Effects of temperature, rainfall and conifer felling practices on the surface water chemistry of northern peatlands

Abstract

Afforested peatlands account for about 25 % of human-affected peatlands worldwide. In some regions, particularly the British Isles, forest-to-bog conversions are now underway in the hope of increasing carbon storage and restoring original ecosystem functions. In this study, the effects on surface water quality of forest-to-bog restoration in the Flow Country peatlands of northern Scotland were monitored during 15 months. The hydrochemistry of rainwater, resurgence ground water, forestry drainage ditch water, open bog surface water, forestry-influenced loch water and stream water was examined before, during and after felling. The seasonal cycles of biologically active (C, Si, P) and organically complexed (Fe, Al) elements were characterised by major changes in concentration. The felling operations amplified these effects due to (i) the decomposition of felling residues (leaching K and C) and (ii) the disturbance and partial mineralisation of shallow peat soils (releasing P, Fe and Al). Shorter term changes (1–10 days) produced by rainfall events and associated changes in hydrological flow paths controlled the concentrations of Na, Ca, Mg, Mn, and to a lesser extent K and Al. There were significant negative correlations with flow of [K], [Ca] and [Mg] but a positive correlation between stream flow and [Al]. Concentration-flow relationships were not significantly altered by the felling operations because of the relatively low volume of water released from the felled area. Fluctuations in the water table represented another important control of surface water composition. Resurgence waters produced high [Fe] and [Si] values when water table levels were low. These contributions were diluted with runoff and surface soil waters enriched in dissolved organic carbon (DOC) when levels rose. Sequential ultrafiltration revealed that Fe, C and P co-occurred in the >30 kDa molecular weight fraction in constant relative concentrations. Since forest-to-bog restoration raises the water table, it can be anticipated to result initially in more intense flushing of DOC, including the >30 kDa molecular weight fraction where Fe and P co-occur.