ReWet - Roadmap for research infrastructure 2020
Store Vildmose is located in Vendsyssel in the north and used to be the largest raised bog in Denmark, with an original area of about 65 km2. The bog is developed on top of the sandy soils on Littorina marine deposits. The oldest part of the bog is approximately 2000 years old, and the peat depth measured between 5-7 m in 1880 in the central part of the bog. The bog drains to the river Rye Å and Lindholm Å. The intensive agricultural use of the bog started with the Danish state acquisition of the area in the 1920s and the construction of 57 farms in the bog. Due to drainage and cultivation the bog has now lost the original character, and only 5 km2 of the original bog surface remains. The bog is intensively drained with ditches and tile drains. The peat depth in the agricultural area is in many places now reduced to less than 1 m. The main land use of the area is perennial grasslands, potato growing and peat extraction.
In Store Vildmose we have two different research sites: one on permanent grassland and one on arable soil. The grassland site has peat with a low bulk density (0.2 g cm-3) and a high soil organic carbon content of 40- 50% C in the upper 1 m. The mean annual groundwater level is ca. 60 cm below soil surface and soil pH is 4.6- 5.3. The arable site is used for potato cropping in rotation with annual grass or barley. The topsoil has a higher bulk density (ca. 0.4 g cm-3) and a carbon content of 30-45%. The peat depth is about 80 cm and the annual groundwater level is 70 cm below soil surface. Soil pH is 4.2-4.9. Both sites are tile-drained. At store Vildmose we will get a site representing highly degraded classical oligotroph raised bog.
Vosborg Enge is a meadow belonging to the historic manor house Nørre Vosborg in western Jutland adjacent to the river Store Å 2 km upstream the river outlet to Felsted Kog. The Store Å river valley is dominated by organic deposits and is located on the border between sandy Saalian deposits to the south (Skovbjerg Bakkeø) and sandy Wechselian meltwater deposits to the north. The Quarternary deposits in the area are underlain by Tertiary clay and sand layers.
Rising groundwater due to rapid sea level rises occurring between ~8000 and ~5000 years ago led to formation of a lake in the western and northern part of the project area and alder swamp in the southeastern part leaving up to 5 m thick lake deposits and 3 m thick peat deposits. The oldest peat deposits in the area are ~6300 years old. When the water level once again declined, the Store Å meandered through Vosborg Enge leaving remnant meander scars in the area, clearly visibly from aerial photographs. The highly fertile soils formed by lake deposits and floodings by the river were the foundation of the monastic gardens whose high hay yields were of great importance for the economy of the manor. The meadow was extensively drained by ditches around year 1800, and in 1945 the meadow was converted to agricultural land by the construction of a dike between the river and the meadow followed by improved drainage of the area and pumping water out of the meadow. Mechanical compression and mineralization of organic sediments have since resulted in subsidence of up to 2 m in in the project area.
The meadows represent a complex geological setting of organic deposits with interbedded sand structures from the meandering river. Yet, the rewetting of the area planned for 2022 does not include a complete removal of the dike allowing for a very controlled monitoring of the water and nutrient balances at the outlet of the complex meadow.
The Nørreå valley is located in the central part of Jutland. The valley is approximately 35 km long and drops only 6 m in elevation. The originally uneven tunnel-valley bottom has in postglacial times been flooded in a transgression and thus covered by the sea. The valley bottom has been levelled out by marine sediments and later, after the marine regression, also by fluvial and organic sediments. Two distinct study sites are available in the Nørreå valley. One site (called Ø) is a 12-ha agricultural field with a high soil organic carbon (SOC) gradient (1.44–42.9%). The western part of the field borders a young moraine landscape with mineral soils and the eastern part has organic soils. The field (averaging 19.5% SOC) includes areas with mainly peat and smaller areas with sandy mineral soils. The field used to be drained by tile drains but is now in the process of rewetting since 2018 as part of the Heltzen pumpelag (drainage association) naturstyrelsen.dk/naturbeskyttelse/naturprojekter/lavbundsprojekt-ved-heltzenpumpelag- og-enge/.
At present, the peat depth varies between 0 and 4 m. Previous GHG measurements with manual chambers have been carried out in parts of the field with >1 m peat depth.
The second site is the Vejrumbro study site, which comprises two ditch-drained fields of 3 and 5 ha cultivated with perennial grasses. A peat layer of about 2 m thickness is found on top of an 8-m gytja layer, except in the northwestern corner of the area. The peat or gyttja are underlain by a sand layer. In 2018, the site was monitored by tTEM and dualEM, and more than 1000 soil samples were taken throughout the field in depth sections of 20 cm and analyzed for chemical composition. More than two hundred piezometers are installed on this site. Despite the intensive drainage the mean groundwater table of the field was (in 2019) about 40 cm below terrain in late spring, tending to flooded conditions in late summer and early autumn. Electromagnetic flow meters and ISCO samplers have been installed at the inlets and outlets of the drainage ditches for quantification of nutrient balances. GHG measurements will be carried out in 2020 with manual chambers under the current shallow drained conditions as part of an ongoing project. The Nørreå Vally sites represents typical large river valleys af Jutland and one of the sites is already heavily instrumented.
Gribskov on Zealand, 40 km north of Copenhagen, represents a young kettled landscape where peatlands are located in numerous natural depressions in the landscape. They are mainly groundwater connected and receive large additional inputs of carbon and nutrients from tree litter compared to peatlands in the open land. In combination, the groundwater contribution and input of nutrient-rich litter types from non-wetland plants make these wetlands relatively nutrient-rich. The wetlands have been intensively drained to increase timber production and today face the same degradation after long-term drainage, subsidence and paludification as the larger wetlands in Danish agricultural landscape.
Several large-scale hydrological restoration projects in Danish state-owned forests have begun. Gribskov has since 1992 undergone successive stages of regeneration of the natural hydrology in previously drained wetlands. It has been estimated that 20% of Gribskov was wetland in 1857, reaching a minimum of 3% in 1988. In 2017 the wetland extent was around 7.5% and the goal is to reach 12% in 2029. Thus, Gribskov represents a current long-term (almost 30 years) wetland development gradient after rewetting. The plans for further rewetting in Gribskov in the coming decade represent an opportunity to study the immediate shortterm effects of rewetting by continuous monitoring at sites before and after rewetting. Gribskov thus supports the purpose of ReWet to cover the diversity of wetland types in Denmark. Using Gribskov we can gather data that are valuable for the future GHG mitigation efforts in existing forests, and we can use the knowledge generated as benchmarks for policy decision and development scenarios for future afforestation projects on peat soils in the agricultural landscape.