Author for correspondence: Christian Fritz Tel: +31 24 3652337 Email: firstname.lastname@example.org
•Vascular wetland plants may substantially increase methane emissions by producing root exudates and easily degradable litter, and by providing a low-resistance diffusion pathway via their aerenchyma. However, model studies have indicated that vascular plants can reduce methane emission when soil oxygen demand is exceeded by oxygen released from roots. Here, we tested whether these conditions occur in bogs dominated by cushion plants.
•Root–methane interactions were studied by comparing methane emissions, stock and oxygen availability in depth profiles below lawns of either cushion plants or Sphagnum mosses in Patagonia.
•Cushion plants, Astelia pumila and Donatia fascicularis, formed extensive root systems up to 120 cm in depth. The cold soil (< 10°C) and highly decomposed peat resulted in low microbial activity and oxygen consumption. In cushion plant lawns, high soil oxygen coincided with high root densities, but methane emissions were absent. In Sphagnum lawns, methane emissions were substantial. High methane concentrations were only found in soils without cushion plant roots.
•This first methane study in Patagonian bog vegetation reveals lower emissions than expected. We conclude that cushion plants are capable of reducing methane emission on an ecosystem scale by thorough soil and methane oxidation.
Effects of vascular plants on methane cycling in wet soils
Wetlands are favourable habitats for methanogenic archaea that form methane during the decomposition of organic material. These methanogens require environments with no oxygen and abundant organic matter, both of which are present in wetland conditions (Segers, 1998). Peatlands dominated by Sphagnum mosses (bogs) are known for the slow decomposition of dead organic matter. The refractory nature of Sphagnum litter is mainly responsible for this slow decomposition, as other plants, including typical bog species, decompose much more rapidly than mosses (Aerts et al., 1999; Woodin et al., 2009). Therefore, if nutrient availability permits the dominance of vascular plants, the potential production of methane is strongly increased by a high production of vascular plant biomass, which results in an increased input of more easily decomposable litter (Whiting & Chanton, 1993; Joabsson & Christensen, 2001). Underground vascular plant tissue can also transport labile carbon compounds into anoxic soil layers (Joabsson & Christensen, 2001; Ström et al., 2003; Chanton et al., 2008). Such increased substrate stocks for methanogenic archaea may be crucial because methane production is frequently substrate limited (reviewed in Whalen, 2005). Moreover, aerenchymatous roots can strongly stimulate the export of methane by creating shortcuts to the atmosphere (van der Nat & Middelburg, 1998; Kutzbach et al., 2004; Whalen, 2005).
The presence of roots, however, may also decrease the release of methane. Oxygen diffuses through the aerenchyma of vascular plants from the atmosphere into the roots and subsequently leaks into the rhizosphere (Armstrong et al., 1991, 2006). Under such oxic rhizosphere conditions, methane production can be reduced by two orders of magnitude (reviewed in Segers, 1998). In addition, when oxygen is present, methane stocks can be decreased by oxidation via methanotrophic bacteria (King, 1994; Sorrell et al., 2002; Raghoebarsing et al., 2005). The passage through a thick aerobic soil–atmosphere interface (i.e. 3–20 cm of aerobic soil) can thus oxidize most of the methane (Roulet et al., 1993; Daulat & Clymo, 1998; Hornibrook et al., 2009). Our study investigated methane release from bog lands that have high root densities, and provides evidence that certain wetland ecosystems do not produce nearly as much methane as do most temperate and tropical wetlands.
The extent to which the rhizosphere can become aerated depends on various conditions: root density, rate of oxygen loss from the roots, soil oxygen consumption and the diffusion coefficient of oxygen in the soil. Under most conditions, the combination of high oxygen consumption (high temperature, suitable substrate and high microbial activity) and limited oxygen release (limited oxygen conduction capacity and low root density) will result in a very thin oxic rhizosphere. Therefore, in wetland soils, a large fraction of the substrate surrounding a root remains anoxic despite root oxygen loss (Armstrong et al., 1991, 1992). Such incomplete oxidation of organic soils promotes the coexistence of roots and methane (Grosse et al., 1996), ultimately resulting in increased emission of methane (Watson et al., 1997; Ding et al., 2005).
