in Water-Saturated Peats Christian Blodau,†,‡,§,* Melanie Siems,‡ and Julia Beer‡ † ‡
School of Environmental Sciences, University of Guelph, Stone Road 50, Guelph, N1G2W1, ON, Canada; Limnological Research Station, University of Bayreuth, Universit€atsstrasse 30, 95440 Bayreuth, Germany;
bS Supporting Information ABSTRACT: A mechanistic understanding of carbon (C) sequestration and methane (CH4) production is of great interest due to the importance of these processes for the global C budget. Here we demonstrate experimentally, by means of column experiments, that burial of water saturated, anoxic bog peat leads to inactivation of anaerobic respiration and methanogenesis. This effect can be related to the slowness of diffusive transport of solutes and evolving energetic constraints on anaerobic respiration. Burial lowered decomposition constants in homogenized peat sand mixtures from about 105 to 107 yr1, which is considerably slower than previously assumed, and methanogenesis slowed down in a similar manner. The latter effect could be related to acetoclastic methanogenesis approaching a minimum energy quantum of 25 kJ mol1 (CH4). Given the robustness of hydraulic properties that locate the oxicanoxic boundary near the peatland surface and constrain solute transport deeper into the peat, this effect has likely been critical for building the peatland C store and will continue supporting long-term C sequestration in northern peatlands even under moderately changing climatic conditions.
’ INTRODUCTION Northern peatlands have accumulated about 1/3 of the global soil carbon (C) store1 and are an important source of methane to the atmosphere.2 Feedbacks between the C cycle and climate thus represent an uncertainty for future climate as the C store can potentially be mobilized, for example by drought and activation of phenoloxidase enzymes,3 increases in temperature and related permafrost degradation,4 and accelerated loss of dissolved organic carbon.5 Other studies point to an ecohydrologically controlled homeostasis of C cycling 6,7 in peat bogs, which largely consist of recalcitrant, anoxic, acidic, and poorly permeable organic deposits. It is of critical importance to resolve the question whether deeper peat remains decomposable in situ, a question that we experimentally address in this study. Methane production and long-term sequestration of C in peat bogs are controlled by organic matter decomposition in the deeper water saturated and anoxic layers (“catotelm”) of peat bogs, which develop toward a dynamic equilibrium between biological C fixation and C release in the aggrading peat masses.8 The catotelm is typically characterized by low hydraulic conductivity9 and an accumulation of solutes stemming from decomposition processes.10 The semiclosed nature of these deposits may lead to a negative feedback on anaerobic decomposition and a “peat inactivation” as accumulation of respiration products lowers Gibbs free energies of key processes toward a biological energy quantum.10,11 This minimum quantity amounts to about 20 to 25 kJ mol1 (CH4) for actively growing methanogenic archaea, equivalent to 1/41/3 of the energy necessary for ATP generation.12 Under microbial starvation conditions and with respect to fermentation processes the energy thresholds may be r 2011 American Chemical Society
more positive,13 whereas threshold values are typically more negative for more potent electron accepting processes.14,15 Generally, microbial kinetics of processes near thermodynamic thresholds is under thermodynamic control.15 The “peat inactivation hypothesis”, as we refer to it subsequently, is difficult to test unequivocally in the field. Peat quality changes with depth, which makes it difficult to attribute vertical change in DIC and CH4 release to accumulation of the dissolved gases rather than change in peat quality. Concentration profiles of decomposition products may be moreover transient, which complicates a quantification of rates of DIC and CH4 release from porewater modeling. To eliminate these difficulties, we designed a simple yet effective column experiment with water saturated and anoxic peat-quartz sand mixtures containing 5%, 15%, and 50% of moderately decomposed bog peat. The mixtures were arbitrarily chosen to create a broad range of potential DIC and CH4 release in the columns. Carbon content in the peat-sand mixture served as substitute for differences in organic matter reactivity in peat soils. In the mixtures differences in peat quality with depth were eliminated by homogenization and pore water profiles reflecting anaerobic decomposition were allowed to approach steady-state. The peat inactivation hypothesis predicts that DIC and CH4 production diminish to similar and very small values deeper into the deposit, regardless of C content, due to the accumulation of metabolic end products. In contrast, production Received: May 24, 2011 Accepted: September 29, 2011 Revised: September 22, 2011 Published: September 29, 2011 9984
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Environmental Science & Technology rates should increase proportionately with C content in the anoxic layers near the column surface, where DIC and CH4 are continuously removed. Furthermore, the decrease in process rates involved in anaerobic respiration should be related to processes approaching their respective biological energy quantum. This study’s objective was to verify these predictions and this way test the peat inactivation hypothesis.
