The direct and indirect role of climate in regulating litter decomposition processes in blanket peatlands

Peatlands are an important component of the global carbon (C) cycle, accounting for around 20-30 % of global soil C. They accumulate C as long as the losses (through decomposition) are less than the inputs (through above- belowground litter production). In the UK, rain-fed blanket peatlands are the dominant peatland type, relying on a large precipitation excess over evaporation to create the waterlogged conditions that promote peat accumulation. This potentially makes them highly sensitive to the effects of climate change. Understanding the sensitivity of blanket peat litter decomposition to climate change is, therefore, of great importance to understand future stability of C stocks, particularly to parameterise dynamic peatland C models. However, to date, there have been relatively few studies of decomposition on blanket peat. In this thesis, a number of experiments primarily focussed on decomposition were carried out with realistic climate treatments simulated in the laboratory and across gradients analogous to near future climate change. The intention was to allow a more accurate quantification of the effects of climate change on peat decomposition processes under ‘realistic’ climatic treatments. By using field sites that lie at the margin of the climatic envelope associated with blanket peatlands, in the south west (SW) of England, we also sought to get a better understanding of how rates of decomposition in other, more northern, blanket peatlands in the UK might be affected by future climate change. A clear finding was that the rate of decomposition varied significantly between different litter types, suggesting an important indirect effect of climate change – shifting vegetation communities – could be one of the most important factors affecting future C accumulation. Despite clear differences in peat temperature between the sites along the altitudinal gradient (330 – 520 m asl), this did not translate into a corresponding reduction in decomposition rate. It appears that other environmental factors (such as increased nitrogen deposition) could be counteracting the effect of decreased temperature, or that peatland litter decomposition rates are relatively insensitive to temperature. That the decomposition did not decrease with altitude also suggests that productivity could be more important in controlling net peat accumulation, potentially increasing with altitude in response to factors such as N deposition. This could have ramifications for the future C balance of these systems, but further work is needed to disentangle the effects of climate from other environmental drivers and determine the balance between productivity and decomposition