Climatic Controls on Organic Matter Decomposition in Boreal Peatlands

Boreal peatlands currently contain 550 Pg C and are located at high latitudes where mean annual temperatures are expected to increase by as much as 7°C by the end of the century. There is growing concern that warming will stimulate decomposition, transforming peatlands from a sink to a source of atmospheric carbon dioxide and accelerating climate change. A primary goal of this dissertation was to evaluate the effect of climate change on organic matter decomposition in peatlands. This was achieved by developing and employing biochemical tracers to indicate the extent of peat decomposition across a range of naturally occurring climatic conditions. First, peat cores were analyzed from a latitudinal transect from the West Siberian Lowland, Russia. Cores in the south of the transect formed with mean annual temperatures as much as 7° warmer than those in the north. However, peat accumulation rates were as much as 5 times higher in the southern cores, leading to faster burial beneath the water table where anoxic conditions generally prevail. The northern cores therefore experienced longer oxygen exposure time than the southern cores. Three independent biochemical indicators (the C:N ratio, hydroxyproline yields, and acid:aldehyde ratios of lignin phenols) all indicated the northern cores were more extensively decomposed than the southern cores. This suggests oxygen exposure time is the primary control on the extent of peat decomposition while temperature is of secondary importance. The importance of oxygen exposure time was supported by assessing temporal changes in decomposition in a peat core from James Bay Lowland, Canada. A reconstruction of the water table based on fossil testate amoebae indicated oxygen exposure time was longest in a 100 cm interval in the center of the core. This interval had lower yields of neutral sugars, lower C:N ratios, and higher amino acid and hydroxyproline yields than the rest of the core, indicating more extensive decomposition. The bottom 50 cm of the core was formed during the Holocene Thermal Optimum under conditions ~2°C warmer than the rest of the core, but was not more extensively decomposed. This supports the conclusion that oxygen exposure time rather than temperature is the main control on organic matter decomposition in peatlands. The low apparent sensitivity of decomposition to temperature is consistent with recent observations of a strong correlation between peat accumulation rates and mean annual temperature, suggesting contemporary warming could enhance peatland carbon sequestration