|dc.description.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 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.||