Why are some peatlands oxic? Developing a molecular model of peatland carbon storage

Biogeochemical Cycles

IAP2-21-269

Overview

Within the terrestrial biosphere, the northern peatlands are the most important terrestrial carbon (C) store: Gorham (1991) has estimated that 20-30% of the global terrestrial carbon is held in just 3% of the land area, i.e. the peatlands. The very existence of peatlands relies on the fate of organic matter and hence the carbon budget is a statement of the ecosystem’s future and the estimation of C budgets has been a common research target (e.g. Nilsson et al., 2008). But what controls the magnitude of the carbon and greenhouse gas sinks and sources within a peatland? Many studies have considered management impacts (eg. revegetation – Rowson et al., 2010) and major drivers (eg. water table – Evans et al., 2021; Abbott et al., 2013), but there is a need now for a broad scale understanding of the processes controlling carbon storage in peatlands.
Peatlands sequester carbon not as an element but as a constituent of macromolecules. Plants sequester CO2 from the atmosphere but, they sequester it to glucose which is then transformed through metabolic cycles and combined with nutrients to build the components of plants – lignin, carbohydrates (cellulose and hemicellulose), proteins, lipids etc. It is this organic matter that is transformed back to CO2 through root respiration, falls as litter, or is released as plant exudates into the soil pores. It is the litter or exudates that then transform to soil organic matter and this transformation favours certain macromolecules. The whole process is the transition of relatively large quantities of highly oxidised carbon (CO2) to a small amount of reduced carbon macromolecules (eg. lignin). Therefore, the actual carbon behaviour in peatlands is the behaviour of molecules and not of elements; and furthermore, it is redox processing of carbon. Biomacromolecular inputs are readily related to vegetation composition and occurrence. There is, therefore, the opportunity to understand carbon and greenhouse gas budgets across regions by understanding the underlying biogeochemical processes.
Therefore, the aim of this project is to build models of the storage of carbon based on the actual forms that carbon takes as it transfers into and through peatlands and relate these to widely mappable features such as vegetation type.

Methodology

The project will utilise peatland sites where elemental and mass budgets already exist, and therefore, sites where molecular measurements can be made within an established context. The field sites will be Moor House, upper Teesdale, where there is a long record of carbon, nitrogen, sulphur and iron budgets and the collection of information is ongoing. To test findings from Moor House two other sites have been chosen (Cors Erddreiniog, Wales; Purgschachen, Austria) where, as with Moor House, there are ongoing carbon flux measurements but the sites are in very different contexts from Moor House and from each other. The fieldwork will be supplemented with the establishment of mesocosm experiments, conducted at Durham University, and in these mesocosm experiments we can test hypotheses generated from the field experiments. The characterisation of the organic matter will include a wide range of techniques that will target major questions:
i) How do biomacromolecular compositions transfer into, through and are stored in peatlands?
ii) What is the source of the redox potential in the peat profile?
The biomacromolecular composition and transformation will be analysed through such techniques as thermogravimetric analysis, TMAH thermochemolysis and Fourier transform infra-red spectroscopy. The characterisation of the redox conditions – within the redox reactions available for a peatland there are both inorganic and organic terminal electron acceptors (TEAs) but organic TEAs are rarely if ever characterised. Here we will use redox probes and 16S rRNA phylogenetic analysis based on next generation sequencing technologies to infer the relative dominance of sulphate reducing bacteria over methanogenic or nitrate reducing bacteria.The biogeochmical studies will be used to inform how we scale up existing peatland carbon models (eg. Worrall et al., 2009) and scale up using vegetation models such as Armstrong et al. (1997).

Project Timeline

Year 1

1. Literature review
2. Establish field trial plots
3. Training in field and laboratory techniques
4. Characterise field trial plots & Run field trial plots for control year

Year 2

1. Carry out treatment interventions
2. Run field trials for first treatment year
3. Establish mesocosm experiments

Year 3

1. Run field trials for second treatment year
2. Conduct mesocosm experiments
3. Present results at international conference (eg. EGU in Vienna)
4. Analyse data for writing up

Year 3.5

1. Complete thesis & Write up papers

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

1. Armstrong, H.M. et al. (1997). A model of the grazing of hill vegetation by sheep in the UK .1. The prediction of vegetation biomass. Journal of Applied Ecology 34, 1, 166-185.
2. Evans, C.D. et al. (2021). Overriding water table control on managed peatland greenhouse gas emissions. Nature DOI: 10.1038/s41586-021-03523-1.
3. Abbott, G.D. et al. (2013). Effect of water-table fluctuations on the degradation of Sphagnum phenols in surficial peats. Geochimica et Cosmochimica Acta 106, 177-191.
4. Gorham, E. (1991), Northern peatlands: role in the carbon cycle and probable responses to climate warming. Ecological Applications, 1, 182-195.
5. Nilsson, M. et al. (2008), Contemporary carbon accumulation in a boreal oligotrophic minerogenic mire – a significant sink after accounting for all C-fluxes. Global Change Biology, 14(10), 2317-2332.
6. Rowson, J.G. et al. (2010), The complete carbon budget of a drained peat catchment. Soil Use and Management, 26(3), 261-273.
7. Worrall, F. et al. (2009). Can carbon offsetting pay for ecological restoration in uplands? Science of the Total Environment 408, 1, 26-36.

Further Information

Prof. Fred Worrall, Dept of Earth Sciences, University of Durham, Tel. no. 0191 334 2295, Fred.Worrall@durham.ac.uk

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