Blog: Trace Gas Dynamics in Tropical Forests

trop

By Bertie Welch. 18th December 2013.

After carbon dioxide, methane is the second most important greenhouse gas, yet how much is produced and transferred to the atmosphere by tropical rainforests is not fully understood, because much of the research in the tropics focuses on rice paddies and wetlands, as these are significant sources of anthropogenic methane. Rice et al. (2010) suggest that globally, hardwood trees could account for emissions of ~60 Tg Ch4 yr-1.

These potentially significant methane emissions are a result of trees allowing the methane produced in deeper, anaerobic soils to bypass the methanotrophs and nitrifying bacteria in the aerobic upper soils (Milich, 1999). We know from temperate forest and mesocosm studies that trees can act as conduits for soil methane into the atmosphere.  Trees can adapt to cope with flooding-induced soil anoxia in a variety of ways: hypertrophied lenticels, adventitious roots and enlarged aerenchyma (Havens, 1994; Kozlowksi, 1997).  Studies of temperate trees such as alders have shown that they use all the above adaptations when in waterlogged soils. This results in increased methane fluxes from the trees as there is increased surface area for emission and easier transmission from soil to atmosphere (Rusch and Rennenberg, 1998; Gauci et al., 2010).

Pangala et al. (2013) showed that in a tropical peat forest in Borneo, tree stems were responsible for 60-80% of total ecosystem methane fluxes, demonstrating that trees can be a significant source of emissions. This new study in lowland evergreen tropical rainforest in Panama aims to expand on this work and investigate the extent of tree stem emissions in an area of relatively free draining soils on a fortnightly basis for 5 months, covering the dry to wet season transition starting March 2014.

There is a second aspect to the study – we don’t know how trace greenhouse gas biosphere-atmosphere exchange will be affected by an atmosphere that is being enriched with CO2. It is thought that rising atmospheric CO2 concentrations will increase primary productivity in rainforests resulting in greater amounts of litterfall. The fieldwork will be done at the Smithsonian Tropical Research Institute, Panama, on plots that are part of an ongoing litter manipulation experiment meant to simulate elevated atmospheric CO2. There are 15 plots in total: 5 control, 5 with litter removed and 5 with litter added. Sayer et al. (2011) discovered soil respiration to be significantly higher in the litter addition plots compared to the control and litter removal plots. We hypothesise that due to increased litter input (and therefore more source carbon for methanogenesis) methane emissions will be greatest in the addition plots.

Gauci, V., Gowing, D.J.G., Hornibrook, E.R.C., Davis, J.M. & Dise, N.B. (2010) Woody stem methane emission in mature wetland alder trees. Atmospheric Environment, 44, 2157-2160.

Havens, K.J. (1994) The formation of hypertrophied lenticels, adventitious water roots, and an oxidized rhizosphere by Acer rubrum seedlings over time along a hydrologic gradient. Virginia Institute of Marine Science, Gloucester Point, Va.

Kozlowski, T.T. (1997) Responses of woody plants to flooding and salinity. Tree Physiology, Monograph No. 1, 29.

Milich, L. (1999) The role of methane in global warming: where might mitigation strategies be focused. Global Environmental Change, 9, 179-201.

Pangala, S.R., Moore, S., Hornibrook, E.R.C. & Gauci, V. (2013) Trees are major conduits for methane egress from tropical forested wetlands. New Phytologist, 197, 524-531.

Rice, A.L., Butenhoff, C.L., Shearer, M.J., Teama, D., Rosenstiel, T.N. & Khalil, M.A.K. (2010) Emissions of anaerobically produced methane by trees. Geophysical Research Letters, 37, L03807.

Rusch, H. & Rennenberg, H. (1998) Black Alder (Alnus glutinosa (L.) Gaertn.) trees mediate methane and nitrous oxide emission from the soil to the atmosphere. Plant and Soil, 201, 1-7.

Sayer, E.J., Heard, M.S., Grant, H.K., Marthews, T.R. & Tanner, E.V.J. (2011) Soil carbon release enhanced by increased tropical forest litterfall. Nature Climate Change, 1, 304-307.

Image: Forest of Taman Negara National Park by Vladimir Yu. Arkhipov, Arkhivov under Creative Commons CC BY-SA 3.0.

