From Drainage to Rewetting—Soil Transformations in European Agricultural Peatlands: A Review

Abstract

European peatlands have been extensively drained for agriculture, resulting in substantial carbon losses and widespread soil degradation. Peatland restoration is therefore a global priority, with rewetting recognised as a key strategy for mitigating greenhouse gas emissions and climate change. This review synthesizes current knowledge on soil transformations following the rewetting of agriculturally drained peatlands in Europe. We describe major degradation processes induced by drainage, including land subsidence, organic matter oxidation, and microbial community shifts from anaerobic to aerobic conditions. We then examine key rewetting approaches—ditch blocking, controlled flooding, and paludiculture—and their intended restoration outcomes. Rewetting fundamentally alters soil physical, chemical, and biological properties by raising and stabilizing water tables, restoring anoxic conditions, and modifying nutrient cycling and microbial processes. Findings indicate long-term stabilization of organic carbon in peat soils under anaerobic conditions, but also reveal trade-offs between reduced CO₂ emissions and increased CH₄ and N₂O fluxes. Vegetation–soil interactions strongly influence recovery trajectories, and paludiculture offers potential to align agricultural land use with climate mitigation objectives. Finally, we evaluate current research methodologies and identify major knowledge gaps, including limited long-term data and insufficient integration of hydrological, chemical, and biological processes. We highlight priorities for future research to support evidence-based rewetting strategies that deliver climate benefits while maintaining ecological and economic sustainability in European peatlands.

1. Introduction

Peatlands are critical ecosystems, covering only 3–5% of the Earth’s terrestrial surface, yet storing a disproportionately large share of global soil carbon and serving as long-term carbon sinks [1,2,3]. Peatlands are wetlands characterised by the accumulation of partially decomposed organic matter (peat) under waterlogged, anaerobic conditions. The term mire is used as a general ecological designation for peat-forming wetlands and includes two major types: bogs, which receive water primarily from precipitation (ombrotrophic), and fens, which are influenced by groundwater and surface water inputs (minerotrophic) [3]. Their exceptional carbon storage capacity results from persistently waterlogged conditions, which limit oxygen availability, slow organic matter decomposition, and promote the accumulation of peat over millennia. Despite their ecological significance, peatlands are increasingly threatened by both natural disturbances and, more importantly, by anthropogenic activities such as drainage for agriculture and forestry. These interventions fundamentally alter peat soil properties, transforming peatlands from long-term carbon sinks into major sources of greenhouse gases (GHGs) [4,5]. Peatlands account for approximately 7% of Europe’s land area, corresponding to nearly 0.5 million km², with particularly extensive coverage in north-western, Nordic, and eastern regions of the continent [6,7,8]. These ecosystems provide a wide range of essential ecosystem services, including carbon sequestration, water regulation, biodiversity conservation, and agricultural production [9]. However, large-scale drainage has severely degraded European peatlands. Approximately 21% of natural peatlands have been drained for forestry and 17% for agricultural and grassland use [10]. As a result, processes such as peat oxidation, land subsidence, nutrient mobilization, and shifts in microbial activity have become widespread, leading to substantial carbon losses a marked decline in peatland-specialist plant and animal species, as well as degradation of characteristic peatland habitats [8,11]. Peatland degradation caused by drainage represents a significant challenge for climate change mitigation. If current trends continue, emissions from drained peatlands could account for 12–42% of the remaining global carbon budget required to limit warming to 1.5–2 °C [12]. Despite national and international commitments to achieve climate neutrality by mid-century [13,14], agricultural drainage of peatlands remains widespread across Europe. In response, rewetting—the cessation of drainage and restoration of high groundwater levels—has emerged as a central strategy for mitigating environmental damage and reversing peatland degradation [14,15]. Rewetting induces profound changes in the physical, chemical, and biological properties of peat soils. Although rewetted peatlands rarely return fully to their pre-drainage state, restoration substantially reduces soil carbon losses and greenhouse gas emissions [16,17,18]. The Intergovernmental Panel on Climate Change (IPCC) recognises peatland restoration, particularly on agriculturally used peat soils, as a key climate mitigation measure [19]. Achieving climate neutrality by 2050 will require rewetting nearly all drained peatlands in Europe. Accordingly, peatland restoration has become a central component of European climate and biodiversity policies. The European Green Deal and the EU Nature Restoration Law establish legally binding targets for peatland restoration, requiring Member States to restore 30% of drained agricultural organic soils by 2030, 50% by 2040, and 70% by 2050, with rewetting forming a major part of these efforts [20]. These commitments represent a paradigm shift in European land management, positioning peatland rewetting as a key component of climate mitigation and biodiversity recovery strategies. Simultaneously, modifications to the Common Agricultural Policy are advocating for payments that are contingent upon outcomes and eco-schemes that incentivise elevated water tables, reduced greenhouse gas emissions, and enhanced biodiversity. Nevertheless, the effectiveness of these changes hinges on scientists possessing a comprehensive understanding of the soil’s response to rewetting.

