Restoration of nutrient-rich forestry-drained peatlands poses a risk for high exports of dissolved organic carbon, nitrogen, and phosphorus

Active afforestation of drained peatlands is not a viable option under the EU Nature Restoration Law

The EU Nature Restoration Law (NRL) is critical for the restoration of degraded ecosystems and active afforestation of degraded peatlands has been suggested as a restoration measure under the NRL. Here, we discuss the current state of scientific evidence on the climate mitigation effects of peatlands under forestry. Afforestation of drained peatlands without restoring their hydrology does not fully restore ecosystem functions. Evidence on long-term climate benefits is lacking and it is unclear whether CO2 sequestration of forest on drained peatland can offset the carbon loss from the peat over the long-term. While afforestation may offer short-term gains in certain cases, it compromises the sustainability of peatland carbon storage. Thus, active afforestation of drained peatlands is not a viable option for climate mitigation under the EU Nature Restoration Law and might even impede future rewetting/restoration efforts. Instead, restoring hydrological conditions through rewetting is crucial for effective peatland restoration.

Consequences of rewetting and ditch cleaning on hydrology, water quality and greenhouse gas balance in a drained northern landscape

Drainage for forestry has created ~ 1 million km of artificial waterways in Sweden, making it one of the largest human-induced environmental disturbances in the country. These extensive modifications of both peatland and mineral soil dominated landscapes still carry largely unknown, but potentially enormous environmental legacy effects. However, the consequences of contemporary ditch management strategies, such as hydrological restoration via ditch blocking or enhancing forest drainage to promote biomass production via ditch cleaning, on water resources and greenhouse gas (GHG) fluxes are unclear. To close the gap between science and management, we have developed a unique field research platform to experimentally evaluate key environmental strategies for drained northern landscapes with the aim to avoid further environmental degeneration. The Trollberget Experimental Area (TEA) includes replicated and controlled treatments applied at the catchment scale based on a BACI approach (before-after and control-impact). The treatments represent the dominant ecosystem types impacted by ditching in Sweden and the boreal zone: (1) rewetting of a drained peatland, (2) ditch cleaning in productive upland forests and (3) leaving these ditches unmanaged. Here we describe the TEA platform, report initial results, suggest ways forward for how to best manage this historical large-scale alteration of the boreal landscape, as well as warn against applying these treatments broadly before more long-term results are reported.

Post-drainage stand growth and peat mineralization impair water quality from forested peatlands

Many recent studies have indicated upward trends in carbon and nutrient concentrations from drained peatland forests over time since their initial drainage, but the mechanisms behind these trends are still poorly understood. We gathered data on nitrogen and phosphorus concentrations discharged from 37 drained boreal peatland forests where we also had data on peat and tree stand characteristics. We found that tree stand volume and peat bulk density were positively correlated with the nitrogen and phosphorus concentrations discharged from particularly the deep-peated sites. We interpret these results to indicate that a plausible reason for the reported upward trends in nutrient concentrations is the maturing and growing of the tree stands over time since initial drainage and the consequent increasing evapotranspiration capacity, which results in lowered soil water levels and enhanced aerobic peat mineralization. We discuss how our results should be considered in the management of drained peatland forests.

Assessing leached TOC, nutrients and phenols from peatland soils after lab-simulated wildfires: Implications to source water protection

Pollutant leaching from wildfire-impacted peatland soils (peat) is well-known, but often underestimated when considering boreal ecosystem source water protection and when treating source waters to provide clean drinking water. Burning peat impacts its physical properties and chemical composition, yet the consequences of these transformations to source water quality through pollutant leaching has not been studied in detail. We combusted near-surface boreal peat under simulated peat smoldering conditions at two temperatures (250 °C and 300 °C) and quantified the concentrations of the leached carbon, nutrients and phenols from 5 g peat L^-1 reverse osmosis (RO) water suspensions over a 2-day leaching period. For the conditions studied, measured water quality parameters exceeded US surface water guidelines and even exceeded EU and Canadian wastewater/sewer discharge limits including chemical oxygen demand (COD) (125 mg/L), total nitrogen (TN) (15 mg/L), and total phosphorus (TP) (2 mg/L). Phenols were close to or higher than the suggested water supply standard established by US EPA (1 mg/L). Leached carbon, nitrogen and phosphorus mainly came from the organic fraction of peats. Heating peats to 250 °C promoted the leaching of carbon-related pollutants, whereas heating to 300 °C enhanced the leaching of nutrients. Post-heated peats leached higher loads of pollutants in water than pre-heated peats, suggesting that fire-damaged boreal peats may be a critical but underappreciated source of water pollution. A simplified Partial Least Squares (PLS) model based on other easily measured parameters provided a simple method for determining the extent of COD and phenolic pollution in bulk water, relevant for water and wastewater treatment plants. Conclusions from this lab study indicate the need for field measurements of aquatic pollutants downstream of peatland watersheds post-fire as well as increased monitoring and treatment of potable water sources for leachable micropollutants in fire-dominated forested peatlands.