’ MATERIALS AND METHODS Column Setup and Sampling. We homogenized commercially available ombrotrophic bog peat with quartz sand and deionized water, and filled the mixture in PVC columns (140 20 cm) equipped with porewater peepers11 of 120 cm length, which were nondestructively vertically retrieved from perforated and meshed frames embedded in the column filling. According to Fourier Transformed Infrared Spectroscopy (FTIR), the peat chemistry was broadly similar as found in depths of about 1 m at a typical bog site (e.g., the Mer Bleue bog, Ontario, where the hypothesis was developed 10,11 (Supporting Information). Columns were kept dark at 18 °C, water table slightly above surface. Peepers were retrieved and replaced after 370 and 550 days (see Supporting Information for additional information). Porewater Chemistry and Modeling. Gaseous CO2 and CH4 concentrations were determined on a 8610C gas chromatograph (GC) with a methanizer and flame ionization detector (FID) from SRI Instruments (U.S.). We used a headspace technique to determine dissolved CO2 and CH4 concentrations as described in ref 16. Pore water concentration of DIC and CH4 were calculated from measured concentrations in the gas phase, using Henry’s law constants corrected for temperature (KCO2 = 0.0389 mol L1atm1, KCH4 = 0.0014 mol L1atm1;17 pH was used to calculate DIC speciation 18). H2 was measured using a Trace Analytical TA 3000 hydrogen analyzer. Concentrations were corrected for the background concentration in the vials and calculated to dissolved concentrations with Henry’s law (KH2 = 8.1 104 mol L1 atm1).17 Inorganic anions (ion chromatography), pH (potentiometry), and H2S (amperometry) were analyzed as previously described,11 and acetate in selected samples by HPLC with UV detection at 208 nm (see Supporting Information). DIC and CH4 production were determined using the PROFILE model by fitting a production-consumption profile to the measured concentration data in an iterative process.19 PROFILE solves the one-dimensional mass conservation equation of a solute, transported by diffusion, by matching and optimizing the production-consumption profile with the measured data numerically in an iterative process. The vertical resolution in the simulation matches the measured data. A series of least-squares fits of simulated and measured concentration data are calculated and compared through statistical F-testing. Effective sediment diffusion coefficients Ds were calculated based on available compilations of diffusion coefficients in water Dw20 and a temperature of 18 °C. Porosity (n) was estimated from bulk density estimated in separate cores with analogous peat-sand mixtures as Ds = n2 3 Dw as proposed by Lerman.20 Chamber Fluxes. CO2 and CH4 fluxes from peat to the atmosphere were measured on three occasions for continuous periods of 5 h at the end of the experiment (day 557559) using static chambers (20 cm diameter, 30 cm height) and hourly measurements and calculated from linear regression of concentration over time (n = 6; R2 > 0.9).