Current estimates indicate that peatland restoration could reduce greenhouse gas emissions by up to 63 million tonnes of CO₂-equivalent annually by 2050, while full restoration could achieve reductions of approximately 185 million tonnes CO₂-equivalent per year [21,22]. Despite growing political and societal momentum, important uncertainties remain regarding how rewetting alters peat soil properties and how these changes affect long-term carbon stabilization and sustainable land use. Effective restoration strategies require a mechanistic understanding of soil responses to rewetting, including shifts in hydrology, redox conditions, nutrient cycling, and microbial processes. Although there is an increasing volume of work on peatland restoration, current assessments predominantly emphasise near-natural systems, with relatively scant focus on the unique biogeochemical, hydrological, and management issues associated with agriculturally drained peatlands. Prolonged draining, intensive agriculture, and significant peat degradation yield restoration trajectories that cannot be directly inferred from natural peatlands. Furthermore, current syntheses often remain disciplinary and weakly connected to European policy and land-use frameworks, limiting their relevance for agricultural management and climate mitigation. This review addresses these gaps by providing an integrated synthesis of soil transformations following rewetting of agricultural peatlands in Europe, linking soil processes to greenhouse gas dynamics, ecosystem recovery, and sustainable land use strategies. This synthesises current knowledge on physical, chemical, and biological transformations in soils following the rewetting of agriculturally drained peatlands in Europe. We further identify critical research gaps and outline priorities for future studies aimed at optimizing climate mitigation and sustainable peatland management.

2. Peatland Soils Under Agricultural Use: Effects of Agricultural Drainage

2.1. Agricultural Use of Peatlands in Europe

Agriculture represents the most widespread form of peatland use in Europe [23]. Over the past two centuries, both the spatial extent and intensity of agricultural peatland utilisation have increased considerably. Currently, approximately 15% of European peatlands are managed for agricultural purposes, predominantly as meadows and pastures [23,24]. In several countries, including Hungary (98%), Greece (90%), the Netherlands (85%), Germany (85%), and Poland (70%), the vast majority of organic soils have been converted to agricultural use [25]. Up to half of all peatlands were used for agriculture in Belarus (~1.5 million ha), Lithuania (~0.6 million ha), and Ukraine (~4 million ha), and up to 35% in Ireland (~0.3 million ha) [26]. In Finland (~0.03 million ha, 2%), the UK (~0.05 million ha, 4%), and Sweden (~0.07 million ha, 5%), only small areas were under agricultural use at that time, according to data reported by Oleszczuk et al. [24]. Agriculture accounts for roughly 50% of peatland conversion in temperate regions, with the most severe impacts occurring in accessible lowland fens and raised bogs [27]. Drainage has enabled conventional agricultural systems, originally developed for mineral soils, to be applied to naturally wet peatlands, thereby creating highly productive agroecosystems [28]. However, despite their relatively small spatial extent, agricultural peatlands contribute disproportionately to global greenhouse gas emissions. It is estimated that peatland agriculture accounts for approximately 32% of total cropland GHG emissions, while producing only 1.1% of global crop kilocalories [29]. Furthermore, carbon stocks lost from drained peatlands are considered irrecoverable on human-relevant timescales, as depleted peat carbon cannot be replenished within decades or centuries [30].

2.2. Impacts of Drainage on Physical Processes (Subsidence)

Drainage profoundly affects peatland functions, with land subsidence representing the most visible and immediate consequence. Subsidence increases flood risk and leads to costly damage to infrastructure, particularly in low-lying agricultural landscapes [31]. The dominant driver of long-term subsidence in drained agricultural peat soils is the mineralisation of soil organic matter (SOM) [32], which results from three primary processes: (i) oxidation, (ii) compaction and shrinkage, and (iii) consolidation [33,34]. Subsidence rates are typically highest immediately following drainage but persist at lower rates for decades, potentially leading to the complete loss of peat layers over time [35,36]. Water table depth is the primary controlling factor, with deeper drainage resulting in higher subsidence rates [37,38]. Additional influences include peat type, degree of decomposition, bulk density, peat thickness, climate, local hydrology, agricultural management practices, and land-use history [39,40,41]. In temperate climates, mineralisation of SOM has been estimated to account for 28–64% of total subsidence [42]. Progressive peat loss eventually reduces peat thickness and carbon content to levels at which soils no longer meet classification criteria for organic soils [43].

2.3. Biogeochemical Consequences (C, N, CH₄ Emissions)

Drainage exposes peat soils (Histosols) or peaty mineral soils to oxygen, accelerating oxidation of organic matter and releasing CO₂ and dissolved organic carbon (DOC) to the environment [44,45]. Mineralisation of SOM also releases nitrogen (N), resulting in high mineral N supplies (e.g., 250–571 kg N ha⁻¹ yr⁻¹ for cropland; Rochette et al., 2010) [46]. Where these exceed crop N demands, substantial N₂O emissions can occur [47]. Nitrogen mineralisation and phosphorus mobilization increase under aerobic conditions, enhancing nutrient leaching [48,49]. Over time, ash content and bulk density rise while peat carbon and nutrient retention decline, marking long-term microbially mediated decomposition and chemical alteration of drained agricultural peat soils [44,50]. Despite these changes, the soils often retain their classification as Histosols or peaty mineral soils, until organic matter content falls below classification thresholds. In drained peatlands, methane (CH₄) oxidation potentials are high throughout the soil profile [51], resulting in near-zero terrestrial CH₄ emissions from croplands. Grassland soil CH₄ emissions are generally low but can spike during periods of inundation and rapid anaerobic decomposition of flood-intolerant plant species [52].

