(1) Restoration of the terrestrialization process and peat accumulation
An interesting aspect of rich fens is their succession from open water to terrestrial vegetation on dead biomass (peat). The restoration of fens is, therefore, not only the rehabilitation of different communities at the spatial scale, but also restoration on the temporal scale including terrestrialization and the peat-forming process. This target can become a greater challenge than species recolonization, especially if high nutrient levels as a legacy of agricultural use speed up decomposition.
The initial situation strongly determines restoration prospects. At Type A locations, former fen locations where all peat has been extracted to other soil layers (sand, clay), mire formation (i.e. with active peat formation) will be much more difficult. The initial stages after rewetting may not directly lead to peat production, but for instance to waters dominated by charophytes. As a result of peat extraction, the historical hydrology has often been heavily modified, and recharge fluxes may have become much larger. In other areas (Types B and C), the original discharge fens have been replaced by high-recharge locations after rewetting measures, or water levels and/or hydroperiodicity have been modified.
(2) Greenhouse gas balance
C losses to the atmosphere of drained peatlands in Europe are high, and measured fluxes range between +80 and +880 g C m−2 year−1, with an average around +400 g C m−2 year−1 (Nykänen et al., 1995; Kasimir-Klemedtsson et al., 1997; Joosten & Clarke, 2002; Byrne et al., 2004; Freibauer et al., 2004; Jacobs et al., 2007; Berglund & Berglund, 2010; Drewer et al., 2010), corresponding to a global warming potential (GWP) of +290 to +3230 g CO2 eq m−2 year−1. Emissions of CH4 are low due to drainage (Gorham, 1991; Freibauer et al., 2004). Subsidence rates measured in Sweden were (after the first consolidation) 0.5 cm year−1 for extensively managed pastures, 1 cm year−1 for fertilized haylands, 1.5 cm year−1 for cereals, and even 2.5 cm year−1 (2.5 m per century) for intensively cultivated crops such as potatoes (Berglund & Berglund, 2010). Average greenhouse gas emissions from agricultural peat soils are 350 g C m−2 year−1 for grasslands and 490 g C m−2 year−1 for croplands. The use of peatlands for row crop production (e.g. potato, sugar beet) shows extremely high C losses of +380–950 g C m−2 year−1, and is therefore by far the least sustainable land use (Freibauer et al., 2004).
Measurements of C fluxes after fen restoration (rewetting) are, however, scarce. For an ex-arable fen meadow that had been rewetted 10 years before and became dominated by Phragmites australis (Cav.) Trin. Ex Steud, Hendriks et al. (2007) measured a NEE of, on average, −280 g C m−2 year−1 (−1030 g CO2 m−2 year−1). Although one has to be careful to draw far-reaching conclusions, also because of uncertainties related to the use of different techniques, these high values indicate there is great potential for C storage on abandoned peatlands (Graf & Rochefort, 2009). Even if rewetting does not lead to the values (−5 to −40 g m−2 year−1) found in undrained fens, C losses to the atmosphere as compared to intensive agricultural use, will still be much lower. An important problem is the fact that peatlands still show hotspots of C loss after rewetting, due to high phenolic oxidase activities and related high rates of organic matter breakdown (Fenner et al., 2011). As a result of the build-up of labile carbon fractions and nutrients during drought, high amounts of carbon can be lost to the atmosphere, groundwaters and surface waters after rewetting (Fenner & Freeman, 2011). This is further enhanced by the increase in pH as a result of alkalinity generation due to anaerobic reduction processes, which may even stimulate decomposition rates. In addition, this decrease in acidity is known to lead to higher exports rates of DOC, as a result of changes in both decomposition rates and DOC solubility. These processes may lead to unexpected and disappointing results in the first years after rewetting. Recent ideas to increase C sequestration in peatlands even suggest the addition of phenolic compounds to reduce decomposition (Freeman, Fenner & Shirsat, 2012), which could be an interesting idea to compensate for high C losses due eutrophication and water table decline before restoration. The benefits of carbon offsetting can be used to finance fen restoration (Worrall et al., 2009). In fen meadows that have become acidic as a result of drainage, however, decomposition rates may also increase initially after rewetting as a result of high nutrient values and increased ANC (Van Dijk et al., 2004).