Depending on the type of vegetation, the potential to lower methane emission by the creation of oxic soil conditions varies from 16% to 95% (Laanbroek, 2009). Extensive rhizospheric oxidation requires a dense root biomass (Grosse et al., 1996; Smolders et al., 2002), which, in turn, provides extra carbon for methane production. However, many studies ignore such additional methane production fuelled by plant litter and root exudates when estimating the oxidation potential of the rhizosphere. The root–methane interaction model of Watson et al. (1997) required high root biomass to find considerable methane oxidation. Only thorough rhizospheric oxidation created a sufficiently large spatial separation of roots from methane to prevent aerenchyma-mediated diffusion (Grosse et al., 1996). Methane emissions can become temporarily decoupled from vascular plant cover when the water levels are low (Bubier, 1995; Couwenberg et al., 2010). At these dry sites, the water table drops below the bulk root mass, so that methane is oxidized before being released via plants.
We hypothesized that in, vascular plant-dominated wetlands, methane release may be decreased or even absent if the soil is thoroughly oxidized by extensive rhizosphere oxygen loss. In search of such wetlands, we targeted methane-producing wetlands (e.g. deep bogs) where oxygen consumption is low. The rainy cold parts of Patagonia harbour pristine bogs with very few nutrients (Kleinebecker et al., 2008; Schmidt et al., 2010). Darwin (1839) described bogs in Patagonia with deep rooting plants growing as dense cushion-like vegetation that formed extensive blanket bogs. Cushion plants, such as Astelia ssp. and Donatia ssp., form dense root systems consisting of shallow tap roots and aerenchymatous roots of > 100 cm in length (Darwin, 1839; Grootjans et al., 2010). Our objective was to elucidate whether these roots negatively affected methane emission by comparing densely rooted sites with sites covered only by moss species (Sphagnum ssp.). Interactions of roots with soil methane cycling were studied by correlating the vertical distribution of methane stock, oxygen availability and methane oxidation potential with the root biomass density of cushion plants.
Materials and Methods
Sampling design and description of experimental sites
The effects of roots on methane (CH4) were studied by comparing methane dynamics in different bog vegetation in southernmost Patagonia: cushion plant lawns in a cushion bog (high root biomass); Sphagnum magellanicum lawns adjacent to the cushion plant lawns mentioned above (nonrooted sites in a system dominated by roots); Sphagnum magellanicum lawns in a control bog with a cover of vascular plants less than 1% (nonrooted sites in a system with only a few roots). Each type was represented by three replicates. In addition, we included two pools surrounded by cushion plants, but without roots in the soil, to estimate methane emissions independent from atmospheric oxygen (oxidation) and roots (gas transport).
Field measurements and experiments were performed in a cushion bog peatland in Tierra del Fuego (Moat, 54°58′S; 66°44′W, 40 m asl) where average daily air temperatures are 5–6°C with cold summers (maximum average temperature, 9°C; R. Iturraspe & C. Fritz, unpublished). July is usually the coldest month at 2°C. The absence of a thermal summer is typical for oceanic bogs in Patagonia (Kleinebecker et al., 2007 and literature therein). The control bog (Andorra, 54°45′S; 68°20′W, 200 m asl) shows slightly higher daily and seasonal temperature differences during the summer because of its location at a valley bottom (Iturraspe et al., 1989). The soil temperature was low and stable at both bogs throughout the growing season, decreasing from 8 to 12°C at 5 cm below the surface to 4–8°C at 100 cm depth. Soil temperature profiles were recorded during expeditions in spring 2006 and summer 2007. Annual precipitation assessed in the 1980s and from 2008 onwards exceeded 60 cm, evenly distributed over the year in both peatlands, providing wet conditions (Iturraspe et al., 1989; R. Iturraspe & C. Fritz, unpublished). Water levels fluctuated between 5 cm above and 20 cm below the surface at all lawn sites from spring to autumn.
The cushion bog was dominated by lawns of evergreen cushion plants intermingled with patches (few square metres) of dominating Sphagnum magellanicum (Bridel) and scarcely vegetated pools (Roig & Collado, 2004; Gebser, 2008). Dominating cushion plants were Astelia pumila (G. Forster) R. Br. and Donatia fasciculares R.R. et G. Forster covering > 70%. The soil below cushion plants was densely packed with tap roots (1–2 mm diameter) and fine roots exceeding depths of 120 cm (Grootjans et al., 2010). By contrast, roots and vascular plants were almost absent at Sphagnum sites. The peat depth was comparable between sites, ranging from 700 to 1000 cm, thus providing large stocks of carbon-rich substrate. The densely rooted cushion plant peat was highly decomposed (H8–H10 on the Von-Post scale), contrasting with the well-preserved Sphagnum peat (Kleinebecker et al., 2007; Gebser, 2008). Peat formed by cushion plants was three to five times denser than Sphagnum peat. At all sites, Sphagnum peat was found at depths greater than 300 cm. The peatlands studied remained unaffected by anthropogenic alteration, such as drainage, agricultural use or elevated atmospheric nutrient deposition. Reviewing scarce deposition data from Patagonia, Godoy et al. (2003) suggested bulk nitrogen depositions below 0.1 g N m−2 a−1 in coastal regions. The substrate was very low in nutrients, with total phosphorus concentrations typically below 0.023% in Sphagnum peat and 0.034% in cushion plant peat. Pore water reflected acidic conditions in both bogs (pH 3.8–4.2) with little variation in the upper 300 cm.
