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Assessing dissolved organic carbon export from Kopuatai bog, New Zealand

Peatlands sequester carbon (C) through CO₂ uptake and, ultimately, storage as peat. Carbon is lost from peatlands though gaseous pathways (as CO₂ and CH₄) and water pathways as dissolved organic C (DOC), dissolved inorganic C (DIC), particulate organic C (POC) or dissolved CO₂ and CH₄. Many studies have shown that the loss of C through DOC export is an important component of the annual C budget, but there are very few published studies where atmospheric fluxes of CO₂ and CH₄ and waterborne fluxes of DOC are measured at the same time. Also, little is known about the spatial variations in DOC concentration within peatlands and the processes leading to the delivery of DOC into rivers. The research described in this thesis focuses on understanding the ways in which vegetation, temperature, peat quality and hydrology interact to determine variability in DOC concentrations and export, both seasonally and spatially, at Kopuatai bog. DOC concentrations were measured at 14 sites across the bog for a year. At each site peat pore water was sampled from three different depths and represented three different vegetation types dominated by Empodisma robustum (jointed wire rush), Leptospermum scoparium (manuka) and Sporadanthus ferrugineus. There were no strong seasonal trends in DOC concentration, possibly due to the small seasonal range in temperatures. DOC concentrations varied spatially, with highest concentrations found under L. scoparium vegetation, likely due to differing chemical properties of plant materials leading to higher decomposition rates. DOC concentration did not vary significantly with depth. DOC export was estimated using a water balance based approach where evaporation (E), precipitation (P) and change in storage were measured to determine water discharge (Q) from the eddy covariance (EC) footprint. Discharge was then multiplied by average DOC concentrations within the EC footprint to derive DOC export. This method had not been directly applied in the literature. It was estimated that 11.7 ± 0.82 g C m⁻² of DOC was exported from the EC footprint from the year 1 February 2012 to 1 February 2013, which is at the lower end of the range when compared to literature values. DOC export had strong seasonal variation with highest export during the winter due to high rainfall and lowest in summer due to low rainfall. A simple method to estimate DOC export was proposed using the strong correlations between DOC export and monthly P and P–E. When annual DOC export was compared to annual net ecosystem exchange (NEE) at Kopuatai, DOC export was equivalent to only 6% of CO₂ sequestered in a year. This is in contrast to the literature where, on average, DOC export is often equal to around 25% of NEE. Preliminary analyses shows that NEE values are significantly higher at Kopuatai than NEE published for Northern Hemisphere peatlands and this is likely an effect of the dominance by vascular plants and the year-round growing conditions at Kopuatai.
Type of thesis
Sturgeon, C. (2013). Assessing dissolved organic carbon export from Kopuatai bog, New Zealand (Thesis, Master of Science (MSc)). University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/7936
University of Waikato
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