As CH4 production increases exponentially with an increase in water level, rewetting of drained fens will lead to a strong increase in CH4 emission, from values around 0 (including negative values due to CH4 oxidation) to values in the range found for peatlands that have been less influenced by human activity. As the 100 year GWP of CH4 is 25 times that of CO2 (on a mass basis; Solomon et al., 2007), it is important to incorporate its impact on global warming. Recently, it has been estimated that the CH4 output of global wetlands reduces the effects of the continental C sink by 25% (Bastviken et al., 2011). Around 90% is emitted by open water, of which half is by ebullition, and 10% through plants (Bastviken et al., 2011). Emergent species strongly facilitate the emission of CH4 from peat by their funneling effect (flow through internal air channels) and by stimulation of methanogenesis, with fluxes ranging from 8 to 260 mg C m−2 day−1 largely determined by temperature and water table (Bubier, 1995; Nykänen et al., 1995; Bellisario et al., 1999; Bastviken et al., 2011). On a yearly base, fens emit 5–50 g CH4–C m−2 (Byrne et al., 2004), which corresponds to a GWP of 165–1650 g CO2 eq m−2 year−1. Post-restoration changes in vegetation have a significant impact on CH4 emission rates (Waddington & Day, 2007; Bhullar et al., 2014), as has herbivory (Dingemans, Bakker & Bodelier, 2011). In addition, water quality can strongly influence CH4 emissions, as concentrations and reduction rates of SO42− can be high after rewetting (Lamers et al., 1998a), and inhibit methanogenesis (Kang, Freeman & Lock, 1998). In restored fens, CH4 fluxes seem to be in the high range of more pristine fens but higher than in cutover minerotrophic peatlands, as shown in Canada (Mahmood & Strack, 2011). Increased sulfate inputs from S deposition, or from surface water or groundwater will strongly suppress CH4 emissions from peatlands (Vile et al., 2003). Atmospheric S pollution may reduce global CH4 emissions from wetlands by 15% (Gauci et al., 2004).
Despite increased CH4 emission and, to a minor extent, N2O emission, rewetting of formerly arable, eutrophic fen peatland can still generate a net sink for greenhouse gases with GWP values around −85 g CO2 eq m−2 year−1 (Hendriks et al., 2007). For undrained European fens, a large range has been reported from −25 (reduction of global warming) to +150 g CO2 eq m−2 year−1 (stimulation of global warming), although many values have a high level of uncertainty (Byrne et al., 2004). It can be expected that high nutrient levels in rewetted ex-arable lands will stimulate decomposition. Literature results, however, are contrasting and include stimulation of decomposition, no effect, and even higher peat accumulation rates (Richardson & Marshall, 1986; Aerts & Toet, 1997; Kalbitz et al., 2000; Sarneel et al., 2010).
The effect of simultaneous increases in temperature and CO2 on peat formation in fens also is a subject of debate. The discussion is due to the fact that these changes may lead to higher decomposition rates as a result of higher temperatures, but also to higher primary production rates, which may offset each other (Gorham, 1991; Kirschbaum, 2000). However, there are also higher risks of drought episodes, stimulating decomposition and reducing fen plant growth (Ise et al., 2008). In addition, CH4 emissions can increase, not only due to higher temperature but also as a result of raised CO2 levels. It is also alarming that the microbial decomposition of biogeochemically recalcitrant organic matter is more sensitive to increased temperature than that of more readily decomposable organic matter (Craine, Fierer & McLauchlan, 2010). More research is needed to be able to estimate and predict the interacting effects of rewetting, eutrophication and climate change on decomposition, net ecosystem carbon exchange and peat formation after restoration of fens, because the interaction between these processes may lead to a trade-off in ecosystem services. With respect to greenhouse emissions, it is clear that the rehabilitation of vegetated areas instead of lakes is preferred. Although peat lakes increase the water storage capacity, they are also hotspots of CH4 emissions with very low CO2-fixation rates. For biodiversity, however, landscape and habitat diversity including open water is important.