Iron in boreal river catchments: Biogeochemical, ecological and management implications

Iron (Fe) is an important element in aquatic ecosystems worldwide because it is intimately tied with multiple abiotic and biotic phenomena. Here, we give a survey of manifold influences of Fe, and the key factors affecting it in the boreal catchments and their waters. It includes the perspectives of biogeochemistry, hydrology, ecology, and river basin management. We emphasize views on the dynamics and impacts of different forms of Fe in riverine environments, including organic colloids and particles, as well as inorganic fractions. We also provide perspectives for land use management in boreal catchments and suggest guidelines for decision making and water management. Based on our survey, the main emphases of water protection and management programs should be (i) prevention of Fe mobilization from soil layers by avoiding unnecessary land-use activities and minimizing soil disturbance in high-risk areas; (ii) disconnecting Fe-rich ground water discharge from directly reaching watercourses; and (iii) decreasing transport of Fe to watercourses by applying efficient water pollution control approaches. These approaches may require specific methods that should be given attention depending on catchment conditions in different areas. Finally, we highlight issues requiring additional research on boreal catchments. A key issue is to increase our understanding of the role of Fe in the utilization of DOM in riverine food webs, which are typically highly heterotrophic. More knowledge is needed on the metabolic and behavioral resistance mechanisms that aquatic organisms, such as algae, invertebrates, and fish, have developed to counter the harmful impacts of Fe in rivers with naturally high Fe and DOM concentrations. It is also emphasized that to fulfil the needs presented above, as well as to develop effective methods for decreasing the harmful impacts of Fe in water management, the biogeochemical processes contributing to Fe transport from catchments via rivers to estuaries should be better understood.

NutSpaFHy—A Distributed Nutrient Balance Model to Predict Nutrient Export from Managed Boreal Headwater Catchments

Responsible forest management requires accounting for adverse environmental effects, such as increased nutrient export to water courses. We constructed a spatially-distributed nutrient balance model NutSpaFHy that extends the hydrological model SpaFHy by introducing a grid-based nutrient balance sub-model and a conceptual solute transport routine to approximate total nitrogen (N) and phosphorus (P) export to streams. NutSpaFHy uses openly-available Multi-Source National Forest Inventory data, soil maps, topographic databases, location of water bodies, and meteorological variables as input, and computes nutrient processes in monthly time-steps. NutSpaFHy contains two calibrated parameters both for N and P, which were optimized against measured N and P concentrations in runoff from twelve forested catchments distributed across Finland. NutSpaFHy was independently tested against six catchments. The model produced realistic nutrient exports. For one catchment, we simulated 25 scenarios, where clear-cuts were located differently with respect to distance to water body, location on mineral or peat soil, and on sites with different fertility. Results indicate that NutSpaFHy can be used to identify current and future nutrient export hot spots, allowing comparison of logging scenarios with variable harvesting area, location and harvest techniques, and to identify acceptable scenarios that preserve the wood supply whilst maintaining acceptable level of nutrient export.

A comparison of porewater chemistry between intact, afforested and restored raised and blanket bogs

