Water stable isotopes as a tool in peatland restoration

Abstract

Peatland ecosystems carry a significant amount of biodiversity and act as carbon sinks. In Finland, peatlands have been drained immensely, especially for forestry, and peaked in the 1960s to 1970s. Finland used to have 10.4 million hectares of mires and an estimated 5.9 million hectares had been drained. Due to the huge drainage, these ecosystems are facing a threat. The biodiversity residing on the brink of extinction and increasing emission of CO2 from degraded peatlands pose a risk of global warming. Restoration is a necessity to return these peatlands to natural forms.

This diploma thesis focuses on the isotope hydrological aspects of peatland restoration, to observe which water sources contribute to the restoration process. Simultaneously, peatlands release dissolved organic carbon (DOC), degrading water quality. To understand the hydrophilic and hydrophobic characteristics of peat pore water, Specific Ultraviolet Absorbance (SUVA) was determined. This study was concluded as a part of “Metsät turvemailla – ratkaisuja päästöjen hillintään ja hiilinielujen kasvattamiseen (TURNEE” -project funded by the Ministry of Agriculture and Forestry of Finland. The study includes 27 restored forested peatland sites and 6 pristine sites. Water samples were collected in the summer and autumn of 2021 and 2022. In all sites, water was collected at two different depths: 50 cm, and 100 cm. The water samples were analyzed with Picarro L2120-I for stable water isotopes δ18O, and UV absorbance was measured.

The median δ18O value for pristine sites was depleted δ18O (-11.79‰) compared to the restored sites δ18O (-11.29‰). The median DOC was comparatively higher in restored sites (59.0 mg/l) than in pristine sites (35.0 mg/l). The median SUVA Index for pristine (4.47 L/m-mgC) and restored sites (4.71 L/m-mgC) showed hydrophilic and hydrophobic characteristics of mixtures. In all sites at 50 cm depth, there was a high DOC content and enriched δ18O signal. In terms of vegetation, threatened vegetation type herb-grass spruce mire (Rhk) from the pristine site had the most enriched median value in δ18O (-11.00‰) and the lowest value of DOC (median 18.5 mg/l). This vegetation type sets a standard and gives important insights into vegetation and water dependency. Sites such as Soljanen 2 (R2 = 0.87) and Haukilammi (R2 = 0.72) showed a significant correlation between δ18O and DOC. Sites such as Kortesalo 1 showed a linear correlation (R2 = 0.60) between δ18O and SUVA Index. For other cases, the correlation varied in different depths for the handful of sites. Water table level data showed great dependency on δ18O for the site Hyytiälä R2 = 0.60. Where great linearity was found in DOC and WTL the R2 = 0.91 for Mustakorpi 1 restored site. Sites restored in the 1990s showed stable and low DOC levels, indicating the potential dependencies of the restoration age, water source, and DOC generation.

Restoration success was visible for the sites for the vegetation type Puolukkaturvekangas (Ptkg2) which had a significantly lower DOC level, increased WTL, and highly enriched δ18O compared to the pristine site vegetation spruce-pine swamp (KR). The peatland's water composition was influenced by precipitation in 2021 and 2022, leaning toward the existing GW δ18O signal. It indicates some sites are not fully restored yet; they are making progress toward restoring the high ground water table. In conclusion, a prediction model can be made that portrays a strong correlation between δ18O and other parameters for all restored peatland sites.