CH₄ and CO₂ production below two contrasting peatland micro-relief forms: An inhibitor and δ¹³C study

Highlights

• Micro-relief significantly affected CH₄ but not CO₂ production in surface peat soil.

• Soil of 10–50 cm depth produced up to 90% of the CH₄ and 50% of the CO₂.

• Hollows' topsoil showed the highest relative contribution of acetoclastic pathway (92%).

• The contribution of hydrogenotrophic pathway of methanogenesis increased with depth.

Abstract

Two peatland micro-relief forms (microforms) – hummocks and hollows – differ by their hydrological characteristics (water table level, i.e. oxic-anoxic conditions) and vegetation communities. We studied the CH₄ and CO₂ production potential and the localization of methanogenic pathways in both hummocks and hollows at depths of 15, 50, 100, 150 and 200 cm in a laboratory incubation experiment. For this purpose, we measured CH₄ and CO₂ production rates, peat elemental composition, as well as δ¹³C values of gases and solids; the specific inhibitor of methanogenesis BES (2-bromo-ethane sulfonate, 1 mM) was aimed to preferentially block the acetoclastic pathway.

The cumulative CH₄ production of all depths was almost one fold higher in hollows than in hummocks, with no differences in CO₂. With depth, CO₂ and CH₄ production decreased, and the relative contribution of the hydrogenotrophic pathway of methanogenesis increased. The highest methanogenic activity among all depths and both microforms was measured at 15 cm of hollows (91%) at which the highest relative contribution of acetoclastic vs. hydrogenotrophic pathway (92 and 8%, respectively) was detected. For hummocks, the CH₄ production was the highest at 50 cm (82%), where relative contribution of acetoclastic methanogenesis comprised 89%. The addition of 1 mM BES was not selective and inhibited both methanogenic pathways in the soil. Thus, BES was less efficient in partitioning the pathways compared with the δ¹³C signature. We conclude that the peat microforms – dry hummocks and wet hollows – play an important role for CH₄ but not for CO₂ production when the effects of living vegetation are excluded.

Introduction

Northern peatlands historically have been a sink for atmospheric carbon dioxide (CO₂). They also have the potential of releasing large amounts of CO₂ and methane (CH₄) into the atmosphere, both naturally or as a result of environmental and anthropogenic forcing (Limpens et al., 2008). Both CO₂ and CH₄ are important greenhouse gases (GHG, IPCC, 2014) whose balance in peatland ecosystems is regulated by multiple environmental factors. Among them are the water table level, which controls the aeration status of the peat (Moore and Knowles, 1989, Moore and Roulet, 1993, Granberg et al., 1997, Kettunen, 2003), the peat quality, which reflects the decomposability of constituent substances (Svensson and Sundh, 1992, Granberg et al., 1997, Yavitt et al., 2000), the vegetation, which regulates peat quality and the transfer of gases belowground and to the atmosphere (Whiting and Chanton, 1993, Bubier et al., 1995), and the temperature, which controls the microbial metabolic reactions (Crill et al., 1993, Granberg et al., 1997, Granberg et al., 2001, Winden et al., 2012). In peatlands, pronounced changes in environmental conditions occur vertically with peat depth and horizontally via micro-relief. This highlights the role of the different locations, each with specific physical and biochemical conditions (Lai 2009). Micro-relief and peat depth determines the interaction between the atmosphere, vegetation and the subjacent peat (Sundh et al., 1994, Granberg et al., 1997, Bergman et al., 2000, Dorodnikov et al., 2011). Such interactions result in the formation of distinct micro-relief forms (microforms). Thus, depending on the surface elevation, three microforms are distinguished: elevated hummocks, depressed hollows and intermediate lawns (Bubier et al., 1993). Two contrasting microforms – hummocks and hollows – distinctly differ by the water table level, i.e. the subsurface of water-logged hollows is typically anaerobic as compared to drier hummocks, thereby stressing the difference in redox processes between the two microforms (Kettunen, 2003). Furthermore, the plant species composition is closely connected with the water table and moisture conditions (Waddington and Roulet, 1997). Vegetation controls the input of plant-derived deposits into the microforms, hence affecting the carbon turnover and the formation and emission of GHG (Ström et al., 2005). For example, in a boreal oligotrophic fen, the hollows-dominating Scheuchzeria palustris contributed 2–4 times more to methanogenesis than the hummocks-dominating Eriophoprum vaginatum. This difference was mainly caused by differences in rhizodeposition, i.e. the release of organic compounds by plant roots into their surrounding environment (Dorodnikov et al., 2011).