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Microbial Biomass C. Extractable microbial biomass C was determined in peat taken from the columns at the end of the experiment. Fumigation with CHCl3 and extraction with 0.5 M K2SO4 was carried out with slight modifications according to Voroney et al.21 in three replicates per depth and column as previously described in ref 22. The method determines dissolved organic cell content lysed from active or inactive living microbial cells by exposure to CHCl3 and has been found to be well suitable to for humic rich and sand bacteria mixtures.23 Dissolved organic carbon (DOC) was measured in extracts from fumigated and nonfumigated samples with a TOC analyzer after acidification and sparging with CO2-free N2 and microbial biomass C calculated by difference between treatments. Due to a handling error only microbial biomass C in the 5% and 15% peat column was analyzed. Thermodynamic Calculations. Gibbs free energy ΔGr (kJ mol1) available for hydrogenotrophic (4 H2(aq) + CO2(aq) f 2 H2O(l) + CH4(aq)) and acetoclastic (CH3COO(aq) + H+(aq) f CO2(aq) + CH4(aq)) methanogenesis was calculated for the smallest integer stochiometry using the Nernst-Equation as previously described:11
ΔGr ¼ ΔG0r þ RT ln
Πi ðproductsÞvi Πi ðsubstratesÞvi
ð1Þ
with ΔGr0 the standard Gibbs free energy of the reaction (kJ mol1) at in situ temperature of 18 °C, R the gas constant (8.314 103 kJ mol1 K1), T the absolute temperature (K), and ν the stoichiometric coefficients. The pressure dependency of ΔGr was neglected and a concentration of 15 μmol L1 acetate used when below the limit of quantification (LOQ, ca. 30 μmol L1); lower levels result in a more positive ΔGr and thus would not alter the interpretations.
’ RESULTS AND DISCUSSION Concentrations of DIC increased steeply to about 30 cm depth and then more gradually to maximum levels of 5800 μmol L1 (5% peat), 8900 μmol L1 (15% peat), and 9500 μmol L1 (50%) peat. In the 5% peat column concentration still increased between 370 and 550 d after column preparation, whereas only little (15% peat) or no increase (50%) occurred in the other columns (Figure 1). Concentration profiles of DIC were thus probably near or already at steady state by the end of the experiment. Concentration of CH4 increased similarly with depth, yet more steeply near the surface and hardly beneath 20 to 25 cm depth, with the exception of the 5% column (Figure 1). The increase in CH4 over time was delayed compared to that of DIC and leveled off at 0.50.6 mmol L1. DIC and CH4 concentration patterns were similar as observed in field studies10 and approached a steady state faster in the columns rich in peat (Figure 1). Production rate profiles and diffusive fluxes of DIC and CH4 were quantified using the program PROFILE19 when steady state was approximately attained after 550days. Also Gibb’s free energies of hydrogenotrophic and acetoclastic methanogenesis were calculated at this point.11 Measured and modeled concentration profiles were in good agreement (R2 of linear regression modeled-measured: 0.950.99) (Figure 1). According to the modeled profiles of DIC and CH4 production, respiration involved an electron acceptor other than DIC, because DIC production strongly exceeded methanogenesis (Table 1). As inorganic electron acceptors such as sulfate were depleted (see Supporting Information) 9985
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Figure 1. Dissolved inorganic carbon (DIC) and CH4 concentrations after 370 and 550 days, modeled concentration profiles (day 550), and resulting depth distribution of DIC and CH4 production in experimental peat-sand mixtures of 5% peat, 15% peat, and 50% peat (mass basis). Steady state was assumed to have been attained by day 550 for application of the model. The vertical dotted line indicates zero production.