2.4. Fire Vulnerability

Anthropogenic drainage reduces peatland moisture content, increasing their flammability and the depth of peat available for combustion [53]. Fire impacts can, therefore, be severe, with a typical uncontrolled fire estimated to emit 122 t CO₂-C ha⁻¹ [54]. Controlled burning still takes place on some mid-latitude peatlands, but the practice is rare, largely due to the air pollution impacts, demonstrated by recent fires in Russia and the United Kingdom [55,56]. Accidental fire risks remain on abandoned sites where drainage may be poorly managed, but these can be reduced by rewetting [57].

2.5. Effects on Microbial Communities

Microbial communities and associated decomposition processes change with depth, as redox conditions and organic matter composition and lability vary along the soil profile [58,59,60]. It is to be expected that susceptibility and resilience of the microbial communities to drainage will differ among peatland types [61,62]. Similarly, the changes in microbial community structure and activity after drainage were more pronounced in fens than in bogs, and the differences in fungal and actinobacterial communities between these peatland types were diminished after long-term drainage [63,64]. Increased aeration of peat can provide more favourable conditions for fungi and certain Gram-negative bacteria [65]. Long-term drainage reduces the activity and diversity of both methanogens and methanotrophs, with more pronounced effects in fens than in bogs [62,66,67]. While methanotrophs oxidize much of the CH₄ produced, their decline is ecologically relevant. It reflects broader shifts in microbial communities, may allow transient CH₄ accumulation during local saturation or rewetting, and can influence nitrogen cycling and other soil processes [66,67,68,69]. While rewetting can partially restore anaerobic processes, full recovery of microbial diversity and carbon cycling remains limited in the long term [49,68,69].

3. Rewetting as a Restoration and Management Strategy

Rewetting of drained peatlands is the process of restoring a stable water table near the surface [70,71]. When peatlands remain waterlogged, anaerobic conditions slow organic matter decomposition and promote long-term carbon storage, although carbon cycling continues through methane emissions [72,73]. The benefits of peatland rewetting include climate protection through the reduction of CO₂ emissions, stabilization of water tables and improved water balance [74], and support for biodiversity through the establishment of peatland-typical plant and animal species [75]. Restoration also enhances ecosystem services such as carbon sequestration, nutrient retention, flood mitigation, and peat fire prevention [3,18,76,77]. Therefore, rewetting frequently forms the first significant step in peatland restoration [78,79,80,81,82,83], providing essential conditions for long-term ecological recovery [84] However, the performance of these services is strictly dependent on the availability of water [82]. Therefore, peatland restoration through rewetting frequently forms the first significant step of the restoration process [85,86,87]. Rewetting aims at reversing the effects of degradation and bringing peatlands’ conditions back to a more natural state [88,89]. The most common and effective peatland restoration measure is to block the drainage ditches with dams, typically constructed from natural materials such as peat, mineral soil, or wood [90]. It involves the emplacement of dams, infilling, or reprofiling of drainage [88,90]. Ditch blocking is used, often combined with controlled flooding, to raise the water table and restore wetland hydrological functioning [91]. In three Norwegian raised bogs, for instance, ditch-blocking increased groundwater levels by ~6 cm on average, extended spatially the influence of higher water tables up to ~17 m from the ditch, and extended the duration of favourable hydrological conditions by ~28% on average [92]. It is also a widely applied first-step restoration action in Polish peatlands in order to raise and stabilise near-surface water tables, reduce lateral losses of dissolved and particulate organic carbon, and re-establish saturated conditions favourable for peat-forming vegetation [93]. Field studies and surveys in Poland report that long-term agricultural drainage has caused measurable peat subsidence. The changed peat properties in the vicinity of ditches and the blocking of these ditches is recommended to limit the further oxidation of organic matter and promote rewetting [94,95,96]. Subsidence and hydrology investigations at Solec and other Polish peatlands have demonstrated faster peat surface lowering near the drainage ditches. They suggest that ditch dams or infilling could diminish further peat loss and hydrological stress in the most affected zones [93]. National NGO guidance and recent practitioner manuals for Poland also present ditch blocking as a standard, low-cost intervention often combined with bunding and small-scale pool creation. These tends to increase surface water retention and to stabilise water tables before longer-term measures such as revegetation or paludiculture are applied [97]. Paludiculture is a way to restore wetlands that combines rewetting peatlands with productive land usage. This lets farmers continue economic activities while restoring wetland ecosystems [96,98,99]. The word “paludiculture”, which denotes the preservation of both the peat body and the production function of the land, is used to describe the environmentally responsible use of wet peatlands [100]. Since drained peatlands are unlikely to revert to their original state even after restoration, they make excellent candidates for paludiculture [3,101]. A fair strategy would be to maintain productive use of rewetted peatlands through paludiculture, allowing farmers to derive some economic benefits while local communities manage their peatland resources. While yields and income are generally lower than under intensive agriculture, paludiculture provides a sustainable compromise between land use and ecosystem restoration [98].