Methane/ethane concentration measurements
Methane and ethane headspace samples were measured on a Hewlett-Packard® (Avondale, California, USA) 5890 gas chromatograph equipped with a flame-ionization detector and a Porapak Q column (80/100 mesh), operated at 120°C with nitrogen as carrier gas, in the laboratory of Radboud University, Nijmegen, the Netherlands (accuracy, 0.2 ppm). The injection volume was 0.1 ml for incubations and pore water samples and 0.5 ml for emission samples to improve the detection of low concentrations.
Methane dynamics (stock and emission)
Methane stock and release were estimated by means of pore water concentration and emission into static chambers, respectively. Sampling took place over the growing season: December 2008 (spring), February 2009 (summer) and late March 2009 (autumn). For logistic reasons, sampling was delayed for 1 wk in the control bog. Insights into inter-annual and seasonal variations in methane stock were addressed by sampling pore water eight times from 2006 to 2009 at one site per vegetation type.
Pore water samples were drawn from eight depths (5, 30, 60, 120, 150, 180, 300 and 600 cm) in the cushion bog and from five depths (5, 50, 150, 300 and 500 cm) in the control Sphagnum bog. Anaerobic peat water samples were taken using 5-cm ceramic cups (Eijkelkamp Agrisearch Equipment®, Giesbeek, the Netherlands), connected to vacuum infusion flasks (40 ml) after sampling 150 ml to exclude internal stagnant sampler water. The 40-ml glass infusion flasks had a sample to headspace ratio of, usually, 1 : 2. As internal standard, 1 ml of ultrapure ethane gas (Airliquide®, Eindhoven, the Netherlands) was added after sampling and flasks were stored at 4°C during < 2 wk until analysis. Microbial modification of samples was hampered by the addition of 0.1 mg HgCl2 (0.1 ml of 0.1 g l−1). Methane and ethane concentrations were measured in the headspace after vigorous shaking, releasing > 96% of methane to the headspace.
Methane emissions were assessed using dark static polyvinyl chloride (PVC) chambers (3700 cm3, 15 cm high) with bleeds of 4-mm PVC hose. PVC frames were installed 2 months before measurement and removable chamber tops were sealed to the frame by the water-filled rim. Gas samples were taken in the morning and in the afternoon on the same day at all sites per peatland. After placing the chambers, the temperature differed by < 3 K between t = 0 and the end of sampling. Gas samples were taken with a double-sided needle for 60 min in 20-min intervals in a pre-vacuumed 12-ml glass vial with a butyl stopper (Exetainer®, High Wycombe, UK). At sites with very low emissions, an additional sample was taken after 360 min. Samples were stored cool and analysed within 1 wk. Emission data are presented for 53 of 62 measurements where the linear slope fitted r² > 0.75 or when the methane headspace concentration stayed constant (zero emissions). Rejected time series, mostly pool sites, were probably subject to ebullition, as observed by unexpectedly high methane concentration at t = 20 min followed by depletion afterwards. After 2 months of additional storage, > 95% of original methane was retrieved in pore water bottles. Emission samples maintained equal concentrations. Methane release by large-scale ebullition was estimated by surface elevation fluctuations measured with water level recorders attached to the surface and a stable benchmark, as described by Fritz et al. (2008). Automatic recorders (Odyssey capacitance probes®, Dataflow, Christchurch, New Zealand) were set up to measure levels in 1-h intervals during 2 yr and confirmed with hand measurements during field visits.