Most studies so far have focused on measuring aboveground GHG flux to the atmosphere as related to microform type. The CH₄ fluxes reportedly decrease in the order hollows > lawns > hummocks, and the highest fluxes from hollows were mainly explained by persistent water-logged conditions (Bubier et al., 1993, Granberg et al., 1997, Saarnio et al., 1997, Forbrich et al., 2010, Aleina et al., 2016). In contrast, the lower water table and higher soil temperatures were proposed as the main factors controlling CH₄ oxidation and CO₂ respiration rates in aerated hummocks (Granberg et al., 1997, Dalva et al., 2001, Becker et al., 2008, Gažovič et al., 2013). Similar to in situ measurements, incubation studies under controlled conditions show that the CH₄ production potential increases from hummocks through lawns to hollows (Saarnio et al., 1997, Juottonen et al., 2015, Robroek et al., 2015). Importantly, the available in vitro studies focused mostly on the top 20–30 cm (down to 1 m) peat profile, where the soil organic matter (SOM) decomposition rates and the vegetation effects are the highest. However, anaerobic deep peat layers (deeper than 1 m) produce and store enormous amounts of GHG and substantially contribute to the surface efflux (Waldron et al., 1999, Glaser et al., 2004, Clymo and Bryant, 2008, Steinmann et al., 2008). Despite its importance, we still insufficiently understand the mechanisms controlling belowground CH₄ and CO₂ dynamics in profile layers deep below the subsurface of microforms.

Generally, CH₄ cycling in peatlands consists of CH₄ production (methanogenesis) in the anoxic parts of the soil by methanogenic archaea (methanogens) and CH₄ oxidation (methanotrophy) in presumably oxic layers (Lai, 2009). Methanogenesis involves two main pathways that can occur solitary or in parallel: (1) acetate cleavage (acetoclastic pathway), which mostly takes place in the presence of fresh soil SOM and (2) CO₂ reduction with hydrogen (H₂) (hydrogenotrophic pathway) when electron acceptors other than CO₂ are not available (Hornibrook et al., 1997, Popp et al., 1999). CO₂ production occurs during all respiratory pathways including anaerobic SOM fermentation and acetoclastic methanogenesis. In the oxic part of the soil it is also released by plant- and microbial respiration, together with methanotrophy. As described above, peatland microforms are distinguished by the thickness of the aerated zone in the peat and plant communities that supply microorganisms with organic substrates. This in turn may affect the proportion of the two methanogenesis types in hummocks and hollows, especially with depth (Dorodnikov et al., 2013). Interestingly, in contrast to the well-defined in situ pattern of an increasing contribution of hydrogenotrophic pathway to overall methanogenesis with depth (based on δ¹³CCH₄ and on δ¹³CCO₂ data, e.g. Hornibrook et al., 1997, Popp et al., 1999, Steinmann et al., 2008), microbial community studies report contradicting results. Thus, the main methanogenic microbial groups present in the upper peat layer (study-defined 10–40 cm) were identified as the hydrogen-utilizing methanogens – Methanomicrobiales (Galand et al., 2002, Galand et al., 2003). In contrast, in deep peat the dominant groups were related to Methanocellales (putative hydrogenotrophs, Liebner et al., 2015) and Methanosarcinales, which can perform both methanogenic pathways (Galand et al., 2002, Putkinen et al., 2009).