In contrast, controlled flooding used in cut-over or previously drained peatlands involves allowing or controlling shallow flooding across low vegetated or bare peat surfaces. It helps in the rewetting of shallow surface peat layers, the redistribution of water, and the formation of pool-like microtopography, enabling ecological restoration and hydrological recovery [92]. To restore natural hydrological regimes, increase water storage, and improve habitat heterogeneity for wetland biota, large regional projects in the Biebrza and other north-eastern wetlands specifically combined ditch blocking with restored floodplain connectivity and controlled inundation [102]. Recent empirical work shows that restored pools and peat pans in temperate peatlands increase retained surface water and stabilise hydrology. Although they can also become hotspots for methane emissions, controlled flooding designs typically balance objectives (water storage, biodiversity, flood buffering) against greenhouse-gas trade-offs by limiting pool depth/area and by promoting rapid recolonisation by peat-forming plants where feasible [103,104]. Furthermore, initial conditions, hydrological processes and, consequently, the possible amount of stored water and responses to drainage and rewetting, vary depending on peatland type. For example, sloping fens have drier peat [105], and they are more sensitive to ditching and groundwater fluxes than flat fens [106,107]. By contrast, drainage of raised bogs, which have less buffering capacity than fens, strongly destabilises water tables, leading to rapid surface drying and shifts in vegetation composition [108].

4. Effects of Rewetting on Soil Properties

4.1. Physical Properties

Rewetting raises and stabilises the water table, reducing the increase of fluctuations caused by precipitation or evapotranspiration [78,109,110,111]. Long-term rewetting of drained agricultural peatlands can modify how groundwater levels respond to precipitation events and enhance the peatland’s capacity to buffer hydrological fluctuations. Such a change is attributed to the restoration of physical properties of the peat body, especially at the upper soil horizon, which increases the water storage capacity of the peat soil allowing it to retain more water and buffer against fluctuations in the groundwater table [78]. In drained peat soils under agricultural management, the surface soil horizons may consist of murshic material (the effect of secondary transformation of peat, due to drainage) [112]. A study by [113] investigating ditch blocking in an abandoned cut-over peatland found that rewetting increased the peatland’s water storage capacity within one year. In systematically drained regions, large-scale rewetting of fen peatlands should be accompanied by an overall rise in groundwater levels in the surrounding landscape; the magnitude of this effect depends on the peatland type [114]. Greater water storage reduces vulnerability during dry periods, which is particularly important for climate adaptation in low-precipitation regions. However, rewetting may not restore natural conditions promptly, especially in long-drained temperate fens. Prolonged drainage and oxidation can alter peat physical properties, leading to higher bulk density, lower porosity, reduced hydraulic conductivity, and decreased storativity. These changes intensify water table fluctuations and limit the peatland’s capacity to buffer hydrological extremes [18]. These tend to reduce shrinkage, and enable organic matter swelling [115,116]. However, the degree of recovery depends on prior land use, drainage intensity, and post-rewetting vegetation succession [17,115,117].

4.2. Chemical Properties

Unlike in natural peatlands, nutrient and C mobilization in rewetted peatlands appears to intensify, even under restored oxygen-free, water-saturated conditions. Increased nutrient availability and low levels of enzyme inhibitors have been identified as drivers of higher mineralisation and turnover during rewetting [89,118,119]. Recent findings suggest that the wide range of phosphorus (P) concentrations in rewetted organic soils is influenced by soil sorption capacity, which depends on the iron (Fe) and aluminum (Al) content, as well as other potential sorbents [120,121,122,123]. Elevated nutrient concentrations influence plant productivity and soil respiration. Phosphorus (P) pollution, in particular, drives species loss more strongly than nitrogen (N) pollution [121]. In rewetted fens, P is often the limiting factor for plant growth, and high P concentrations can hinder recovery of plant biodiversity [122]. Consequently, microbial and plant communities recover toward their pre-drainage state only in ecosystems that were previously weakly disturbed [123]. Rewetted peatlands can act as nutrient sinks, particularly for nitrate (NO₃⁻) [124]. Rising water tables reduce aerobic nitrification and overall mineralisation rates, while enhancing denitrification where NO₃⁻ is present [125]. As a result, NO₃⁻ concentrations typically decline in the medium to long term after rewetting, although brief pulses may occur during the transition phase [126]. In agro-managed peatlands, rewetting often mobilises dissolved nitrogen, notably dissolved organic nitrogen (DON), which can accumulate due to long-term drainage, peat decomposition, and fertiliser inputs [127]. DON may persist even as inorganic N decreases and can contribute to downstream eutrophication after microbial transformation. The magnitude of DON release depends on peat degradation, nutrient legacy, and rewetting strategy, with highly degraded agricultural peats showing the strongest responses [128,129]. Drainage of peatlands generally increases DOC concentrations and fluxes in streams and rivers [130,131]. Rewetting can alter DOC dynamics in complex ways. Some studies report that rewetting following drought can reduce water quality by releasing DOC [132], while others show that highly degraded peatlands experience substantial DOC increases within one year of rewetting [119], or after ten years compared with pristine and drained peatlands [133]. Conversely, other studies have observed lower DOC in rewetted than drained peatlands [134], or lower DOC in degraded compared with intact peatlands. These differences highlight that DOC responses depend on peat degradation status, restoration strategy, and environmental conditions [135,136,137,138,139]. There are a few contradicting studies that claim that peat rewetting increases DOC concentrations in waterbodies and promotes an increase in CH₄ emissions [124]. Short-term negative impacts of rewetting may include enhanced greenhouse gas emissions, particularly methane [16,83,136]. Although peatland chemical properties may gradually recover following rewetting [140,141].

