Impacts of rewetting on peat, hydrology and water chemical composition over 15 years in two finished peat extraction areas in Sweden

Wetlands Ecol Manage DOI 10.1007/s11273-016-9524-9 ORIGINAL PAPER Impacts of rewetting on peat, hydrology and water chemical composition over 15 years in two finished peat extraction areas in Sweden Lars Lundin . Torbjörn Nilsson . Sabine Jordan . Elve Lode . Monika Strömgren Received: 18 July 2016 / Accepted: 8 December 2016  The Author(s) 2016. This article is published with open access at Springerlink.com Abstract Restoration of wetlands is a high priority world-wide. Peat extraction areas can be restored by rewetting, however affecting the environment. It could be expected to turn the drained peat-cutover area from a source to a sink of most elements. This study examined effects of such rewetting on peat, hydrology and water chemistry over 15 years at two sites in Sweden; the nutrient-poor Porla peatland and the nutrient-rich Västkärr peatland. Rewetting caused minor changes to peat chemistry, but at the Västkärr site ammonium concentrations increased in superficial peat layers while nitrate decreased. In terms of hydrology, rewetting of the Porla site decreased annual runoff and both high and low discharges. Water pH at the Porla site stayed fairly stable, but at the Västkärr site pH, after an initial 4 years dip, gradually increased to higher values than before rewetting. Water colour and organic matter content were fairly stable, but slightly lower values were found after 15 years than in initial 4–5 years. The concentrations of base cations and of inorganic N were lower after rewetting, while total P was higher. However, these impacts could change from an initial phase as the L. Lundin (&)  T. Nilsson  S. Jordan  E. Lode  M. Strömgren Department of Soil and Environment, Swedish University of Agricultural Sciences, P.O. Box 7014, 750 07 Uppsala, Sweden e-mail: wetlands in the long-term perspective develop into mires. Keywords After-use  Hydrochemistry  Peatland  Restoration  Wetland Introduction In total, four million km2 of the Earth’s surface (circa 3% of the land area) are covered with peatlands, and peatlands are found in almost every country of the world (Schumann and Joosten 2008). It is especially abundant in vast areas of North Europe and Canada. The peatlands and peat resource is considered a societal asset and is used in agriculture, forestry, horticulture and as an energy source. These uses have resulted in numerous remnants of damaged mires, for which restoration is a high concern. Peatland restoration aims to improve environmental service values for biodiversity objectives, ecosystem functions, carbon storage, flood mitigation, deposition purification, etc. (Rochefort et al. 2003; Vasander et al. 2003; Ramchunder et al. 2012). Peat extraction has been carried out in many countries for at least 200 years. In Europe, peat losses have been considerable and in more than 50% of the original natural mire area peat organic material is no longer accumulating (Joosten and Clarke 2002). For a number of decades, it has been mandatory to find a suitable use for the remaining land 123 Wetlands Ecol Manage after extraction ceases. In 1998, a new environmental law was established in Sweden, giving increased support for restoration. There are multiple possibilities for wise after-use of former peat extraction areas (Joosten and Clarke 2002). On sites with low potential for forests or crop production, rehabilitation activities can seek to improve especially biodiversity values. Ecologically, wetland restoration has the main goal of bringing back disturbed terrestrial ecosystems and it aims to restore natural wetland hydrology (water level, flow paths and water chemistry) and to re-establish characteristic peatland plant cover (Vasander et al. 2003). For success in such operations, the starting conditions, i.e. the state of the peatland after peat extraction, should be considered, since it provides the main potential for successful end-results (Blankenburg and Tonnis 2004). Suitable indicators of wetland generation success include stable hydrology, appropriate water quality and vegetation development (Lode 2001). Rewetting of cut-over peat would be the reverse of drainage; so many findings in forest drainage research could apply to conclusions for rewetting turning the effects of drainage back to the pre-drainage situation (Lundin 1988). Drainage also has effects on the water balance, providing space for precipitation in temporary surface water storage to be used in prolonged evapotranspiration and runoff. Such storage mitigates fast discharge responses. However, the storage capacity could be limited in situations when groundwater levels are high. In such circumstances, the ditches provide rapid channel flow capacity and possibly enhanced discharge peaks (Iritz et al. 1994). However, in general the effects of drainage on hydrology are lower peak flows, but somewhat higher annual runoff (Braekke 1970), accompanied by higher low discharge because the ditches promote release of water (Lundin 1988). In the case of rewetting, the reverse influence on discharge could be expected. Not only hydrology is affected by peatland drainage, but also hydrochemistry. In the peatland formation process, when peat is accumulating and peat organic matter storage is increasing, there is sequestration of most chemical elements in the organic material. Drainage alters this process to decomposition when peat oxidation occurs and the stored elements are released (Sallantaus 1989). In drained conditions, higher outflow of most elements occurs, 123 but the concentrations of dissolved organic matter (DOC) and protons in the leaching decrease, providing higher pH. Nitrogen (N) content changes from being dominated by the organic fraction (Norg, 80%) to inorganic nitrogen (Ninorg) release that can reach a level of 60% of total N (Ntot) (Lundin 1988). In the case of rewetting, a change from release to retention could be expected. In modern industrial peat harvesting, the total peat resource is often extracted, with only thin remnants of peat left on top of mineral soil. This creates conditions strongly deviating from those in areas where a rather thick ([1 m) layer of peat still remains (Eggelsmann et al. 1993). There is probably some benefit in complete peat removal at one site, instead of affecting many sites by removing the top peat layer only. Such upper layer extraction keeps remnant peat conditions favourable and resembling the natural wetland properties, facilitating vegetation restoration. In the case of total peat extraction, the new, or actually very old, bottom peat layers and mineral soil, which have long been largely preserved from surface water and atmosphere interface processes, fall under the influence of new environmental processes. This situation gives rise to new hydro-chemical and biological conditions (Wheeler 1995). This study primarily examined the impacts of peatland rewetting on properties of the peat, hydrology and water chemical conditions. (...truncated)