Blog: Ditch Blocking in a Welsh Peatland

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By Mike Peacock. 9th September 2011.

Northern peatlands are typically nutrient-poor, acidic ecosystems.  They store one third of global soil carbon, and one tenth of global freshwater.  However, over the past century they have been damaged in numerous ways; they have been burnt for agricultural management, drained for forestry and agriculture, and harvested as a fuel source.  In the UK the major change in peatland management has been through the digging of drainage ditches to lower the water table.  These ditches vary, but are typically half a metre wide and a metre deep, often in dense networks across large areas.

Decades of research into drained peatlands has created a large knowledge base on the effects of altered water tables.  Most studies report an increase in carbon dioxide emissions from respiration. Methane emissions, on the other hand, decrease.  This is due to an ingress of oxygen into the peat leading to increased methanotrophy due to a larger and more continuous oxic zone  Most studies agree that taken as a whole the biogeochemical changes following drainage lead to a decreased carbon store in the peat, and a net increase in greenhouse gas emissions.  With current concerns for climate change and carbon, this is clearly a hot topic.

Now, peatland restoration is in vogue.  In the UK the favoured method is to block the man-made drainage ditches, thus restoring the water table to approximately its original level.  A research project involving Bangor University, Leeds University, the Open University and the Centre for Ecology and Hydrology is based on one such example.  Following the collection of baseline data, hundreds of miles of ditches were blocked on the Migneint, a large blanket bog in north Wales.  The aim of the project is to examine the effects of two types of ditch blocking on greenhouse gas fluxes, as well as changes in hydrology and water chemistry.  As blocking took place in February 2011, the experimental phase is now well underway.

In addition to being drained, the study site is near the crest of a hill and so was originally moderately dry.  Methane fluxes were therefore found to be generally low from both the drained landscape, and have so far remained low from the intact blanket bog areas between the blocked ditches.  However, both types of blocking being trialled use peat dams to restrict water flow down the ditches.  Pools of varying sizes have formed behind these dams, and large methane fluxes have been recorded from their surfaces.  It appears that the removal of the oxic zone has limited the niche for aerobic methanotrophs, thus allowing more methane to reach the atmosphere.  Although these fluxes are generally fairly steady over short periods, intense pulses have also been observed, indicating methane ebullition (bubble emissions) from the peat.

Further to this, preliminary CEH/Bangor University work by Mark Cooper at an adjacent site found that blocked ditches were rapidly recolonised by Eriophorum.  These plants were large methane hotspots as they act as ‘chimneys’ to allow methane to bypass methanotrophs and enter the atmosphere.  It seems probable that the newly blocked ditches will also be colonised by Eriophorum, as this has been seen at numerous peatlands.

Hopefully the study will be able to untangle the complex effects of vegetation and hydrology on gas fluxes and water chemistry, to elucidate whether expensive ditch blocking is a cost-effective method for reducing greenhouse gas emissions, in the UK and elsewhere.

Anaerobic Methane Oxidation Coupled to Denitrification

lake constance

10th December 2014.

Nitrate/nitrite-dependent anaerobic oxidation of methane (n-damo) is a recently discovered process that is, to some extent, responsible for limiting emissions of methane from water bodies to the atmosphere. Despite its potential importance for carbon cycling, numerous questions remain unanswered about the process. A new paper published in PNAS addresses some of these questions.

The researchers investigated the microorganisms responsible for n-damo, which are bacteria belonging to the candidate phylum NC10 and related to Candidatus Methylomirabilis oxyfera. Through the analysis of sediments from Lake Constance in Germany, it was discovered that the abundance of M. oxyfera-like bacteria was greatest where methane and nitrate profiles intersected, lending support to the hypothesis that these bacteria carry out n-damo. Furthermore, results suggested that denitrifying methanotrophs were more numerous than aerobic methanotrophs at deep, undisturbed locations in the lake.

The authors conjectured that the need for a stable environment could drive the within-lake distribution of n-damo bacteria. For instance, numerous processes could introduce oxygen to sediments in shallow areas of the lake, such as waves, bioturbation, or the actions of algae and macrophytes. Because the bacteria grow relatively slowly, such disturbances would be difficult to recover from. Additionally, due to biological factors, nitrate concentrations are depleted in the upper layers of the lake, but remain higher in the lower layers, thus favouring n-damo in deeper waters.