4.3. Biological and Biochemical Properties

4.3.1. Microbial Community Structure Following Drainage and Rewetting

Microorganisms play a central role in organic matter (OM) transformation in peat soils [142], and changes in hydrology strongly influence both microbial activity and community composition. Drainage of fens generally increases microbial activity and alters the active microbial community, reflecting enhanced oxygen availability and substrate turnover [143,144]. In contrast, rewetting has been shown to promote a partial recovery of peatland microbial communities towards near natural conditions. In a geographically extensive study of thirteen European fens, ref. [89] demonstrated that rewetted sites were more similar to near-natural fens than drained sites in both taxonomic and functional diversity. However, responses differ among peatland types. In bogs, several studies using community fingerprinting and 16S rRNA gene sequencing reported little to no effect of drainage on microbial diversity and composition [145,146]. Peat decomposition rates and overall microbial activity have even been shown to decline under drained bog conditions [142,147], contrasting with responses observed in fens and challenging the hypothesis of drought-induced positive feedbacks in peat decomposition [148]. On block-cut bogs, rewetting exerted limited influence on microbial community structure, with peat depth emerging as a stronger determinant than restoration status [145], consistent with other findings [89].

4.3.2. Aerobic Versus Anaerobic Decomposition Processes

Decomposition processes in peatlands are strongly structured by oxygen availability. Aerobic respiration occurs in oxic compartments of the peat profile, including layers above the water table, oxygenated porewater zones, and the rhizosphere [149]. Following rewetting, a short-term priming effect may occur due to the reactivation of microbial communities and enhanced substrate availability, leading to temporarily increased aerobic respiration in surface and root-influenced layers. Saprotrophic fungi, actinobacteria, and methanotrophs dominate aerobic decomposition in peatlands [59,149,150], and their diversity and abundance can serve as indicators of decomposition pathways. Higher richness of saprotrophic fungi has frequently been reported under drier conditions, potentially reflecting increased oxygen availability or the high drought tolerance of fungi [151,152,153,154,155]. Under rewetted conditions, aerobic processes are generally restricted, but they remain important in surface layers and the rhizosphere, where fungal abundance may increase after rewetting, although often not reaching levels observed in pristine systems [156]. With increasing water saturation, anaerobic processes progressively dominate, including fermentation, anaerobic respiration, and methanogenesis, thereby regulating long-term peat carbon turnover and greenhouse gas production. Drainage also affects symbiotic fungi through associated changes in vegetation composition. Defrenne et al. [157] reported a shift from ericoid mycorrhiza to ectomycorrhiza near drainage ditches, linked to the replacement of dwarf shrub-dominated vegetation by tree species, suggesting that changes in mycorrhizal assemblages may signal peat degradation and carbon losses.

4.3.3. Enzymatic Controls on Organic Matter Mineralisation

Microbial carbon mineralisation is mediated by extracellular and intracellular enzymes, with phenol oxidase traditionally considered a key regulator of peat decomposition. According to the enzymic-latch theory, oxygen limitation suppresses phenol oxidase activity, leading to the accumulation of phenolic compounds that inhibit hydrolase enzymes and reduce heterotrophic respiration [158]. Drainage alleviates oxygen limitation, enabling phenol oxidase activity and accelerating soil organic carbon (SOC) decomposition. However, the enzymic-latch theory has been increasingly questioned. Multiple studies report contradictory patterns, suggesting that oxygen availability alone cannot explain enzyme dynamics in peatlands [53,159,160,161,162]. Other interacting factors, including pH, which broadly regulates peatland biogeochemistry, as well as substrate quality, nutrient availability, and microbial community composition, exert strong controls on enzymatic activity [163,164]. Integrating enzymatic assays with molecular approaches, such as microbial community profiling, has therefore been proposed as a more robust framework for assessing peatland decomposition status.

4.3.4. Links Between Microbial Processes and Greenhouse Gas Dynamics

Microbial processes directly regulate greenhouse gas (GHG) emissions from peatlands. Rewetting of drained peat soils is widely recognised as an effective strategy to reduce agricultural CO₂ emissions by suppressing aerobic peat decomposition [165,166]. A recent meta-analysis showed that rewetting reduced CO₂ emissions by an average of 1.43 ± 0.35 Mg CO₂-C ha⁻¹ yr⁻¹, although responses varied with climate zone and nutrient status [17]. Emission factors for rewetted peatlands differ regionally, with higher CO₂ emissions generally associated with nutrient-rich temperate peatlands [15,167,168]. Rewetting also alters nitrogen cycling, largely mediated by microorganisms. Denitrification becomes the dominant N₂O producing pathway under water-saturated, carbon-rich conditions [169,170,171]. Meta-analyses indicate that N₂O emissions from rewetted European peatlands range from −1.08 to 5.27 kg N₂O–N ha⁻¹ yr⁻¹, with overall reductions compared to drained conditions [70,172,173]. Nevertheless, high N₂O emissions may persist in fertilized or highly degraded sites [174,175]. Together, these findings highlight the importance of linking microbial community structure, enzymatic controls, and biogeochemical processes to understand GHG trade-offs following peatland rewetting.

