Landuse change and organic carbon exports from a peat catchment of the Halladale River in the Flow Country of Sutherland and Caithness, Scotland

Abstract

Upland peat catchments are usually assumed to function as carbon sinks, however, there have been extensive studies witnessing increasing trends in concentrations and fluxes of organic carbon in UK rivers over the last few decades. A number of controls on dissolved organic carbon (DOC) release from peatlands, such as climatic changes and landuse management, have been proposed. This study examines the effects of land use and management on organic carbon exports in the Dyke catchment of the River Halladale (northern Scotland) with a nested catchment approach. This study provides insight into the processes controlling the DOC dynamics in the Dyke catchment, and the impact of disturbance caused by landuse changes such as afforestation and tree felling for restoration. The results from factor analysis, end-member mixing, absorbance (E4/E6), and hysteresis analyses on stream water chemistry from individual sub-catchments identified the major hydrological pathways during storm events, and based on these results, conceptual models were developed to explain DOC evolution during storm events. At all the sites studied, nearsurface soil water was identified as the major controlling end-member for stream DOC concentrations. The calculated annual flux of DOC from the Dyke catchment, up-scaled from the results of the individual sub-catchments, is 521.6 kg C ha⁻¹ yr⁻¹, which is significantly (~5 times) higher than the previously published value (103.4 kg C ha⁻¹ yr⁻¹) for the River Halladale catchment (Hope et al., 1997). In this study, it is shown that about 57 - 95% of the DOC export occur during 5 - 10% of the high flows, therefore, it is crucial that quantitative records of DOC export are developed using high frequency storm event measurements, as well as lower frequency low flow sampling. Climatic changes related to precipitation, temperature, coupled with water yield capacity of the sub-catchments, are identified as significant controls on DOC fluxes, rather than landuse change, as the intact site releases more organic carbon per unit area compared to the disturbed and re-wetted site undergoing restoration. In addition, the results from this study provide landowners, policy makers and organisations with the evidence they require for initiating future peatland restoration works, as felling of forestry coupled with drain-blocking is shown to be an effective restoration technique that may help a catchment to eventually return to a near-pristine state.