The authors conclude by stating that n-damo is currently an overlooked process, and that it functioned as the dominant methane sink in Lake Constance. They also include the caveat that care must be taken when studying this pathway, as the close juxtaposition between oxygen and nitrate profiles could result in n-damo being misidentified as aerobic methane oxidation.

Reference: Deutzmann, J.S., Stief, P., Brandes, J., Schink, B. 2014. Anaerobic methane oxidation coupled to denitrification is the dominant methane sink in a deep lake. PNAS, 10.1073/pnas.1411617111.

Photo: Satellite image of Lake Constance, NASA.

Oxic Methanogenesis and Aquatic Methane Fluxes

800px-Canada_Island,_Lake_Lila

24th November 2014.

Recent research published in Nature Communications has shed new light on aquatic methane dynamics by demonstrating that methanogenesis occurs at the ecosystem-scale in oxic lake waters.

Despite being a recognised mechanism of methane production, the significance of methanogenesis in oxic waters has yet to be determined.  The researchers, from the Université du Québec à Montréal, used floating mesocosms that were closed at the bottom but open to the atmosphere, thus excluding non-pelagic methane sources.

During the 28-day experiment, dissolved methane concentrations increased in the mesocosms, and water/air fluxes ranged from 0.07 to 0.36 mmol m-2 day-1.  Furthermore, methane production was higher in mesocosms treated with nutrient enrichment, and lower in those treated with dissolved organic carbon (DOC).  The difference in methane concentrations and fluxes between treatments was linked to algal dynamics, specifically both gross primary production and net ecosystem production.  Analysis also suggested that the methane was produced through acetoclastic methanogenesis.  Considering the lake as a whole, mean fluxes from the control (i.e. not-treated) mesocosms accounted for approximately 20% of diffusive fluxes measured during summer, and were of a similar value to ebullitive fluxes.

The authors stress that this is not a missing measurement from aquatic carbon budgets, as this component is captured during standard flux measurements.  However, they point out that “incorporating the origin of the methane that outfluxes to the atmosphere from these lakes is critical to our understanding of the regulation of these fluxes and our capacity to predict their future change. In this regard, there are potentially large global implications for algal-driven oxic-water methanogenesis.”  Additionally, because methane dynamics differed in response to nutrient and DOC treatments, it is likely that aquatic methane emissions will be affected by environmental changes.

Reference:

Oxic water column methanogenesis as a major component of aquatic CH4 fluxes. 2014. Bogard, M.J., del Giorgio, P.A., Boutet, L., et al. Nature Communications, DOI: 10.1038/ncomms6350.

Photo: Canada Island, in Lake Lila, William C. Whitney Wilderness Area, Adirondack Park, Long Lake, NY, USA, by Mwanner. CC BY-SA 3.0.

A Synthesis of Methane Emissions from Wetlands

wetland pic

5th May 2014.

An analysis of 19,000 methane flux measurements from 71 sites has revealed new detail on controls driving methane emissions from wetlands. The new paper is by a group of international researchers and is published in Global Change Biology. The study examined bogs, swamps, poor fens and rich fens across a range of regions (subarctic to subtropical) in the northern hemisphere. The investigation also considered sites that had been drained (either solely by drainage or by peat harvesting) and those that had been wetted (either through experimental flooding or permafrost thaw in thermokarst collapse scars).

The analysis yielded numerous interesting findings. For instance, large methane fluxes (above 500 mg CH4 m-2 d-1) were observed more frequently in bogs and poor fens when compared to swamps and rich fens. The effect of wetland type on methane flux changed with region; emissions from bogs and rich fens decreased in the higher latitudes, whilst poor fens in the subarctic region displayed large fluxes. When results for all wetland types were averaged according to region, it was found that temperate sites emitted the largest amounts of methane; a mean flux of 125.4 mg CH4 m-2 d-1.

Examinations of the controls on methane flux revealed that vascular plants played a significant role in contributing to emissions. Large fluxes were associated with sampling locations rich in sedges and grasses, whilst locations with trees or where vascular plants were absent showed low fluxes. This result is consistent with the frequent observation that vascular plants with aerenchymatous tissue can transport methane from the soil directly into the atmosphere. Furthermore, vascular plants can contribute labile carbon compounds into the soil, thus stimulating methanogenesis.