5. Greenhouse Gas Fluxes and Biogeochemical Implications

A primary inquiry about the impact of peatland restoration on the services they offer is how to reconcile greenhouse gas (GHG) emissions with their function in regulating the global temperature. Drainage and restoration influence the production and emission of carbon dioxide (CO₂) and methane (CH₄) from peat soils [176]. The CO₂ to CH₄ exchange ratio indicates the extent to which a wetland contributes to atmospheric radiative forcing [177]. Rewetting drained peatlands is widely recognised as an effective method to reduce CO₂ and N₂O emissions; nevertheless, it typically results in increased CH₄ emissions [178]. IPCC [86] states that a drained peatland can be rewetted when the average annual groundwater level (GWL) is 30 cm or more beneath the peat surface. Elevated groundwater levels expand the anoxic catotelm and reduce the oxic acrotelm. This halts aerobic breakdown and accelerates methanogenesis [179], as oxygen diffusion into the peat matrix is restricted, leading to a shift in dominant microbial pathways from aerobic respiration toward anaerobic fermentation and methanogenic processes. Empirical investigations consistently reveal an inverse relationship between CO₂ emissions and GWL, showing diminished CO₂ emissions from rewetted peatlands compared to drained ones [180,181,182], primarily due to suppressed aerobic decomposition and reduced enzymatic oxidation of organic matter. Conversely, CH₄ emissions generally increase upon rewetting due to enhanced anaerobic processes [183,184], including methanogenesis fuelled by increased substrate availability and limited methane oxidation resulting from reduced oxygen penetration. Numerous synthesis studies performed in tropical, temperate, and boreal peatlands have validated similar trends [185,186,187]. Prolonged site-specific investigations reveal the diversity in greenhouse gas reactions subsequent to rewetting. For example, strategic water retention rapidly transformed a rewetted Irish raised bog from a significant source of CO₂ to a net sink of CO₂ within six years, despite an increase in CH₄ emissions [188,189]. Conversely, a nutrient-dense post-extraction site in the Irish Midlands featured extensive open water regions that remained significant sources of both CO₂ and CH₄, compensating for the almost neutral fluxes from vegetated zones [83], likely reflecting spatial heterogeneity in redox conditions, organic substrate quality, and plant-mediated gas transport pathways. Nevertheless, assessments of global warming potential (GWP) indicated that the cumulative impact on the climate was diminished compared to the period prior to rewetting. Comprehensive research at the Himmelmoor peatland in Germany further demonstrates the influence of site factors and management practices. Three decades after rewetting, several vegetation groups persisted in emitting CO₂ and exhibited increased CH₄ emissions, associated with sporadic floods and fluctuations in the water table [190], which periodically reintroduce oxygen into surface peat layers and stimulate alternating aerobic and anaerobic microbial activity. Subsequent research revealed that peat dams and inundated regions exhibited elevated emissions of CO₂ and in certain instances, N₂O. Nonetheless, CH₄ fluxes were at the lower end of the spectrum observed for rewetted peatlands [191,192], suggesting that peat structure, pore connectivity, and gas diffusion constraints strongly regulate emission dynamics. Recent data indicate that CO₂ emissions are often reduced by approximately 40% relative to drained circumstances, despite rewetted sites remaining net GHG sources [192,193]. A comprehensive study by Minkkinen et al. [173] demonstrated that draining markedly increases N₂O emissions in nutrient-rich peatlands across Europe, an effect that can be largely alleviated through rewetting. N₂O emissions from rewetted sites were often modest and comparable to those from undrained sites, consistent with existing knowledge regarding N₂O production in anaerobic conditions [194], where complete denitrification is favoured, and nitrate availability is limited. Studies on forestry-drained peatlands underscore the efficacy of rewetting for mitigation purposes. Measurements conducted prior to and during rewetting in both fens and bogs revealed substantial reductions in ecosystem respiration, particularly in bare soil plots, alongside improved growing season carbon balances after vegetation regeneration [195]. Similar trends were observed in ombrotrophic peatlands characterised by thin organic layers. Rewetting sites exhibited ecosystem respiration rates comparable to undrained peatlands and significantly lower than those of drained sites [196]. Methane emissions post rewetting vary significantly, mostly influenced by vegetation type and water availability in the region. Restored nutrient-rich fens in Belarus and Poland exhibit significant CH₄ emissions, particularly under wet conditions and with predominant sedge vegetation [197,198], as aerenchymatous plants facilitate direct methane transport from soil to atmosphere. Nutrient-deficient systems and artificial wetlands with open water ponds have comparatively reduced CH₄ emissions [183,199]. Numerous studies indicate that N₂O emissions from rewetted peatlands are generally negligible, supporting the claim that rewetting suppresses N₂O production by restoring anaerobic soil conditions [15,200]. Generally, rewetting elevates CH₄ emissions; however, the significant reductions in CO₂ and N₂O emissions typically surpass this impact. This results in a reduced GWP₁₀₀ and a progressive transition towards carbon neutrality. The magnitude of this advantage is significantly influenced by the type of peatland, its nutritional content, the health of plant growth, and the stability of the water levels.

6. Soil–Plant Interactions

Peatlands must remain saturated and have slow decomposition rates to enhance peat accumulation. Alterations in vegetation due to climate change, fertiliser enrichment, drainage, or modifications in management practices can disrupt these circumstances and initiate peat deterioration. Holden et al. [201] and Klimkowska et al. [90] demonstrated that an increase in shrubs and trees within mires can elevate evapotranspiration, decrease near surface water saturation, and promote localised peat oxidation and subsidence. Woody plants such as Betula pubescens and Salix cinerea alter root depth and water absorption levels upon establishment. This diminishes the water tables and exposes the top peat strata to aerobic decomposition [68,202]. Alterations in vegetation influence the quality of litter and its decomposition process. Woody litter generally decomposes more slowly and promotes aerobic microsites, whereas litter from sedges creates anaerobic conditions conducive to carbon sequestration [203]. Eventually, these vegetation hydrology interactions may transform peat-forming systems into carbon sources [204]. Maintaining plant functional diversity is crucial, particularly for functional groups such as Sphagnum mosses, sedges, and Eriophorum species, which retain water and produce slowly decomposing litter essential for peat formation [205,206,207]. If vegetative succession promotes woody species, rewetting alone will not restore peat-forming processes. Active management of vegetation is often essential to safeguard soil carbon reserves and hydrological stability, promoting peat-forming species while restricting the proliferation of shrubs and trees [208,209]. Plant communities directly regulate oxygen levels, litter chemistry, and microbial processes, rendering them essential for peatland carbon sequestration and GHG exchange [210].

