Methane (CH4) and nitrous oxide (N2O) emissions from surface of deciduous tree stems and soil in forests with drained and naturally wet organic soils

In general, the groundwater level (GWL) in forests with organic soils plays a crucial role in greenhouse gas (GHG) emissions from both the soil and the surface of tree stems. High GWL typically leads to anaerobic conditions in the soil, reducing the emission of GHG like nitrous oxide (N2O) and increasing methane (CH4) emissions. Understanding and managing GWL in such ecosystems is therefore essential for mitigating GHG emissions. The scope of the study was to evaluate magnitude of fluxes of GHG (specifically CH4 and N2O) from the surface of deciduous tree stems and soil, as well as to find interrelationships with environmental factors like GWL and the soil temperature. CH4 and N2O fluxes from tree (black alder, silver birch and aspen) stems and soil were monitored in 10 study sites in forest stands with drained and naturally wet organic soils in Latvia in 2022 and 2023. We found that the surface of tree stems was a net source of CH4 and N2O emissions in forests with both drained and naturally wet organic soil. MeaN·magnitude of CH4 fluxes from tree stems were 0.017 ± 0.003 mg CH4-C·mE−2·hE−1 in forest stands with drained organic soil and 0.032 ± 0.010 mg CH4-C·mE−2·hE−1 in forest stands with naturally wet organic soil, while meaN·magnitude of N2O emissions were 0.002 ± 0.001 and 0.007 ± 0.002 mg N2O-N·mE−2·hE−1, respectively. Thus, emissions from the surface of tree stems under naturally wet soil conditions were higher than under drained conditions; a statistically significant difference was found only for N2O. Drained organic soils in the study sites were a net sink of CH4 (small removals were observed), while naturally wet soils created emissions (mean 1.15 ± 0.89 mg CH4-C·mE−2·hE−1). The magnitude of N2O emissions from soil was similar for naturally wet and drained conditions (0.034 ± 0.017 and 0.028 ± 0.014 mg N2O-N·mE−2·hE−1, respectively). In general, the study showed that drainage of organic soils in forest lands can reduce CH4 and N2O emissions; however, due to high variation in studied GHG fluxes, the number of measurement points should be increased to prove the significance of the difference.