Changes in Pore Water Quality After Peatland Restoration: Assessment of a Large‐Scale, Replicated Before‐After‐Control‐Impact Study in Finland

Abstract

Drainage is known to affect peatland natural hydrology and water quality, but peatland restoration is considered to ameliorate peatland degradation. Using a replicated BACIPS (Before‐After‐Control‐Impact Paired Series) design, we investigated 24 peatlands, all drained for forestry and subsequently restored, and 19 pristine control boreal peatlands with high temporal and spatial resolution data on hydroclimate and pore water quality. In drained conditions, total nitrogen (Ntot), total phosphorus (Ptot), and dissolved organic carbon (DOC) in pore water were several‐fold higher than observed at pristine control sites, highlighting the impacts of long‐term drainage on pore water quality. In general, pore water DOC and Ntot decreased after restoration measures but still remained significantly higher than at pristine control sites, indicating long time lags in restoration effects. Different peatland classes and trophic levels (vegetation gradient) responded differently to restoration, primarily due to altered hydrology and varying acidity levels. Sites that were hydrologically overrestored (inundated) showed higher Ptot, Ntot, and DOC than well‐restored or insufficiently restored sites, indicating the need to optimize natural‐like hydrological regimes when restoring peatlands drained for forestry. Rich fens (median pH 6.2–6.6) showed lower pore water Ptot, Ntot, and DOC than intermediate and poor peats (pH 4.0–4.6) both before and after restoration. Nutrients and DOC in pore water increased in the first year postrestoration but decreased thereafter. The most important variables related to pore water quality were trophic level, peatland class, water table level, and soil and air temperature.

The impact of ditch blocking on fluvial carbon export from a UK blanket bog

“…A comparison of DOC radiocarbon ( 14 C) measurements from intact, drained, and re‐wetted peatlands from the boreal to tropical climate zones (which included our study catchments) showed highly variable sensitivity to drainage‐induced loss of DOC from deeper peat (Evans et al, ), with tropical peatlands showing the greatest sensitivity and blanket bogs the least. These differences are reflected in literature showing clear evidence of DOC response to drainage and re‐wetting in boreal, continental temperate, and tropical peatlands (Glatzel et al, ; Haapalehto et al, ; Menberu et al, ; Moore et al, ; Strack et al, ; Urbanová et al, ), but not in blanket peatlands (see above). One possibility, given the observed subsidence at our site, is that blanket bog drainage causes an initial flush of DOC, which dissipates as the peat surface subsides and ditches infill.…”

“…The lack of apparent reduction in DOC concentrations following ditch blocking is somewhat surprising, given that many studies have observed higher DOC concentrations in porewaters or run‐off from drained peatlands, and/or reduced concentrations after re‐wetting (e.g., Frank, Tiemeyer, Gelbrecht, & Freibauer, ; Haapalehto, Kotiaho, Matilainen, & Tahvanainen, ; Höll et al, ; Menberu et al, ; Urbanová, Picek, & Bárta, ; Wallage et al, ). Overall, published data suggest that peatland drainage increases DOC concentrations by around 50% and that re‐wetting reduced concentrations by around 25% (Evans et al, ).…”

“…Blanket bog data presented by Luscombe et al (2016) (Williamson et al, 2017) also suggests that, since ditches were dug in the 1980s, peat adjacent to many ditches has subsided by 10 cm or more, effectively following the water table down (Joosten, 2016). Although unquestionably an impact of drainage, this subsidence means that the volume of aerated peat has declined with time since drainage, and it is also probable that peat bulk density has increased, and hydraulic conduc- showing clear evidence of DOC response to drainage and re-wetting in boreal, continental temperate, and tropical peatlands (Glatzel et al, 2003;Haapalehto et al, 2014;Menberu et al, 2017;Moore et al, 2013;Strack et al, 2008;Urbanová et al, 2011), but not in blanket peatlands (see above). One possibility, given the observed subsidence at our site, is that blanket bog drainage causes an initial…”

“…This perception has contributed to a number of water industry‐linked initiatives aimed at improving raw water quality by re‐wetting drained blanket bogs (e.g., Anderson, Worrall, Ross, Hammond, & Keen, ; Grand‐Clement et al, ; RSPB, ). Although there is reasonable evidence to suggest that peat drainage does indeed increase lateral DOC export and that re‐wetting can reduce these losses (Evans et al, ; Menberu et al, ), much of this evidence derives from studies of continental raised bog and fen systems, rather than UK blanket bogs, for which findings have been more equivocal. Here, some studies have reported clear effects (Wallage, Holden, & McDonald, ), but others have observed only limited impacts (Armstrong et al, ; Gibson et al, ; Turner, Worrall, & Burt, ), indicating that caution may be needed regarding the water quality outcomes of peat restoration (Armstrong et al, ).…”

Peatland ditch blocking has no effect on dissolved organic matter (DOM) quality

“…In addition, there are concerns that drainage leads to increased concentrations and fluxes of dissolved organic carbon (DOC) in streams and rivers. This effect has been observed in tropical (Moore et al, ), temperate (Moore & Clarkson, ; Strack et al, ), boreal (Menberu et al, ), and subarctic peatlands (Lou, Zhai, Kang, Hu, & Hu, ) and has been recognised in the U.K. uplands for several decades (Mitchell, ; Naden & McDonald, ).…”