Redox potential and oxygen measurements
Redox potential measurements were taken at five depths (30, 60, 120, 150 and 200 cm), 2–3 d after gas sampling. Per depth, four platinum electrodes were gently pushed into a pre-made hole and allowed to equilibrate. Stable readings were generally obtained after 30–60 min. In most cases, the drift was smaller than 1 mV min−1 within 10 min. The redox potential (E7) corrects the field measurements (Efield) for pH (pHsoil), absolute temperature in K (T) and the potential of the 3 M AgCl/Cl reference electrode (Eref = 217 mV at 10°C) using the following relationship:
Literature on redox processes (e.g. Laanbroek, 1990) suggested E7 values of > 330–350 mV as an indication for free oxygen in soils. In figures showing redox data, we highlight 350 mV as a threshold for occurrence of free oxygen, also used by similar studies (Visser et al., 2000). However, other studies found some nanomoles of oxygen for E7 just above 300 mV (Lloyd et al., 1998). Oxygen content in the soil was measured polarographically at 30 and 70 cm below the water table in the cushion bog in February 2009. For oxygen measurements, we deployed platinum needle electrodes with a sensing tip of < 0.1 mm embedded in stainless steel (Microscale Measurements, The Hague, the Netherlands). The platinum tips remained protected by cellulose-nitrate membranes. Oxygen electrodes were connected to a custom-made nA-meter (Electronic Workshop, University of Groningen, the Netherlands) and an AgCl/Cl reference electrode. To calibrate, we used oxygen-saturated bog water in the field. Zero point calibration was performed in laboratory demineralized water flushed with nitrogen for at least 24 h. Persistent precipitation prevented frequent measurement of oxygen and also the establishment of polarograms at various depths. We measured currents at some 450-mV pre-settings of the equipment obtained from polarograms in Dutch bogs.
Root density at the cushion plant site was determined by sampling in a piston corer [internal diameter (ID), 10 cm] to a depth of 80 cm and from 50 to 250 cm using a D-Section corer (ID, 4.7 cm; Eijkelkamp Agrisearch Equipment®, Giesbeek, the Netherlands). Roots were dried at 70°C for 2 d. Root density is expressed in gram per litre of soil. The presence of living fine roots of cushion plants (1–2 mm in diameter) usually coincided with a sharp change from black coloured peat to yellow–brown peat below the (oxygenated) rooting zone. In this article, the rooting zone comprises the entire volume of soil down to the maximum root depth. The proportion of rhizosphere to rooting zone depends on the root density and the space that is affected by the activity of individual roots. Integration of the root density over the entire rooting zone rendered the total dry root biomass expressed as g m−2. The porosity of root material was determined in 1-cm increments using the microbalance method (Visser & Bögemann, 2003). To visualize oxygen loss in the rooting zone, we exposed cushion plants to an anaerobic methylene blue solution (25 mg l−1 methylene blue, 0.5 g l−1 agar, 5 mM KCl, 0.05 mM CaSO4) filled in glass cuvettes in the laboratory. Sodium dithionite (Na2S2O4) was used to decolorize the dye. The leaves projected into the air, but the surface of the solution was protected from the air by plastic and gently flushed with nitrogen (adapted after Armstrong et al., 1992).
To highlight the functional role of cushion plants with respect to the redox state of the soil, the oxygen transport below the water table was hampered by removing the green parts of cushion plants in January 2008. We chose to remove four large areas of 2 × 2 m to reduce the effects of surrounding cushion plants. Differences in methane dynamics were documented by methane pore water samples taken at three depths (60, 150 and 300 cm) after 1 month, 13 months and 26 months, as described above. The redox potential (E7) was measured before and 13 months after the removal of green parts. Methane emissions were measured at three clipped sites 2 yr after clipping. Regrowth was minimal within 2 yr, which highlights the harsh growing conditions.
Methane production and consumption
To estimate differences in potential methane production and consumption, we took peat cores in the cushion bog (6 December 2008). Samples (100 ml) were placed in airtight plastic bags in the field and stored at 4°C before being processed in the laboratory. Differences in potential production were measured in one pure Sphagnum magellanicum and one cushion plant site dominated by A. pumila at 20, 70 and 120 cm depth. The activity of methanotrophs was estimated by incubating peat along a profile at cushion plant site 1. Samples were collected at depths of 70, 120, 140 and 150 cm. The maximum rooting depth was 140 cm at this site.
