The effects of long-term drainage and subsequent restoration on water table level and pore water chemistry in boreal peatlands

Abstract

Degradation by drainage threatens biodiversity and globally important peatland ecosystem functions such as long-term carbon sequestration in peat. Restoration aims at safeguarding peatland values by recovering natural hydrology. Long-term effects of drainage and subsequent restoration, especially related to within-site variation of water table level and pore water chemistry, are poorly known. We studied hydrological variation at 38 boreal Sphagnum peatland sites (pristine, drained and restored) in Finland. The average water table level was significantly lower at Drained than Pristine sites especially near the ditches. We also observed large pore water chemical differences between Drained and Pristine sites, such as higher DOC concentration at the sites drained several decades earlier. Furthermore, there were large differences in water chemistry between the samples collected from ditches and from the peat strips between the ditches. For example, the ditch water had apparently higher minerogenic influence, while DOC concentrations were highest in peat strips. The water table level was, on average, at the targeted level of Pristine sites at 5 years ago restored (Res 5) and 10 years ago restored (Res 10) sites. The Res 10 sites were more similar to the Pristine sites in water chemical composition than were the Drained sites. Water chemical differences between ditches and peat strips were smaller at the Res 5 and Res 10 than at Drained sites indicating, on average, successful decrease of drainage-induced within-site variation in water chemistry. Our results suggest more pronounced water table inclination towards the old ditches at Res 10 than at Res 5 sites. While this pattern may be an early warning sign for incomplete recovery of hydrology in long-term, we found no chemical evidence supporting this assumption yet. Our study suggests that restoration can result in significant recovery of peatland hydrology within 10 years, while some deviation from pristine hydrology is still typical. Restoration appears to have potential to reduce leaching of nutrients and DOC to downstream waters in the long term, but practitioners should be prepared for temporary increase of leaching of N and P for at least 5 years after restoration of boreal Sphagnum peatlands.