Paludiculture, the productive utilisation of wet or rewetted peatlands, has emerged as a method to reconcile peatland rewetting with ongoing land use [211]. Paludiculture prioritises the conservation of peat bodies by maintaining elevated water tables and anaerobic conditions, in contrast to conventional agriculture on drained peat soils [101,211]. In temperate peatlands, where subterranean biomass frequently influences peat formation, it is feasible to partially harvest aerial vegetation without substantially impacting peat accumulation [99]. Paludiculture systems include the extraction of Phragmites australis, Carex spp., Typha spp., Sphagnum mosses, or wet forestry with Alnus glutinosa, alongside substantial grazing systems [3]. Biomass can be utilised to produce construction materials, biofuel, or horticulture substrates. This aids in achieving climate objectives while reducing land subsidence and greenhouse gas emissions [212,213,214]. Although paludiculture is not primarily focused on habitat restoration, it can significantly enhance biodiversity and water flow. Many peatlands with significant agricultural backgrounds have adjusted to recurrent disturbances, and abandonment following rewetting may lead to the prevalence of a restricted array of competing species, resulting in a decline in biodiversity [183,215]. In these environments, mowing, grazing, or biomass harvesting may be essential to manage eutrophication and maintain species-diverse fen ecosystems [216,217]. Studies consistently demonstrate that paludiculture sites promote enhanced wetland biodiversity relative to drained peatlands, benefiting both specialist wetland species and open landscape taxa [218,219]. Paludiculture can be implemented at a landscape scale in conjunction with untouched areas and buffer zones to support broader objectives for peatland conservation and restoration [220]. Paludiculture stabilises carbon and nitrogen cycles while facilitating land usage by maintaining elevated water levels and promoting functional plant groupings that produce peat. [99,214].

7. Challenges, Knowledge Gaps, and Research Perspectives

Peatland rewetting has become a key way to reduce carbon losses, but there are still big questions about how well it will work in the long run and how well it can be used on a larger scale. Rewetting can lower CO₂ emissions because the water table rises, but the overall effect on the climate can be very different depending on the site because of possible increases in CH₄ emissions, different responses of plants, and different uses of the land before rewetting. Moreover, the lack of standardised monitoring protocols and long-term data series makes it hard to get a good picture of how rewetting works in different climate zones. These gaps in knowledge make it harder to include rewetting in national GHG inventories and make it harder to find reliable ways to reduce emissions. This shows that we need coordinated research, standardised measurement frameworks, and regionally calibrated emission factors right away. In the last twenty years, our understanding of the biogeochemistry of peatlands and the dynamics of greenhouse gases (GHGs) has improved a lot. However, significant uncertainties persist regarding long-term trends, model representation, and management outcomes. Recent syntheses underscore inadequate monitoring coverage, absence of standardised metrics, and suboptimal integration among hydrological, ecological, and biogeochemical processes [204,221]. One ongoing problem is that there are not any continuous, multi-decadal observations of carbon fluxes, water tables, and vegetation dynamics across different types of European peatlands and climate zones. Europe has some of the longest running programs for monitoring flux and hydrology (like those in the UK, Finland, Germany, and the Netherlands), but many datasets are still broken up, short-term, or only available for areas that have been studied a lot in north-western and northern Europe. Peatlands in central, eastern, and southern Europe, especially fens and rewetted agricultural peats, are still not well represented in long-term monitoring networks [204,208,210].

Methodological diversity continues to pose a significant challenge. Different studies have different definitions of peatland, ways to measure the depth of the water table, and signs of restoration success, which makes it harder to put all the information together [207,222]. Meta-analyses and upscaling are still uncertain because there are no common metadata standards or centralised data repositories [221]. To make sure that data from different studies and regions can be combined, we need to create internationally accepted core indicators for the state of peatlands, the success of restoration, and the carbon balance. Most GHG budgets focus on vertical fluxes and do not consider how dissolved and particulate organic carbon moves sideways to streams and rivers. These lateral losses can counterbalance carbon gains resulting from rewetting [208,210]. Models and monitoring frameworks seldom incorporate fire disturbances, erosion, and fluvial processes [221]. Consequently, it is essential to quantify lateral fluxes and incorporate them into peatland carbon accounting to yield comprehensive ecosystem-scale greenhouse gas budgets. Rewetting usually lowers CO₂ emissions because it brings back anoxic conditions, but it can raise CH₄ emissions depending on the type of plants that grow there, the amount of nutrients in the soil, and the water flow. The overall effect on the climate depends on how much CO₂ is lost over time and how much CH₄ is released in short bursts, as well as how the climate changes in the future.