The interior of the bulk peat was subsampled by taking 20 g fresh weight of soil [some 1.2 g dry weight (DW)], which was incubated in 100-ml grey rubber-stoppered glass flasks threefold at 22°C. For potential methane production, flasks were flushed with nitrogen and vacuumed eight times to remove methane and oxygen. For aerobic production, we incubated with ambient air. Methane consumption incubations contained a headspace of ambient air and methane was added to a final concentration of 1.2–1.5%. Methane headspace concentration was frequently determined over 7 wk in both production and consumption incubation. Rates of methane production/consumption were derived from the linear part of the slope related to the weight of the sample after drying at 70°C for 48 h. For the cushion plant site, we present methane production rates for the beginning and end of the incubation period because rates differed by one order of magnitude. Oxygen depletion was regularly controlled by means of CO2 headspace concentrations determined on an infrared gas analyser (IRGA, ABB Advance Optima, Zürich, Switzerland). Bulk density samples were taken at the same locations using the D-Section corer (ID, 4.7 cm) mentioned above, and dried at 70°C for 48 h. Rates of methane consumption and production are related to volume and to DW, respectively. Rates can be related to either the surface (volume) or substrate quality (DW).
Methane emission and physical factors (temperature, water table)
Methane (CH4) emissions were low, but significant, reflecting the low temperature and nutrient status of the bog sites investigated (Fig. 1). In cushion plant lawns, however, emissions approached zero. One cushion plant site exhibited emission rates below 1 mg CH4 m−² d−1 on two occasions. In contrast with these virtually zero emissions, in Sphagnum lawns of the cushion bog the methane emission rate was 1–14 mg CH4 m−2 d−1 (95% confidence interval), and similar to the control bog (1–11 mg CH4 m−2 d−1). Emissions of pools were in the same range as those of Sphagnum lawns (Fig. 1). The highest emissions were found where cushion plants had been clipped. The average water levels during the measurements were comparable between the different sites (c. 5 cm, with cushion plant lawns being slightly drier), but fluctuated seasonally by some 10 cm from the mean water level (9 cm below the surface). Therefore, differences in water level did not correlate with methane emission rates (r² = 0.05, n =53). The temperature was 8–12°C in air and 10°C in the first 10 cm of the soil, varying by < 2°C between measurements on the same day. Monitoring of surface elevation in cushion plant and Sphagnum lawns gave no indication of lifting of the peat surface by several centimetres within hours, which is associated with large-scale ebullition. Weak ebullition events could only be triggered in pools and Sphagnum growing in pools by jumping of the observer in the direct vicinity. The low frequency of ebullition generally indicates a low concentration of methane in the upper peat layers.
Methane stock (pore water profiles)
Similar to emission rates, the methane stock varied strongly between different vegetation types, reflecting the presence of cushion plants and their deep roots (Fig. 2). Importantly, no methane (< 1 μmol l−1) was found in the rooting zone of cushion plants, whereas Sphagnum lawns stocked less methane in the cushion bog compared with the control bog. The linear increase in methane concentration with depth was similar between the cushion bog and control bog; however, there was a 170-cm offset between the two bog types, which coincided with the maximum depth of the rooting zone in the cushion bog (Fig. 2). Unexpectedly, an offset in methane stock at this depth was also found in Sphagnum lawns < 3 m adjacent to cushion plants. In the upper 170 cm, the mean methane pore water concentration of 166 μmol l−1 (SD = 46, n =45) remained stable with depth. This plateau of intermediate methane concentrations differed from the generally increasing methane concentrations with depth (Fig. 2). It needs to be stressed that Sphagnum patches formed small islands closely surrounded (< 3 m) by cushion plants and their rooting zone. Lateral gradients of methane pore water decreased over the same order of magnitude (50–150 μmol CH4 l−1 m−1) as the gradients in depth (220–320 μmol CH4 l−1 m−1). A levelled surface of 1% and hydraulic head differences of < 0.2% vertical (Gebser, 2008) suggested a substantial horizontal water movement. The horizontal methane gradients and water flow underline the connectivity of Sphagnum patches with their surrounding rooting zone of cushion plants. In addition, the intrusion of methane-depleted, oxic rain water most probably occurs in the sponge-like upper peat of Sphagnum lawns.
The absence of methane from the rooting zone of cushion plants becomes more visible when zooming at its lower boundary. At all three cushion plant sites, the presence of methane was tightly linked to the lower boundary of the rooting zone of cushion plants. Remarkably, methane was always found only 5–10 cm below the maximum root depth, which differed between sites (Fig. 3a,b). Below 300 cm, many pore water samples indicated supersaturation of methane (mean, 1499 μmol l−1) in the control bog. Methane stock measurements taken between 2006 and 2009 revealed the same patterns. In general, seasonal and inter-annual variations were minor compared with the striking differences between the rooting zone of cushion plants and samples from nonrooted layers.