Afforestation is a significant cause of global peatland degradation. In some regions, afforested bogs are now undergoing clear-felling and restoration, often known as forest-to-bog restoration. We studied differences in water-table depth (WTD) and porewater chemistry between intact, afforested, and restored bogs at a raised bog and blanket bog location. Solute concentrations and principal component analysis suggested that water-table drawdown and higher electrical conductivity (EC) and ammonium (NH₄-N) concentrations were associated with afforestation. In contrast, higher dissolved organic carbon (DOC) and phosphate (PO₄-P) concentrations were associated with deforestation. Drying-rewetting cycles influenced seasonal variability in solute concentrations, particularly in shallower porewater at the raised bog location. WTD was significantly deeper in the oldest raised bog restoration site (~9 years post-restoration) than the intact bog (mean difference = 6.2 cm). However, WTD in the oldest blanket bog restoration site (~17 years post-restoration), where furrows had been blocked, was comparable to the intact bog (mean difference = 1.2 cm). When averaged for all porewater depths, NH₄-N concentrations were significantly higher in the afforested than the intact sites (mean difference = 0.77 mg L^-1) whereas significant differences between the oldest restoration sites and the intact sites included higher PO₄-P (mean difference = 70 μg L^-1) in the raised bog and higher DOC (mean difference = 5.6 mg L^-1), EC (mean difference = 19 μS cm^-1) and lower SUVA₂₅₄ (mean difference = 0.13 L mg^-1 m^-1) in the blanket bog. Results indicate felled waste (brash) may be a significant source of soluble C and PO₄-P. Mean porewater PO₄-P concentrations were between two and five times higher in furrows and drains in which brash had accumulated compared to other locations in the same sites where brash had not accumulated. Creating and maintaining brash-free buffer zones may therefore minimise freshwater impacts.

Restoration of afforested peatland: Immediate effects on aquatic carbon loss

Peatland restoration is undertaken to bring back key peatland ecosystem services, including carbon storage. In the case of drained, afforested blanket peatlands, restoration through drain blocking and tree removal may impact upon aquatic carbon concentrations and export, which needs to be accounted for when considering the carbon benefits of restoration. This study investigated concentrations and export of aquatic carbon from a drained, afforested blanket bog catchment, where 12% of the catchment underwent drain blocking and conifer removal (termed 'forest-to-bog' restoration), and from two control catchments: one in open bog and one that remained afforested. Using a before-after-control-impact (BACI) design, we found no significant increases in concentrations or export of aquatic carbon (DOC, POC or DIC) in the first year following forest-to-bog restoration (i.e. across the whole post-restoration period). However, increased DOC concentrations were observed in the first summer (2015) post-restoration, and seasonally increased DOC export was noted during storm events in the autumn of the same year. The lack of significant effects of forest-to-bog restoration on aquatic carbon export may be a consequence of the small proportion of the catchment (12%) undergoing management. In terms of management, the removal of more of the forestry residues (i.e., brash) may help to mitigate effects on aquatic carbon, by removing a potential DOC and POC source. Restoring small areas at a time (≤12%) should result in minimal aquatic carbon export issues, in contexts similar to the current study.

Browning of freshwaters: Consequences to ecosystem services, underlying drivers, and potential mitigation measures

Browning of surface waters, as a result of increasing dissolved organic carbon and iron concentrations, is a widespread phenomenon with implications to the structure and function of aquatic ecosystems. In this article, we provide an overview of the consequences of browning in relation to ecosystem services, outline what the underlying drivers and mechanisms of browning are, and specifically focus on exploring potential mitigation measures to locally counteract browning. These topical concepts are discussed with a focus on Scandinavia, but are of relevance also to other regions. Browning is of environmental concern as it leads to, e.g., increasing costs and risks for drinking water production, and reduced fish production in lakes by limiting light penetration. While climate change, recovery from acidification, and land-use change are all likely factors contributing to the observed browning, managing the land use in the hydrologically connected parts of the landscape may be the most feasible way to counteract browning of natural waters.

The effects of forest clearance for peatland restoration on water quality

Recognition of the importance of peatlands has led to increased efforts to protect and restore these environments but there are concerns about the impact of restoration on water quality, particularly in terms of sediment delivery, nutrient transport and carbon losses. In this study we present the results of almost 10 years of water quality monitoring in 3 catchments at a lowland raised bog that was afforested with conifers in the 1960s and 1970s and cleared over the 2010s. Phosphate concentrations increased after clearfelling with the main peaks seen in the summer after forest clearance; the use of low ground impact harvesting methods, removal of forest residues and especially phased felling tempered phosphate losses. Annual mean phosphate concentrations returned to pre-felling levels 3-5 years after felling finished. Nitrate concentrations increased slightly from baseline levels during and after felling. DOC concentrations increased at two sites with mean annual concentrations still elevated at one site after 4 years; little increase in DOC was recorded with phased felling. Colour levels increased and remain elevated at all sites relative to pre-felling. In only one stream, pH increased improving the WFD water body status from 'poor' to 'high' in the space of 3 years. Our results show that forest clearance for peatland restoration can impact negatively upon water quality due to phosphate, DOC, colour and suspended sediment releases. The mechanisms by which the releases occur require further investigation but are thought to be driven by nutrient leaching from forest residues, soil disturbance by machine trafficking and indirect effects of forest clearance, such as water table rise following cessation of forest water use. Climatic effects also play a significant role, particularly drying-rewetting cycles and periods of extended drought. To prevent negative impacts, phased felling and less intensive forest harvesting techniques should be employed.