and fermentation products did not accumulate, DIC was likely produced by electron transfer to humic substances16,24 although we cannot ascertain this possibility. An electron acceptor capacity of solid-phase humic bearing wetland sediments has been identified.25 Even if these capacities were small, as cautioned by the authors, they may have supported DIC production given the large peat C reservoir of 220850 mmol C L1 in the columns. The patterns of DIC and CH4 production between and within columns support the peat inactivation hypothesis and a negative feedback on decomposition by metabolic product accumulation. Production rates declined deeper into the peat-sand mixtures where DIC and CH4 accumulated, whereas rates were roughly proportionate between the 5%, 15%, and 50% peat columns in the surface layers and when integrated over depth (Figure 1 and Table 1). We have confidence in the robustness of these results as the inverse modeling approach was safe-guarded by flux measurements of CO2 and CH4 (static chamber measurements) from the columns. The modeled diffusive and measured fluxes of CO2 (Table 1) were in a similar magnitude and CH4 emission could not be detected. Ebullition of CH4 should have been identified by a stepwise jump in CH4 concentrations during the chamber measurements but could not be found. Concentration profiles of CH4 concentration (Figure 1) and H2S (Supporting Information) further indicated anoxic conditions almost up to the surface, with the exception of the 5% peat column, where a substantial sink for CH4 existed in the peat (Figure 1). Aerobic respiration was thus unlikely to significantly contribute to DIC production in the columns.
Table 1. Depth-Integrated Production of DIC and CH4 as Determined by PROFILE and Flux at the Upper Boundary of the Calculation Domain (mmol m2 d1). a treatment
5%
15%
50%
5%
15%
50%
production
DIC
370 days
0.32
0.79
1.86
550 days
0.42
0.70
1.98
0.012
diffusive flux 370 days 0.32
0.85
1.95
0.0016 0.079 0.200
550 days
0.76
2.10
0.094
0.39
CH4 0.073 0.200
chamber flux 0.75 ( 0.30 1.15 ( 0.11 1.37 ( 0.48 nd
0.082 0.231
0.084 0.224 nd
nd
Chamber fluxes (n = 3) were recorded at the end of the incubation period over a 300-minute period. (nd, not detectable). a
The anaerobic decomposability of the material strongly decreased with the hydrochemical conditions developing deeper into the peat-sand mixtures. Estimated C based peat decomposition constants, calculated from C store divided by DIC production, were 1.6 105 yr1 (5%), 2.4 105 yr1 (15%), and 2.0 105 yr1 in the active surface layer, but only 4.9 107 yr1, 3.5 107 yr1, and 7.1 107 yr1 in the deeper layer. This finding experimentally confirms earlier and more uncertain observations at the Mer Bleue bog.10,11 Based on the experimental design the decline in decomposition constants with depth can be linked to the accumulation of DIC and CH4, which could not be clearly demonstrated in the earlier field study. 9986
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Environmental Science & Technology The previously reported anaerobic decomposition constants decreased from about 104 yr1 near the surface to 107 yr1 in the deeper catotelm, including potential changes in intrinsic peat decomposability with depth. The in situ decomposability of buried, water saturated, and anoxic peat is thus likely considerably smaller than previous estimates of decomposition constants between 104 yr1 to 105 yr1 26,27 have suggested. In line with the DIC and CH4 production, total microbial biomass C in the columns also decreased with depth, albeit not as steeply (Figure 2). The extracted peat microbial C content decreased with depth from 855 ( 66 μg g1 (dry weight) to 444 ( 12 μg g1 (15% peat) and 451 ( 21 μg g1 to 271 ( 18 μg g1.
Figure 2. Extractable microbial biomass C in peat of the 5% peat and 15% peat columns after 550 days. Error bars represent standard deviation (SD) for three replicates.
