IAP2-22-400

How come peatlands exist when there are not enough electrons?

There is not enough electron capacity provided by inorganic electron acceptors in peatlands to give rise to the amount of carbon storage we observe in the very same peatlands.
The most efficient carbon (C) store in the terrestrial biosphere are our peatlands, for example, UK peatlands store more carbon than the forests of UK and France combined, and furthermore, unlike forests, peatlands can be perpetual greenhouse gas (GHG) sinks. The problem is that we do not know what controls the amount of carbon stored in peatlands.
Peatlands are a competition for energy and electrons and the fate of the organic matter – the very reason for the existence of the peatlands – is the balance between the rate of primary production and the rate of oxidation. Oxidation of organic matter can occur via a range of terminal electron acceptors (TEA) and will occur via the most energetically favourable TEA available. The most favourable TEA in natural environments is O2, followed in succession by NO3, Mn, Fe, SO4 and ultimately methanogenesis. Which TEA is used is not only a matter of energy release but also supply of the TEA and in the waterlogged conditions the supply of O2 is restricted. If O2 availability is restricted then less energetically efficient TEAs are utilised and rapidly used up in the peat profile. After O2, NO3 is removed before the root zone; the amount of Fe and Mn is limited and so SO4 becomes an important electron acceptor. Boothroyd et al. (2021) have shown that a peatland could reduce all the SO4 supplied to it even if that did not actually happen. Worrall et al. (2022) have shown that methanogensis is so energy consumptive that methane formation does not result in any long term deep peat formation. When comparing the SO4 budget, the C budget and the CH4 budget there is a gap – the amount of SO4 removed and the amount of CH4 produced and of peat formed do not match and means there is not enough electron capacity to explain the amount of peat formed. Therefore, where is the electron capacity coming from that explains the existence of peat?
This project proposes that the solution to this observed deficit in electron supply lies in dissolved organic matter (DOM) acting as an electron shuttle. Worrall et al. (2022) have shown that DOM leaving the peat is very oxidised in contrast to the DOM in the peat soil water which is comparatively reduced. Furthermore, DOM is present in at least an order of magnitude greater concentration than either SO4 or NO3 and flux of DOM is up to 85 tonnes C/km2 larger concentrations – there is enough oxic DOM leaving a peatland that if only 8% would exchange with deep peat pore water then there would be sufficient electrons to explain the peat carbon storage.
To answer to test this hypothesis this project will measure the pathways of DOM relative to redox conditions of the peat.

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Methodology

The approach will be to use multi-level piezometers in replicated transects across a blanket bog slope from the watershed to a first-order stream.
The first purpose of the multilevel piezometers will allow access to the peat porewater at multiple depths meaning that the redox sequence can be studied relative to the flow and diffusion of DOM. In the sampled porewaters we will measure: pH, specific conductance and redox potential; concentrations of the inorganic redox species – NO3, Fe, Mn, and SO4; and DOC concentration and composition. The composition of the DOC will be analysed for its oxidation state and also its age.
The second purpose of multi-level piezometers will be to measure the potential of a vertical flux of water and of redox active ingredients. For the project’s hypothesis to be true, the oxic DOM observed in first-order streams has to exchange with the reduced DOM in the porewater and so there must either be flow of water or diffusion in the vertical profile.
The data from the multilevel piezometer network will be supported by analysis of the peat profile. In particular, it is important to know the age profile of the peat so that it is possible to assess the age contrast between porewater and its surrounding peat.
Mesocosm and lab experiments will be established in Durham to support the findings from the fieldwork. In Durham we have established a set of 1 m deep peat cores outside of the department where we can control the water table and these enable us to performed controlled experiments and to monitor carbon fluxes.
Pyrolysis-GC/MS and LC-MS will be used at Newcastle University to characterise the molecular composition of the DOM to ascertain whether it is rich in oxidants or reductants (Abbott et al., 2013; Williams et al., 2016).

Project Timeline

Year 1

1. Literature review
2. Establish field transect
3. Training in field and laboratory techniques
4. Characterise field transect
5. Run field transect for control year

Year 2

1. Carry out treatment interventions
2. Run field transect
3. Establish mesocosm experiments

Year 3

1. Run field transect for second treatment year
2. Examine macromolecular fate across European peatlands
3. Conduct mesocosm experiments
4. Present results at international conference (eg. EGU in Vienna)
5. Analyse data for writing up

Year 3.5

1. Complete thesis
2. Write up papers for publication

Training
& Skills

The studentship will involve full training in the necessary field, laboratory and data analysis techniques needed. The field techniques include: formal experimental design; the use of gas analysers; systematic sampling; and total greenhouse gas budgeting. Laboratory analysis will include: gas calibration; water quality analysis and mesocosm experiments.

References & further reading

Abbott GD, Swain EY, Muhammad AB, Allton K, Belyea LR, Laing CG, Cowie GL. Effect of water-table fluctuations on the degradation of Sphagnum phenols in surficial peats. Geochimica et Cosmochimica Acta 2013, 106, 177-191.

Boothroyd, I.M., Worrall, F., Moody, C.S., Clay, G.D., Abbott, G.D. & Rose, R. (2021). Sulfur Constraints on the Carbon Cycle of a Blanket Bog Peatland. Journal of Geophysical Research – Biogeosciences, 126, 8, G006435.

Williams JS, Dungait JAJ, Bol R, Abbott GD. Contrasting temperature responses of dissolved organic carbon and phenols leached from soils. Plant and Soil 2016, 399(1-2), 13-27.

Worrall, F., Boothroyd, I.M., Clay, G.D., Moody, C.S., Clay, G.D., Heckmann K., Burt, T.P., & Rose, R. (2022). Constraining the carbon budget of peat ecosystems: application of stoichiometry and enthalpy balances. Journal of Geophysical Research – Biogeosciences,

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