Presence of oxygen and roots
All three cushion plant sites were characterized by a dense root biomass (Fig. 3a,b). The average root biomass density was 2.15 g DW l−1 (SD = 0.33, n =3) in the upper 170 cm. Integration of the root density along the rooting zone revealed that cushion plants maintained a total root biomass of 3590 g DW m−2 (SD = 550, n =3). The porosity of roots of the dominating cushion plant A. pumila was 60–70%, providing sufficient aerenchyma for rapid diffusion of oxygen. Lower porosity was only found within 5 cm from the root tip (apex). Staining experiments with methylene blue suggested modest oxygen release rates along the length of the root, being highest around the root tips. Root tips could be found scattered over the entire depth profile. However, the largest densities of root tips were confined to the upper 70 cm, resulting in the highest potential to release oxygen in the upper half of the rooting zone (Fig. 3c). Less than 1% of the fine root biomass was located close (< 15 cm) to accumulated methane in the soil (Fig. 3b). At Sphagnum sites, the very few roots growing down to 30 cm reflected well the very sparse cover of vascular plants.
The decrease in redox potential mirrored the increase in methane, being highly sensitive to the presence of roots of cushion plants (Fig. 4). Free oxygen in the rooting zone to a depth of 120 cm was indicated by redox potentials higher than E7 = 330–350 mV (cf. Laanbroek, 1990). The presence/activity of roots resulted in an increase in the redox potential of c. 170 mV compared with the Sphagnum site. Beyond the maximum rooting depth, the redox potential decreased rapidly to values comparable with Sphagnum sites. Root densities decreased strongly with depth, whereas redox potentials varied little in the upper 120 cm. By contrast, a tight relationship between root density and redox potential was found at the bottom of the rooting zone. At 150 cm depth, redox potentials varied substantially, c. 364 mV (SD = 61, n =9), suggesting that a smaller proportion of the substrate remained aerated. Here, the coexistence of oxic and anoxic patches correlated with small numbers of root tips and low root densities (Fig. 3b). In the lower rooting zone, only two to five root tips were found per litre of peat substrate. Hence, a surplus of oxygen (leading to the presence of free oxygen) became more variable at these depths, leading to a high spatial variation in redox potential. Seasonal variations of the redox potential were small compared with differences related to the density/presence of roots. In February 2009, oxygen-sensitive mini-electrodes gave further evidence of oxygenated conditions. Oxygen concentrations up to 5 μmol l−1 were found at 30 cm and 70 cm below the water level in cushion plant lawns. Below Sphagnum vegetation, oxygen was absent when measured by mini-electrodes.
Clipping of cushion plants caused a significant change in soil processes. Within 1 month after clipping, 3 μmol l−1 (SE = 1.4, n =4) methane accumulated in the uppermost rooting zone (Fig. 5a). After 13 months, the pore water concentration increased from zero to c. 78 μmol CH4 l−1 (SE = 49) at 60 cm and 102 μmol CH4 l−1 (SE = 39) at 150 cm. This increase in methane concentration was accompanied by a drastic decrease in the redox potential to < 300 mV, suggesting a depletion of oxygen within a year (Fig. 5b). After 26 months, mean methane emissions (10 mg CH4 m−2 d−1, n =6) exceeded those of Sphagnum lawns (Fig. 1) and methane accumulated in the pore water to a concentration of 706 μmol CH4 l−1 at 60 cm (SE = 136, n =4).
Methane oxidation and production
The activity of methanotrophs was found in the entire rooting zone of cushion plants. Mean activities ranged from 10 to 86 μmol l−1 d−1 and 0.2 to 1.2 μmol g−1 DW d−1, respectively (Fig. 6). The highest oxidation rates were found in the lower part of the rooting zone where oxygen was in the vicinity of methane-containing substrates (Figs 3, 4). The activity of methanotrophs (10–15 μmol l−1 d−1) was also found above the methane–oxygen interface. In the rooting zone, methanotrophs are methane limited (Fig. 3a), whereas, below the roots, methane consumption became oxygen limited (Fig. 4). Oxidation rates in the field may be 62% lower, assuming an average soil temperature of 8°C and a Q10 of 2, found for methanotrophs in the control bog by Kip et al. (2010).
