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Carbon dynamics in restiad peatlands across different timescales A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Sciences at The University of Waikato by Joshua Lee Ratcliffe 2019 I would like to dedicate this thesis to my friend and former mentor; Dr. Richard Payne who died in an avalanche on the 26th of May 2019 while attempting to climb an un-named peak upon Nanda Devi mountain. Abstract Peatlands contain one of the largest terrestrial carbon stores on the planet, and one which is known to interact with climate and global biogeochemical cycling of nutrients. Peatlands maintain their carbon primarily through a high and stable water table which restricts decomposition, and large amounts of carbon can be lost upon drying. However, peatlands are also characterised by non-linear responses to external forcing with a complex array of internal feedbacks which tend to dominate ecosystem response over long-timescales and may amplify or dampen external influences. This has made predicting the effects of environmental change, beyond a few years, highly challenging. Here we use CO2 fluxes and down-core measurements of carbon accumulation to study the drivers of peatland carbon exchange across a wide range of timescales. First of all, we compare contemporary CO2 fluxes at two raised bogs, one of which is extremely dry in an international context (Moanatuatua), with the summer water table drawdown approaching one metre, and another where the water table is high and stable (Kopuatai). We found that despite the low and fluctuating water table in the impacted bog the site remained a sink for CO2 which is strong in an international context, but reduced compared to the wet bog. A key factor in the wet bog being able to maintain a net sink for CO2 was the enhanced photosynthetic capacity compared to the wetter bog, especially in summer, which was able to partially compensate for enhanced ecosystem respiration. There was a clear difference in how the two sites responded to contemporaneous water table drawdown, which was consistent with differences seen across wet and dry bogs in the international literature. We found plant productivity to be restricted at the dry bog, at both wet and dry extremes, while at the wet bog water table lowering stimulated ecosystem respiration, with neither effect being consistent across both bogs. To further investigate how the dry bog, Moanatuatua, has adapted to low water tables we re-analysed CO2 flux data from 1999 and 2000, a period at least several years after the water table initially dropped, and we compared this flux to the present day (after a 16 year gap). Re-analysis of the older data showed that in 1999 and 2000 Moanatuatua was a moderate source for CO2 with elevated ecosystem respiration and lower photosynthetic capacity compared to the present i day. We attribute the change in photosynthetic capacity to the increased cover of the woody shrub Epacris Pauciflora, while a long term decline in ER would be consistent with changes in peat physical and chemical qualities as peat degrades making it further resistant to microbial decay. In order to contextualise the CO2 flux records and assess the drivers of carbon accumulation at the 50-100 year timescale we measured down-core C accumulation in unprecedented detail for a New Zealand bog, with supporting records available for fires, climate, eruptive events and plant species changes available from published and unpublished records at Moanatuatua. Contrary to our initial expectations, we found elevated C accumulation to be associated with at least three separate eruptive events, and possibly two others, with carbon accumulation rates increasing rapidly from a baseline typical of long-term accumulation rates, e.g. ~22 g C m-2 yr-1 to one which is more typical of contemporary uptake, 80-140 g C m-2 yr-1. The complex eruptive history of the bog at this time makes it difficult to isolate any other effects, for instance climate. Geochemical analysis of the peat suggests increased phosphorus inputs as a mechanism for rapid C accumulation, with the peat stoichiometry recording a shift towards phosphorus abundance relative to carbon and nitrogen. The eruptive events linked to elevated carbon accumulation rates are also known to contain phosphorus within the volcanic glass and the mineral apatite, both of which would be expected to weather and become biologically available due both to processes within the eruptive plume and within the bog post-deposition. As such we have found the restiad bogs studied to be highly resilient to water-table drawdown in the long-term but in contrast, the C sink is highly sensitive to phosphorus inputs. ii Acknowledgements I would like to acknowledge my chief supervisor David Campbell, for his friendship and excellent tutelage. Dave’s students were always first priority for him and his technical expertise and attention to detail was exactly what I needed to develop my skills as a young scientist. In addition, Dave was always there for advice and support both professionally and personally. Louis Schipper kept me on track, taught me that less is more, that good science is not creating a baroque edifice, but rather that good Science is carried out incrementally, with precision and clarity. David Lowe was generous with his time and advice, displaying a enthusiasm and passion which was infectious; I especially appreciated his good- natured intelligence and subtle sense of humour, which was always present in my interactions with him. Beverly Clarkson lived up to her formidable reputation as a passionate and deeply knowledgeable peatland scientist. Many of the ideas in this thesis originated or were shaped by conversations with Bev and her enthusiasm and conviviality made her a joy to work with. Aaron Wall played an important role in the data analysis component of this thesis, and was always willing to offer help and advice, often combined with a humorous quip. Aaron had a leading role in designing and optimising many of the systems I used to process data. I found Aaron to be highly skilled and competent, I doubt it is possible to find a researcher who achieves a more perfect balance between attention to detail and efficiency and I hope to emulate his approach to data in my future work. I would also like to thank Dr Roxane Anderson and (the late) Dr Richard Payne for there earlier mentorship and for investing so much time and effort into helping me navigate the pitfalls and challenges faced by a young scientist. Richard Payne, for whom I dedicate this thesis, always viewed science as a fundamentally collaborative endeavour. He taught me that science is often at its best when a diverse range of minds and people work together, and that the rewards of working with people from different fields and different backgrounds are nearly always worth the costs. Richard invested so much of himself in his students and his work. He will live on through our shared labour and passion. Many of the support staff at the University of Waikato offered invaluable assistance with this thesis; the librarians are truly exceptional and are a real asset to the University. I am grateful to Grace for keeping the coffee and conversations iii flowing. Annie Barker has an amazing ability to find anything you ask for and generally makes the chemistry lab a pleasant and welcoming place to be. Noel Bates was also very accommodating. He kept everything running smoothly in the soils lab and was an extremely pleasant person to be around due to his cheerful and friendly personality. Chris Morcom provided lots of muscle and good company, helping out with field work. Amanda French has an inside out knowledge of the ICPMS and was generous with her time and advice. She exudes competence, and I am sure the quality of my ICPMS data is in no small part due to her professionalism and care. Jude Hoult was fun to work with and helped me out considerably by weighing samples for stable isotope analysis. Dean Sandwell was always willing and able to help out with occasional requests and problems and kept the field gear clean and well organised. Alan Hogg offered me advice on radiocarbon dating, paired with tea and cake. While I did not work directly with Annette Rodgers, she knows the University inside out and frequently helped me with the small day to day issues which crop up. The SciAdmin team Fiona, Gloria, Vicky and Helen were extremely helpful and my interactions with them were always enjoyable, they solved many a problem and were generous with access to the faculty thesis store and stationary cupboard. I would further like to thank Rewi Newnham, Matt Amesbury and Tom Roland, who visited on a separate research project, and involved me in their research in the early days of the PhD when I was still figuring out what I was doing.