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Such contents are similar to those of anoxic peat at depths of 5070 cm depth obtained from column experiments with peat from the Mer Bleue reference site and a further poor fen site at the Experimental Lakes Area, and considerably lower than in aerated peat surface layers (3.78.0 mg g1).22 The “background” microbial biomass C obtained from larger depths, where steady state respiration rates were very low (Figure 1), may represent build-up of biomass C after column preparation, when DIC and CH4 were still actively produced in the deeper peat. Nonsteady state production, before respiration processes were inhibited, still occurred between day 370 and 550 in the 5% peat column, as indicated by DIC and CH4 concentration increase over time (Figure 1). The negative feedback of peat burial, and the involved anoxic and stagnant conditions, on DIC and CH4 release can be related to changes in Gibbs free energy with DIC and CH4 accumulation. Hydrogenotrophic methanogenesis was, in this particular case, infeasible in the bulk peat, as ΔGr values were mostly positive after 370 days and only approached 10 kJ mol1 (CH4) in the 5% and 50% peat treatments after 550 days (Figure 3). Levels of H2 concentration (∼0.1 to 3 nmol L1, Figure 4), which were too low to support methanogenesis, hence equilibrated with a thermodynamically more potent electron acceptor than DIC. Unlike H2 levels, concentrations of acetate, or alternative substrates, apparently did not adjust to consumption of this electron acceptor, because this would have resulted in absence of methanogenesis altogether.28 Even considering uncertainty about geochemical microheterogeneities that have been identified,29 lacking free energy of hydrogenotrophic methanogenesis suggests that CH4 was produced via the acetoclastic pathway or possibly a pathway that we did not analyze. This finding differs
Figure 3. Gibbs free energies of acetoclastic and hydrogenotrophic methanogenesis in experimental peat-sand mixtures of 5% peat, 15% peat, and 50% peat (mass basis) after 370 and 550 days. Methane production rapidly proceeded only in the uppermost peat layers, where Gibbs free energies of the acetoclastic pathway were , 25 kJ mol1 (CH4). 9987
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Figure 4. Hydrogen concentrations in the porewater in experimental peat-sand mixtures of 5% peat, 15% peat, and 50% peat (mass basis). Peaks in H2 concentrations were recorded where DIC production was fastest and organic electron accepting moieties with higher redox potential potentially already exhausted.
from previous in situ measurements at the Mer Bleue site, where H2 levels in deep peat deposits were considerably higher.10,11 In line with the peat inactivation hypothesis, declining rates of methanogenesis in the columns with depth occurred when the acetoclastic pathway of methanogenesis approached 25 kJ mol1 (CH4). A similar effect occurred over time before steady state concentration profiles were attained: In the 5% peat treatment, CH4 concentration increased rapidly when Gibbs free energies were still more negative between day 370 and 550. They did not in the 50% peat treatment during this period any more, arguably because the theoretical thermodynamic threshold was approached earlier (Figures 1 and 3). At the threshold, which is close to a theoretical minimum energy quantum conserved in ATP generation, the driving force of acetoclastic methanogenesis approaches zero and the process should cease.15 A thermodynamic constraint cannot be quantified regarding DIC production because the distribution of thermodynamic stability constants of the utilized electron acceptors is unknown. As humic substances represent a redox ladder,30,31 we expected molecular hydrogen concentrations to increase over time with exhaustion of thermodynamically more potent moieties.14,15 This expectation was in agreement with the increasing H2 concentration peak at depths of 1025 cm over time (Figure 4). We speculate that the exhaustion of such electron accepting organic moieties and the concurrent accumulation of DIC lead to the negative feedback on anaerobic respiration. Alternative explanations for the slow-down of DIC and CH4 release in deep peat, such as a decrease in organic matter reactivity over time after preparation of columns and establishment of anoxic conditions,32 and a deactivation of phenoloxidase enzymes by anoxic conditions,3 can neither explain the observed decline of DIC and CH4 production deeper into the homogeneous peat-sand mixtures (Figure 1), nor the decrease in extractable microbial biomass C (Figure 2). The peatsand mixtures were homogeneous throughout the cores and a slow-down in rates due to such effects should have thus been visible at all depths, rather than only in the deep peat. The findings have implications with respect to the stability of the peat C storage and CH4 production and emission. From the experiments it is obvious that accumulation of DIC and CH4 slowed anaerobic respiration and methanogenesis when steady state concentration levels were approached. This effect occurred at DIC concentrations of 610 mmol L1 and CH4 concentrations of 0.40.6 mmol L1, which in presence of acetate concentrations
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