Empirical studies that examine integrated hydrology, vegetation, and fluxes over decadal timescales are still uncommon. Most peatland models depict hydrology, vegetation, or microbial dynamics in isolation, as evidenced by their structural deficiencies and parameter uncertainties. Many models simulate short term water balance or CO₂ fluxes but do not accurately depict long-term peat accumulation, vegetation shifts, and redox-mediated CH₄ dynamics. Long-term, paired monitoring of rewetted and drained sites in different climates is needed to measure how GHG trade-offs and ecosystem resilience change over time. It is also important to create modular, open-source process models that combine hydrological, biogeochemical, and ecological dynamics. These models should be supported by standardised benchmarking and model intercomparison projects.

The makeup of the plants is still a major factor in the carbon balance of peatlands in Europe. Functional groups like Sphagnum mosses, sedges (Carex spp.), and Eriophorum species help peat form by holding a lot of water, making the soil more acidic, and making litter that breaks down slowly [205,207]. Nonetheless, extensive datasets correlating alterations in plant functional composition to ecosystem carbon budgets remain limited [204]. Recent research indicates that vegetative alterations resulting from rewetting or climate warming can significantly influence CH₄ and CO₂ fluxes by modifying litter quality and redox conditions [223,224]. Drainage, nitrogen deposition from the atmosphere, and rising temperatures have sped up the growth of woody plants in many European peatlands, especially in lowland fens and cut-over bogs [36,225]. Shrubs and trees that are closer together increase evapotranspiration, lower the water table in the summer, and speed up aerobic decomposition, which in the long-run lowers the ability of the soil to store carbon. Even with these new findings, there are not many quantitative studies that show how functional diversity affects peatlands’ ability to recover from rewetting and warming. The majority of current research is localised and of limited duration (under 10 years). To understand how plant functional diversity affects peat accumulation and greenhouse gas dynamics as climate change and restoration continue, we need to perform trait-based monitoring and experiments over several decades. Comparative studies throughout Europe should identify thresholds at which woody encroachment converts peatlands from net carbon sinks to sources, and examine management strategies (e.g., controlled shrub removal, Sphagnum reintroduction) to preserve peat-forming conditions. Conduct trait-based, long-term field experiments to evaluate the influence of vegetation functional diversity on carbon sequestration and greenhouse gas balance across different hydrological regimes. In addition to biophysical and hydrological constraints, social acceptance, farmer engagement, and economic feasibility remain critical factors influencing the successful implementation of peatland rewetting strategies across European agricultural landscapes.

8. Conclusions

Drained peatlands are responsible for significant carbon loss, as well as loss of biodiversity and ecosystem services. Understanding the dynamics of GHG fluxes caused by land use change is essential for successful peatland restoration. Uncertainties in long-term estimations of C storage and carbon loss dynamics are remarkably high and do not allow for exact predictions. However, even educated guesses can be valuable for decision-making on further management of peatlands. To make accurate estimations, it is crucial to investigate the full combined impact of hydroclimate change, microbial processes, and vegetation on GHG emissions from restored peatlands. Peatland protection and restoration towards natural functioning is essential for cost-efficient climate change mitigation and for maintaining biodiversity and water-related services. Scientific evidence showing that peatlands restoration re-establishes ecological processes, protects biodiversity and improves carbon storage is accumulating, but further evidence is still needed. Aligning peatland rewetting with emerging European policy frameworks, specifically the EU Nature Restoration Law and CAP eco-schemes, presents a distinctive opportunity to integrate climate mitigation, biodiversity restoration, and rural development. The efficacy of these initiatives will rely on scientifically based management strategies that consider site-specific soil processes, hydrological limitations, and socio-economic conditions.

Both raising the water level in managed peatlands for more sustainable use in forestry and agriculture, and restoration for protection, should be considered across the region. In many cases, return to natural peatland may not be possible due to severe degradation, but partial restoration of peatland ecosystem functions such as reduced carbon emissions and regulation of water flow/sedimentation retention may.

Rewetting of managed peatlands for paludiculture of agricultural crops or native trees has the potential for a more sustainable use of degraded peatland ecosystems as it provides relatively rapid greenhouse gas mitigation while maintaining income for farmers and landowners. Therefore, it may provide an immediately available, socially acceptable pathway to achieve the goal of carbon neutrality on large land areas. Management with the aim to rise the water level in peatland forests and agricultural peatlands that decreases but does not halt peat loss, i.e., by reducing drainage intensity in situations where full rewetting is not possible, provides some climate benefit. Engagement of, and support to, local communities in making use of new policies and initiatives for sustainable peatland use are relevant for enabling transition to a climate-neutral and resilient society. In countries with small-scale landownership settings, e.g., Ireland, Poland and Ukraine, dedicated local peatland management is particularly vital. Rewetting of drained peatlands is very positive for the climate. Restoration for nature conservation should restrict the intensity and frequency of the interventions. The increasing demand for biomass implies that drainage-based land use may have to be replaced by paludiculture. Water levels that are too low are the central cause of peatland degradation. The presumption that without action peat growth will eventually recover spontaneously is questionable. In most cases, active intervention is required to raise the water table again to around or over the peat surface.

Scaling up restoration requires integrating hydrological, ecological, and socio-economic knowledge to achieve measurable climate mitigation, carbon storage, and ecosystem service recovery. The scientific consensus indicates that without active interventions to restore water levels and peat-forming vegetation, spontaneous recovery is often too slow to prevent continued carbon loss, particularly in agriculturally degraded peatlands. Therefore, restoration strategies must be science-informed, site-specific, and adaptive, combining both conservation and sustainable-use approaches to maximize ecological and climate benefits.