Blog: UV Irradiation of Aquatic Organic Carbon: An Overlooked Source of Methane?

black burn_0

By Amy Pickard. 3rd December 2013.

Amy Pickard1*, Andy McLeod1, Kate Heal1 and Kerry Dinsmore2

1 School of GeoSciences, University of Edinburgh, UK *amy.pickard@ed.ac.uk  2Centre for Ecology and Hydrology, Bush Estate, Penicuik, UK

Wetland environments, including northern peatlands, are a globally significant source of methane, releasing in the order of 100 Tg CH₄ yr-1 (Wuebbles and Hayhoe, 2002). Emissions from these systems have typically been attributed to microbial metabolism of organic carbon into methane in anaerobic conditions.

However a seminal study by Keppler et al. (2006) showed that methane was produced in aerobic conditions when plant matter was subjected to stress. UV irradiation is a known source of plant stress that was later shown to initiate methane production (McLeod et al., 2008). Further follow up studies indicated that plant pectin was a possible source for the emissions and confirmed that methane could be produced from detached leaf components (McLeod et al., 2008; Vigano et al., 2008). The key outcome of this body of research was the incorporation of aerobic methane production from UV irradiation of plant foliage into the global budget (Bloom et al., 2010). Nevertheless, to date, this pathway of aerobic methane production has only been investigated in the terrestrial environment.

The hypothesis of my research is that plant-derived material transported from the terrestrial environment to aquatic systems may release methane when exposed to UV irradiation. The NERC-funded PhD that I am currently undertaking aims to further investigate the aerobic production pathway as a potential source of methane in aquatic systems.

In order to test this hypothesis, water samples rich in dissolved organic carbon (DOC) were collected from a stream draining Auchencorth Moss (Fig. 1), an ombotrophic peatland area in south east Scotland which is one of the Centre for Ecology & Hydrology’s Carbon Catchments. Upon returning to the laboratory samples were filtered and decanted into quartz vials. They were then exposed to an ambient dose of UV irradiation for 4 hours. After irradiation, vial headspace methane concentrations were measured using gas chromatography.

These initial experiments have demonstrated that aquatic systems contain sufficient levels of plant derived material to stimulate aerobic methane production.  UV irradiation resulted in increased CH₄ production rates of 63.2 ± 16.4 nmol L-1 (mean ± standard deviation for 4 replicates) relative to unirradiated control samples. The increase in gaseous production in the headspace of irradiated vials was coupled with a decrease in DOC concentration in the water sample. This finding is in agreement with the hypothesis that dissolved plant matter acts as the aerobic methane source material and adds weight to the suggestion that UV irradiation plays an as yet overlooked role in the aquatic methane budget.

Increased losses of DOC from catchments in the northern hemisphere have been well documented (Clark et al., 2010) and are projected to accelerate with climate change (Worrall et al., 2004). This creates an interesting setting for aquatic methane research, as it follows that in catchments where more DOC is delivered to aquatic systems, methane emissions stimulated via UV irradiation of organic matter will increase concurrently. Whether this hypothesis holds true in either laboratory tests or field based experiments is yet to be determined, however our initial data present plenty of options to explore. The challenge now is to understand what environmental factors affect aerobic methane emissions and to combine both laboratory and field based experiments to determine the importance of this process in catchment level biogeochemical cycles.

Photo: The Black Burn at Auchencorth Moss. Photo courtesy of Fraser Leith

References

Bloom, A.A., Lee-Taylor, J., Madronich, S., Messenger, D.J., Palmer, P.I., Reay, D.S., McLeod, A.R., 2010. Global methane emission estimates from ultraviolet irradiation of terrestrial plant foliage. New Phytol 187, 417-425

Clark, J.M., Bottrell, S.H., Evans, C.D., Monteith, D.T., Bartlett, R., Rose, R., Newton, R.J., Chapman, P.J., 2010. The importance of the relationship between scale and process in understanding long-term DOC dynamics. Sci Total Environ 408, 2768-2775

Keppler, F., Hamilton, J.T.G., Brass, M., Rockmann, T., 2006. Methane emissions from terrestrial plants under aerobic conditions. Nature 439, 187-191.

McLeod, A.R., Fry, S.C., Loake, G.J., Messenger, D.J., Reay, D.S., Smith, K.A., Yun, B.W., 2008. Ultraviolet radiation drives methane emissions from terrestrial plant pectins. New Phytol 180, 124-132.

Vigano, I., van Weelden, H., Holzinger, R., Keppler, F., McLeod, A., Rockmann, T., 2008. Effect of UV radiation and temperature on the emission of methane from plant biomass and structural components. Biogeosciences 5, 937-947

Worrall, F., Burt, T., Adamson, J., 2004. Can climate change explain increases in DOC flux from upland peat catchments? Sci Total Environ 326, 95-112

Wuebbles, D.J., Hayhoe, K., 2002. Atmospheric methane and global change. Earth-Sci Rev 57, 177-210

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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.

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.

Anaerobic Oxidation of Methane in Peatlands

bog pool photo

12th July 2013. A new study published in Environmental Science and Technology has shed light on anaerobic oxidation of methane (AOM) in peatlands.  The importance of AOM as a methane sink has been recognised in marine sediments for a number of years, but the process is only poorly understood in peatlands, and has received relatively little attention.

In the latest study, researchers collected soil samples from 15 different peatlands and carried out lab incubation experiments.  The addition of 13C-CH4 isotope tracers to the anaerobic incubations resulted in the production of 13CO2 for samples from all 15 peatlands, implying that AOM occurred at all sites.  Rates of AOM were sustained for longer in fens compared to bogs, and the authors hypothesised that this was due to groundwater inputs supplying the electron acceptors needed to maintain the process.  However, the second part of the experiment that attempted to determine the relevant electron acceptor found that additions of nitrate, sulphate and iron had no effect on AOM.  It therefore appears that the pathway of AOM in peatlands is fundamentally different to that in other systems.

As a final piece of work, the authors calculated the amount of methane consumed through AOM for northern peatlands using low, average, and high estimations.  The results from their average scenario suggested that AOM could act as a sink of 24 Tg of methane each year; a significant amount.  Methane fluxes in peatlands are traditionally modelled using variables such as temperature, vegetation and water table, with methane consumption and production being considered to be oxic and anoxic processes.  However, the growing body of research on peatland AOM may well have implications on the use of such simple models.

Reference: Gupta, V., Smemo, K.A., Yavitt, J., Fowle, D.A., Branfireun, B.A., Basiliko, N. 2013. Stable isotopes reveal widespread anaerobic methane oxidation across latitude and peatland type. Environmental Science and Technology, in press.  DOI: 10.1021/es400484t