Among other factors controlling CO₂ and CH₄ production in peatlands, GHG fluxes could be altered by the deposition of compounds and availability of anions such as ammonium (NH₄⁺), nitrate (NO₃⁻), sulfate (SO₄^2 −), metals, e.g. iron (Fe) (Granberg et al., 2001, Eriksson et al., 2010, Sutton-Grier et al., 2011, Lozanovska et al., 2016). Peatlands are supplied with N and S compounds mainly through anthropogenic eutrophication of inland waters and/or acidic deposition from the atmosphere (Sutton-Grier et al., 2011). Along with the nutrition effect of N, S and Fe compounds for the plant- and microbial communities, these elements participate in redox reactions as alternative electron acceptors when oxygen availability is low. The presence of alternative electron acceptors can reduce CH₄ production due to a combination of inhibition and competitive effects between methanogens and other microorganisms for electron donors (Bodegom and Stams, 1999, Eriksson et al., 2010).

Under laboratory conditions, the mechanisms involved in CH₄ and related CO₂ dynamics can be studied using an approach with a specific inhibitor of methanogenesis, 2-bromo-ethane sulfonate (BES). BES is known to inhibit the reductive demethylation of methyl-Coenzyme M (Müller et al., 1993), a coenzyme required for methanogenesis. BES added at a certain concentration reportedly inhibits the acetoclastic – but not the hydrogenotrophic – pathway of CH₄ production (Zinder et al., 1984). Therefore, amending peat soil with BES may help to reveal the distribution of methanogenic pathways between microforms and with depth. Another method to partition methanogenic pathways is based on the stable C isotope signatures (represented as δ¹³C values) of CH₄ and CO₂, which reflect the CH₄ pathway formation (Whiticar, 1999, Conrad, 2005). Accordingly, CH₄ produced by the acetoclastic pathway is less ¹³C depleted (e.g. shows higher δ¹³C values) than CH₄ produced by the hydrogenotrophic pathway (lower δ¹³C values). This is because methanogens more strongly discriminate against heavier ¹³C during the latter process (Whiticar et al., 1986, Avery et al., 1999). The combination of both methods is assumed to provide strong evidence for the respective methanogenic pathway. If the inhibitor BES blocks CH₄ production by the acetoclastic pathway, then the respective δ¹³C–CH₄ signature should decrease due to a higher contribution of ¹³C–depleted CH₄ produced by the hydrogenotrophic pathway as compared to the control (without inhibitor). Nonetheless, other important factors influencing δ¹³C in CO₂ and CH₄, e.g. the δ¹³C value of the organic substrate, fractionation during gas diffusion, CH₄ oxidation, must also be considered. Avery et al. (1999), Steinmann et al. (2008) and Clymo and Bryant (2008) gained valuable information about vertical and seasonal changes in the isotopic composition of CH₄ in peat profiles. Nonetheless, very little information is available about the effects of peatland micro-relief on the patterns of CH₄ and CO₂ isotopic signatures (Dorodnikov et al., 2013).

This study was designed to cover two aspects. Firstly, to estimate the production potential of CH₄ and CO₂ in the whole depth profile (down to 200 cm) below two contrasting microforms – wet hollows and dry hummocks – and to link this potential with the peat elemental composition. Secondly, to identify the contribution of the two methanogenic pathways in hummocks and hollows with depth by amending a specific inhibitor of methanogenesis and by measuring δ¹³C in CH₄, CO₂ and peat organic matter. The following hypotheses were tested:

• Under controlled anaerobic conditions, naturally wetter hollows will show an overall higher CH₄ but lower CO₂ production potential as compared with drier hummocks. This relationship should preferentially be caused by the adaptation mechanisms of microbial communities in microforms and be more pronounced in the upper peat layer.

• With depth, the CH₄ and CO₂ production potential will decrease under both microforms, predominantly due to the decomposition state of the peat organic matter.

• Due to higher availability of fresh plant-derived deposits in the upper vs. deeper peat layers, the contribution of the acetoclastic pathway to overall methanogenesis will decrease with depth below both microform types; between the microforms, more intensive rhizodeposition in hollows will promote the contribution of the acetoclastic pathway, as compared to hummocks, at least in the topsoil.