Land management impacts on the Carbon Cycle in UK Blanket Peats

Peatlands occupy a mere 3 % of the world's land mass, but store up to one third of terrestrial carbon stocks. Peatlands are widely regarded as carbon sinks owing to their ability to sequester more carbon than is released. Carbon cycling in peatlands is driven by environmental conditions e.g. water table levels, temperature and pH; substrate quality i.e. the ease with which microbes can synthesise the carbon; nutrient availability and the composition of the microbial community. Peatlands are valued not only for their ability to sequester carbon, but also for the range of ecosystem services which they provide including the provision of food, recreation and leisure, a source of income for rural communities, water supply and as habitats for a range of flora and fauna. As a result, management of peatlands is widespread, with the four most common methods of management of upland blanket bogs being afforestation, drainage, grazing and burning. To date, little work has been carried out on the effects of such management practices on carbon losses or drivers of the carbon cycle. The aim of this research was to identify how these management practices influenced losses of carbon from peatlands as well as the chemical and physical drivers of the peatland carbon cycle. A combination of field and laboratory work was carried out on managed peats with an unmanaged site at the Moor House National Nature Reserve in Cumbria. Field monitoring involved measurement of dissolved organic carbon (DOC) in the peat solution, water table levels and carbon dioxide gains and losses. Laboratory analysis was carried out on cores of peat to examine nutrient concentrations, the structure of the peat in terms of porosity and density; carbon stocks and the quality of the carbon. The results of this research demonstrated that all sites including the unmanaged site acted as carbon sources. Greatest losses occurred from the afforested site, where losses of DOC were significantly higher than all other sites and some of the highest losses of carbon dioxide were found. In contrast, the site that was burnt on a 10 year rotation was found to be a very slight carbon sink, held the most carbon within the peat and lost the least amount of DOC. Few significant differences in the chemical composition of the peat were observed between the sites, however, lignin, the most recalcitrant fraction was found to be significantly lower in the burnt (every 10 years) site, which had the highest carbon content. Lignin was identified as the dominant constituent of the peat for all the sites, with highest concentrations present in the afforested site. The high lignin content of the peats from all the sites indicated that the peats are in the latter stages of decomposition, and are thus fairly recalcitrant. The higher lignin content in the afforested site, coupled with the highest losses of DOC, some of the highest CO2 losses through ER (ecosystem respiration), however, suggest that the chemical composition of the peat is not a strong a driver of the peatland carbon cycle. Temperature was found to be the dominant driver of ER, accounting for between 54 and 92 % of variation in the data. The afforested site was the only treatment where a significant relationship between temperature and ER was not identified. Rates of primary productivity were highest in the burnt and grazed sites indicating that regeneration of the vegetation through management is of key importance in terms of sequestering carbon. The lowest primary productivity was identified at the drained site, where concentrations of nitrogen were also lowest. In terms of the structure of the peat, the air filled porosity of the burnt and grazed (every 20 years) site was greatest, however no linkages were established between the structure of the peat and gaseous carbon losses. This thesis has provided a unique insight into the effects of land management on the drivers of the peatland carbon cycle, carbon dioxide gains and losses, and DOC production. Further work should focus on examining the effects of the intensity of land management practices on peatland carbon budget for example, comparing low and high temperature burns, or closely spaced drains with drains that are located far apart. The results of this thesis suggest that future management needs to focus on encouraging increased PP by managing water table levels and promoting growth of peat forming species of vegetation such as Sphagnum. Light burning was also found to increase water table levels and peat solution acidity, thus reducing losses of DOC into the peat solution. The results demonstrated that temperature is the most important control on ER, and under climate change losses are likely to increase, therefore, the need twas found to be strongly linked to water table levels, pH and the carbon quality, with higher concentrations of holocellulose resulting in reduced losses of DOC.