Peatlands are important terrestrial stores of carbon and a principal source of dissolved organic carbon (DOC) to the fluvial environment. Whilst often regarded as a net carbon sink, enhanced DOC concentrations and an associated rise in the level of water discolouration observed in many artificially drained peatland catchments across Europe and North America suggests that continued degradation may shift the balance of the carbon budget, such that they become a net carbon source. Peatland restoration, in the form of drain blocking, is currently being undertaken in a number of these locations. However, a great deal of this work has been carried out on a pragmatic or even an ad-hoc basis, with a distinct lack of process-baseda ssessmentT. hus, very
little is known about how such changes in land management affect DOC and colour dynamics. In order to bridge this knowledge gap, this thesis examines a range of processes known to influence DOC/colour production and release. A variety of field monitoring and laboratory measurements were undertaken to assess the upland blanket peat within the Oughtershaw Beck catchment in the Yorkshire Dales National Park, UK.
The installation of drainage ditches was found to reduce both the carbon storage potential of the peat and the quality of upland catchment waters. Drainage lowered the depth of the water table across the peat by an average of 10 cm, enhanced the rate of microbial activity by 33 % and increased DOC and colour production in soil water solutions by 35 %, relative to an adjacent intact site that had not been drained. The greater level of aeration in the upper peat layers associated with a lowered water table also appeared to reduce the degree of surface saturation and the occurrence of overland flow (OLF), resulting in a greater volume of water being drawn down into the peat body. The reduced saturation levels caused the subsidence and compaction of the upper soil layers, which increased the bulk density and ultimately reduced the degree of macroporosity within the soil. In turn, this is thought to have increased the residence time and surface area over which percolating waters flow, which is likely to have enhanced the degree of interaction with decompositional products, and thus the mobility of DOC/colour.
Drain blocking proved to be a highly effective technique for improving the carbon storage potential of blanket peat and ameliorating upland water quality. Blocking, using regularly spaced peat dams, successfully raised the height of the water table across the peat by an average of 4 cm, relative to the drained site. This increased the level of surface saturation and occurrence of OLF, whilst reducing rates of microbial activity and DOC production by 50 %. However, the volumetric changes associated with drainage appear to have resulted in permanent modifications to the structural and infiltration properties of the peat. Both the water table and the proportion of macropore flow at the drain-blocked site were reduced relative to the intact site, as were microbial activity rates, DOC concentration and colour levels. In addition, DOC composition was markedly different to that produced in the soil waters of both the intact and drained sites. The evidence suggests that a greater volume of percolating water travels through the peat matrix, relative to the intact site, which results in a pore water flushing mechanism and the preferential removal of labile un-coloured DOC components. Furthermore, the lower rate of microbial activity relative to the
drained site provides evidence against the commonly quoted hypothesis that an enzyme-latch reaction may be sustained in a peat that has been re-wetted following water table drawdown.
Although there was a strong association between DOC and colour, the relationshipvaried significantly between peat layers, land managements, and through time. This challenges the use of spectrophotometric analysis as an indirect method of DOC determination in peat soil waters as the use of a single regression equation resulted in the miscalculation of DOC concentrations by more than 50 %, as it failed to account of the fact that the fraction of coloured DOC components could vary significantly due to modifications made to microbial decomposition and mineralisation pathways and hydrological routing mechanisms.