Ditch network maintenance in peat-dominated boreal forests: Review and analysis of water quality management options

The objective of this study was to evaluate the potential of different water management options to mitigate sediment and nutrient exports from ditch network maintenance (DNM) areas in boreal peatland forests. Available literature was reviewed, past data reanalyzed, effects of drainage intensity modeled, and major research gaps identified. The results indicate that excess downstream loads may be difficult to prevent. Water protection structures constructed to capture eroded matter are either inefficient (sedimentation ponds) or difficult to apply (wetland buffers). It may be more efficient to decrease erosion, either by limiting peak water velocity (dam structures) or by adjusting ditch depth and spacing to enable satisfactory drainage without exposing the mineral soil below peat. Future research should be directed towards the effects of ditch breaks and adjusted ditch depth and spacing in managing water quality in DNM areas.

Measuring restoration progress using pore- and surface-water chemistry across a chronosequence of formerly afforested blanket bogs

During the restoration of degraded bogs and other peatlands, both habitat and functional recovery can be closely linked with nutrient cycling, which is reflected in pore- and surface-water chemistry. Several peatland restoration studies have shown that the time required for recovery of target conditions is slow (>10 years); for heavily-impacted, drained and afforested peatlands of northern Scotland, recovery time is unknown. We monitored pore- and surface-water chemistry across a chronosequence of formerly drained, afforested bog restoration sites spanning 0-17 years, using a space-for-time substitution, and compared them with open blanket bog control sites. Our aims were to measure rate of recovery towards bog conditions and to identify the best suite of water chemistry variables to indicate recovery. Our results show progress in recovery towards bog conditions over a 0-17 year period post-restoration. Elements scavenged by trees (Mg, Na, S) completely recovered within that period. Many water chemistry variables were affected by the restoration process itself, but recovered within 11 years, except ammonium (NH₄⁺), Zn and dissolved organic carbon (DOC) which remained elevated (when compared to control bogs) 17 years post restoration. Other variables did not completely recover (water table depth (WTD), pH), exhibiting what we term "legacy" effects of drainage and afforestation. Excess N and a lowered WTD are likely to slow the recovery of bog vegetation including key bog plants such as Sphagnum mosses. Over 17 years, we measured near-complete recovery in the chemistry of surface-water and deep pore-water but limited progress in shallow pore-water. Our results suggest that at least >17 years are required for complete recovery of water chemistry to bog conditions. However, we expect that newer restoration methods including conifer harvesting (stem plus brash) and the blocking of plough furrows (to increase the WTD) are likely to accelerate the restoration process (albeit at greater cost); this should be evaluated in future studies. We conclude that monitoring pore- and surface-water chemistry is useful in terms of indicating recovery towards bog conditions and we recommend monitoring WTD, pH, conductivity, Ca, NH₄⁺, phosphate (PO₄^3-), K, DOC, Al and Zn as key variables.

Nitrogen and phosphorus concentrations in discharge from drained peatland forests are increasing

The current understanding, based on previous studies, is that increased discharge nutrient concentrations from boreal peatlands drained for forestry return to similar levels as those of pristine peatlands within about 20years after their drainage. As an implicit consequence of this finding, it has been assumed that there are no long-term increasing trends in nutrient exports from these peatlands after the establishment of forestry. We analysed discharge total nitrogen (TN) and phosphorus (TP) concentration data from 54 catchments with undrained pristine peatlands and 34 catchments with drained peatlands using data with considerably longer drainage history than in previous studies. Our results agree with previous studies in that discharge TN and TP concentrations in areas drained 20-30years ago did not differ much from those in pristine sites. However, we also observed that the TN and TP concentrations were increasing with years since drainage of these catchments. Discharge TN and TP concentrations were over two times higher in areas drained 60years ago when compared with more recently drained areas. Our results challenge the current perceptions by showing that forestry-drained peatlands may contribute to water eutrophication considerably more than previously estimated.