Mean potential methane production in the upper 120 cm ranged from 1 to 20 μmol l−1 d−1 and 0.04 to 0.36 μmol g−1 DW d−1, respectively (Fig. 6). Based on volume, the highest production was found in the most recently accumulated parts: dense peat that was little decomposed and had the highest nutrient content. Because Sphagnum peat has a three to five times lower density (some 20–30 g l−1), Sphagnum sites had a lower methane production potential per surface area or volume compared with cushion plants (density of 50–120 g l−1). In aerobic incubations, methane production was below the detection limit. Thus, the actual methane production is assumed to be negligible in the rooting zone of cushion plants because of aerobic conditions (Fig. 4).
Samples taken from the rooting zone of cushion plants showed a time-lagged increase in production, exceeding the volume-based rates of Sphagnum sites (Fig. 6). The time-lagged increase indicated that the community of methanogenic bacteria had adjusted to the anoxic conditions of the incubations. As the substrate from the rooting zone of cushion plants had been subjected to oxygen release, a low presence and activity of methanogens can be anticipated. Edwards et al. (1998) found that substrates from aerobic environments or exposed to oxygen after sampling exhibited hampered activity of methanogens.
In this study, methane dynamics revealed a tight but inverse link to the presence of vascular plant roots. We found evidence that the specific conditions in cushion bogs lead to high oxygenation of the wetland soil well beyond the rhizosphere (> 150 cm), thus limiting methane production and methane release via plants. Crucial for extensive oxygenation are the nutrient-poor conditions of these sites, limiting soil oxygen demand, combined with high densities of very long and aerenchymatous roots. These data are the first on methane emissions and stocks in temperate bogs in the Southern Hemisphere and Patagonia.
The larger part of the cushion bogs consists of cushion plants, which are characterized by high densities of long aerenchymatous roots. These roots may function as a conduit for methane release. However, in the rooting zone of cushion plants, no methane was present, and only around this zone did methane levels increase steeply with (both horizontal and vertical) gradients of 200–300 μmol l−1 CH4 m–1 (Figs 2, 3). Therefore, although a large methane stock was present at the ecosystem scale, methane emissions at cushion plant sites were low, approximating zero (Fig. 1). The main reason was the oxygenation of the rooting zone by oxygen loss from the roots (Fig. 4). Root-derived oxygen suppressed methane production and increased methane oxidation, thus diminishing methane stocks in the rooting zone (Fig. 6). By contrast, studies on root–methane interactions revealed that roots remained in contact with methane. This resulted in methane emission rates exceeding those common for Sphagnum vegetation (Popp et al., 2000; Ding et al., 2004; Strack et al., 2006). Our study suggests that densely growing cushion plants have a higher potential to oxidize soil and methane than do common wetland species, for example Phragmites ssp. (van der Nat & Middelburg, 1998), Oryza ssp. (Frenzel, 2000), Carex ssp. (Popp et al., 2000; Ding et al., 2004) and Sphagnum ssp. (Larmola et al., 2010). As a consequence of incomplete oxygenation, the methane production in anoxic parts of the soil becomes fuelled by easily decomposing root exudates and litter (Joabsson & Christensen, 2001; Juottonen et al., 2005). This ‘fuelling-effect’ of vascular plants was tested in this study by long-term clipping of cushion plants. After cutting off the oxygen supply to the roots, the redox potential dropped well below 330 mV, indicating anoxic conditions in the rooting zone (Fig. 5). Consequently, a substantial methane stock built up within a few months in the upper 150 cm fuelled by decomposing roots. After 2 yr without oxygen supply, methane stocks exceeded those of Sphagnum vegetation (Figs 1, 6). Part of the built-up methane stock may have resulted from decaying roots, especially at the beginning of the experiment.