As would be expected, both temperature and water table played a part in controlling methane fluxes. For bogs and poor fens it was found that mean water table was a significant predictor of flux, with fluxes increasing as the water table came closer to the peat surface. Regarding temperature, for bogs and swamps a significant positive relationship was found between methane flux and mean annual temperature. However, the authors stress that these relationships are not straightforward; for instance, flooding can suppress emissions due to reduced diffusion of methane through the water.

Finally, the analysis suggested that wetland drainage lead to a reduction in methane emissions. Additionally, drainage was found to alter the relationship between methane flux, soil temperature and water table. In both drained bogs and fens, neither short-term warming nor wetting resulted in a high flux, though this response was observed for pristine sites.

Lead author, Merritt Turetsky of the University of Guelph, said: “Our analyses show that northern fens, such as those created when permafrost thaws, can have emissions comparable to warm sites in the tropics, despite their cold temperatures. That’s very important when it comes to scaling methane release at a global scale.”

Reference: Turetsky, M.R., et al. 2014. A synthesis of methane emissions from 71 northern, temperate, and subtropical wetlands. Global Change Biology, DOI: 10.1111/gcb.12580.

Photo: Bog – St-Daniel sector – Frontenac National Park (Québec, Canada) – July 2008, by Boréal.

Temperature Dependence of Methane Fluxes Across Different Scales

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21st March 2014. A new paper published in Nature has shed light on the subject of the temperature dependence of methane fluxes measured across three different scales: cultures of methanogens, laboratory sediment incubations, and field measurements.  The meta-analysis considered wetlands (such as peat bogs), aquatic systems (such as lakes and rivers), and rice-paddy fields.

The analysis showed that the temperature dependence for the three different ecosystems was indistinguishable, with an average activation energy of 0.96 eV.  Furthermore, temperature dependence at the ecosystem scale was statistically identical to that of cultured methanogens (1.1 eV) and sediment incubations (0.93 eV).  The authors therefore suggest that the ecosystem-scale seasonal temperature dependence of methane fluxes is driven primarily by the methanogenic community.

Lead author, Dr Yvon-Durocher said: “This is important because biological methane fluxes are a major component of global methane emissions, but there is uncertainty about their magnitude and the factors that regulate them. This hinders our ability to predict the response of this key component of the carbon cycle to global warming. Our research provides scientists with an important clue about the mechanisms that may control the response of methane emissions from ecosystems to global warming.”

This similarity in temperature dependence across scales and ecosystems has important ramifications, as discussed by the paper.  For instance, the calculated temperature dependence of methane emissions is higher than that of carbon dioxide fluxes from both respiration (0.65 eV) and photosynthesis (0.3 eV).  The authors investigated the implications of this difference in activation energy in relation to a warming climate.  They found that the contribution of methane fluxes to total carbon emissions increased at elevated temperatures.  As methane is an extremely potent greenhouse gas, this finding suggests that positive feedbacks between climate change and the carbon cycle could be stimulated in future.  However, substantial variation existed in the temperature dependence of methane fluxes at the ecosystem scale, and the authors hypothesise that other factors such as water table depth, methanotrophy, substrate supply and microbial community structure could constrain changes on methane emissions in the long-term.

Dr Yvon-Durocher, from the Environment and Sustainability Institute (ESI) at the University of Exeter’s Penryn Campus in Cornwall, added: “The discovery that methane fluxes are much more responsive to temperature than the processes that produce and consume carbon dioxide highlights another mechanism by which the global carbon cycle may serve to accelerate rather than mitigate future climate change. However more research, using our results as a platform for refining Earth system models, is required to fully explore the consequences of our findings for future levels of climate change”.

Reference – Yvon-Durocher, G., Allen, A.P., Bastviken, D., Conrad, R., Gudasz, C., St-Pierre, A., Thanh-Duc, N., del Giorgio, P.A. 2014. Methane fluxes show consistent temperature dependence across microbial to ecosystem scales. Nature, doi:10.1038/nature13164

Photograph – a Welsh peatland on the island of Anglesey. Mike Peacock.