Pyrogenic carbon characteristics and short-term degradation trends in UK peatland catchments

  • Oscar Kennedy-Blundell

Student thesis: Phd

Abstract

UK peatlands are important carbon storing ecosystems. They have a long history of prescribed burning and are periodically subjected to wildfires. Whilst fires in such settings may release significant amounts of carbon if peat is combusted, there is also the potential for charred material to contribute to long term carbon storage. Combustion of peatland vegetation produces a range of charred materials, known as pyrogenic carbon (PyC). PyC is the continuum of materials resulting from the incomplete combustion of organic matter (OM) and is of major research interest owing to its recalcitrance, high carbon content, degree of aromatic condensation and high sorption potential. These physiochemical characteristics mean that PyC has the potential to significantly contribute to carbon cycling in peatlands. Using laboratory produced PyC samples, this thesis aims to improve the understanding of UK peatland PyC, with specific focus on understanding: (i) the characteristics of the PyC formed in peatland fires; (ii) how these characteristics change with terrestrial exposure; (iii) how degradation of PyC affects water quality; and (iv) how combined terrestrial and aquatic processes alter PyC characteristics. Fuel type and burn severity significantly influenced PyC characteristics. Different combinations of fuel type and severity produced PyC with distinct elemental (e.g. H/C and O/C) and molecular characteristics (e.g. C=O/C=C and CH/C=C). Elemental and molecular ratios decreased with increasing burn severity and were lower for woody fuels than non-woody fuels. Surface area increased with increasing severity, with higher values for woody than non- woody fuels. Burn severity proves to be an effective means by which to categorise distinct PyC groups. When left at the peat surface, PyC showed rapid changes in leachable carbon, which then declined following early field exposure (≤1 month). There were strong indications that mineral and OM interactions occurred (e.g. increased spectral absorbance at 1060 cm-1). Vegetation species significantly affected O/C, C/N and C-O/C=C ratios, with non-woody fuels returning higher values than woody fuels. Burn severity had a significant effect on all measured variables, suggesting a greater degree of aromaticity in high severity samples than low severity samples. PyC degradation in UK peatlands occurs in multiple phases, with long term exposure (12 months) resulting in stabilisation through soil matrix interactions. When it enters an aquatic environment, fresh PyC releases substantial amounts of dissolved organic carbon (DOC), indicating rapid changes to water quality. In aquatic PyC degradation experiments, DOC concentrations were greatest and aromaticity was lowest with low temperature woody PyC and high temperature grassy PyC in their fresh state. There was also evidence of enhanced DOC leachability from PyC that had been subject to longer terrestrial exposure before entering the aquatic system. Overall, the greatest effects of PyC degradation on water quality comes from PyC that enters the aquatic system immediately following a fire, and within the first week of aquatic exposure. PyC that enters the aquatic system immediately after a fire event undergoes different degradation trends compared to PyC that is terrestrially exposed prior to entering the aquatic system. The greatest losses of carbon were seen with fresh PyC, whilst PyC that had been exposed for 3 – 6 months gained carbon. FTIR analyses indicated sorption of OM and minerals following terrestrial exposure. Aromatic condensation declined following early terrestrial exposure, and subsequent aquatic exposure resulted in further declines in the degree of condensation. PyC mobilisation from terrestrial peatland settings into the aquatic system exerts a significant effect on degradation. From these findings, it is clear that PyC undergoes rapid degradation when exposed to environmental conditions, which is then followed by longer term stabilisatio
Date of Award31 Aug 2021
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorJames Rothwell (Supervisor), Emma Shuttleworth (Supervisor) & Gareth Clay (Supervisor)

Cite this

'