By contrast, living cushion plants can thoroughly oxygenate the organic peat soil through oxygen leakage from hundreds of root tips per litre of soil. Highly decomposed cushion bog peats are likely to consume little oxygen because of low soil temperatures (4–10°C) (Haraguchi, 1995; Chapman & Thurlow, 1998; Allen et al., 2002) and carbon densities (50 g C l−1). The recalcitrant nature of highly decomposed peat (Chapman & Thurlow, 1998) and the low nutrient availability, such as total P <0.02% (Reddy et al., 1999), further reduce oxygen consumption. The aerobic state of the soil prevailed in the upper 120 cm despite a decrease in root density with depth (Figs 3, 4). At root densities as low as two to five tips per litre, found in the bottom 10 cm of the rooting zone, an oxic state is unlikely to be maintained far beyond the root surface. However, this zone of low root density separates the bulk root surface from methane. We suggest that, in the upper profile, oxygen release rates exceed consumption. The surplus oxygen is transported by infiltrating rain water down the profile, where deeper root layers thus receive additional oxygen, next to the in situ oxygen leakage. Lateral groundwater flow can convey fairly oxidizing conditions beyond the rooting zone. This is indicated by lower methane stocks at the ecosystem level, as suggested by low methane concentration in the upper 200 cm below Sphagnum patches (Fig. 2) and pools (data not shown) in the cushion bog. This is further indicated by methanotrophic activity, which is maximal in the bottom 10 cm of the rooting zone (Figs 3b, 6). Methanotrophic activity is highest where upward diffusion of methane meets available oxygen (Watson et al., 1997; Edwards et al., 1998). Despite the low root density, sufficient oxygen is present in the bottom 10 cm of the rooting zone to maintain methane oxidation (Figs 4, 6), which results in a spatial separation of roots and methane. When soil is aerated by deep drainage, a similar separation of roots and methane results in low or zero emissions (Roulet et al., 1993; Bubier, 1995; Couwenberg et al., 2010). A thorough oxygenation of the rooting zone is essential for sufficient separation between roots and methane stock and, consequently, for complete cessation of methane emission. This has also been suggested by physical models of root–methane interactions (Watson et al., 1997; Segers et al., 2001). Such oxygenation of wetland soils by an extensive and deep root biomass requires sufficient nutrients (van Bodegom et al., 2005; Koelbener et al., 2010) that pristine bogs usually lack (van Breemen, 1995; Kleinebecker et al., 2008). Higher nutrient levels also increase oxygen consumption because litter/peat formed under nutrient-rich conditions breaks down more rapidly than recalcitrant Sphagnum litter from pristine sites (Aerts et al., 1999; Chapin et al., 2003). Incomplete oxygenation because of high soil oxygen consumption then permits the co-existence of roots and methane, resulting in methane emissions.
However, cushion plants, such as Astelia ssp. and Donatia ssp., have specific traits which allow them to develop a dense root system whilst still out-competing Sphagnum at low nutrient levels. These traits are a dense apical growth, high root to shoot ratio, very porous roots, low intrinsic growth rate, their evergreen nature and their efficient nutrient recycling (Gibson, 1990; Schmidt et al., 2010; C. Fritz, unpublished). As a result of the high nutrient use efficiency and a low biomass turnover, a dense root system can be maintained even in the very nutrient-poor Patagonian bogs (Kleinebecker et al., 2008; Schmidt et al., 2010). We show that cushion plants can form two to four times more biomass of fine roots (3590 g DW m−2) than other bog vegetation (Moore et al., 2002). A similar growth strategy is known from isoetid species growing at the bottom of nutrient-poor, soft-water lakes. Isoetid species (such as Littorella uniflora and Lobelia dortmanna) exhibit a dense root system whilst growing slowly, and also oxidize entire mineral soil layers (Smolders et al., 2002). The mutual interaction between plant (traits) and soil conditions warrants further investigation.
From our study, we conclude that, under specific circumstances, vascular plants are capable of oxidizing the bulk of soil methane that might otherwise be released via the root aerenchyma. We have highlighted an outstanding example of an inverse relation between root density and methane release. The clear spatial separation of methane from cushion plant roots resulted from the low oxygen consumption that was exceeded by oxygen loss from the roots. The influence of vascular plants on methane cycling depends on traits such as the formation of a dense root biomass in spite of nutrient-deficient conditions. Rising temperatures and habitat losses of cushion plants are expected to stimulate future methane emissions from Patagonian bogs.
For their invaluable support during field campaigns, all authors are much indebted to Victoria Surrur, Hernán Dieguez, Pablo Huelin Rueda, Hermen Keizer, Ronny Gebser and many others who made field work successful in extreme weather. We are grateful for the valuable comments of three anonymous referees. Facilities offered by the staff of the Prefectura Naval Argentina were highly appreciated. Important to us was the refrigerator, working space and logistics offered by Lucas Varela of the La Posta Hostal family. We would like to thank Gerard Bögemann for offering his redox equipment and expertise in measuring the porosity of roots. Susanne Abel is acknowledged for sharing her experience in taking root samples from cushion bogs. Assistance in methane incubation and isolation of methanotrophs by Nardy Kip was highly appreciated. Collaboration with CONICET was conducted within the Convenio XXI.