Annual greenhouse gas fluxes from drained transitional bog and raised bog forest soils with different tree species composition

Peat bogs are terrestrial wetland ecosystems where waterlogging prevents the complete decomposition of plant material. Therefore, organic matter production exceeds its decomposition, resulting in net peat accumulation. However, anthropogenic pressures, such as drainage for forestry, significantly affects those systems' biogeochemistry. Drainage lowers the originally high water table, increasing the oxic peat layer depth, which changes the dynamics of peat soil greenhouse gas (GHG) fluxes. Moreover, change dynamics can differ in peatland types. While GHG fluxes from drained minerotrophic and ombrotrophic peatlands are relatively well studied, drained transitional peatlands require additional accurate data for different spatio-temporal conditions.
This study aims to estimate the magnitude and temporal variability of soil GHG fluxes in three drained transitional bog forests in southeastern Estonia with different tree compositions, dominated respectively by Downy Birch (Betula pubescens), Norway Spruce (Picea abies) and Scots Pine (Pinus sylvestris), in addition to one drained raised bog forest dominated by Scots Pine. Ongoing sampling campaigns run twice a month from April 2022 to March 2023. Soil CO₂ fluxes (heterotrophic soil respiration; n=6) are measured using a dark dynamic chamber connected to EGM-5 Portable CO₂ Gas Analyzer. To estimate soil CO₂ (forest floor respiration), N₂O and CH₄ fluxes, gas concentration samples are collected at 20-minute intervals during an hour-long session using manual static chambers (n=6) and are analyzed with Shimadzu GC-2014 gas chromatography. Soil environmental parameters (water table depth, soil temperature and moisture) are measured simultaneously with GHG measurements at each site.
Preliminary results (April 2022 – December 2022) show that sites with greater depth of oxic peat layer were, on average, stronger emitters of CO₂ (forest floor respiration) and net CH₄ sinks. The birch site had the highest average CO₂ flux (103.6 ± 9.96 mg C m⁻² h⁻¹, mean ± SE), while the drained raised bog pine forest site had the lowest (59.9 ± 4.82 mg C m⁻² h⁻¹). The transitional bog sites were net CH₄ sinks, with the birch site being the largest (−85.85 ± 7.41 μg C m⁻² h^–1), in contrast to the drained raised bog pine forest being a net source (33.92 ± 20.38 μg C m⁻² h⁻¹). The nitrogen-rich spruce site had the largest N₂O emissions (27.64 ± 9.88 μg N m⁻² h⁻¹), with the highest fluxes in April and May (with a maximum of 309.84 μg N m⁻² h⁻¹). Further analysis of soil GHG fluxes and linkage to soil chemical, physical and environmental parameters will help determine and explain the magnitude and temporal variability of drained transitional bog forest's GHG fluxes and, consequently, highlight the importance of disturbance of these sensitive ecosystems.