Synthesis of greenhouse gas emission factors for forest organic soils in the Dfb zone of the Köppen–Geiger climate classification

Organic soils, especially peat soils, store large amounts of carbon and are therefore considered a significant source of greenhouse gas (GHG) emissions, disproportionately contributing to land-use emission estimates in countries with extensive organic soil areas. When synthesising emission factors (EFs), data are typically stratified by broad climate zones such as temperate and boreal. However, given the spatial distribution of available forest-soil GHG data, such stratification is suboptimal. Temperate forest soils remain less studied than boreal soils, although recent research has expanded the number of temperate estimates from 8 used in the IPCC default EF compilation to at least 91, with the most recent flux data originating from the Baltic states (56 sites). When deriving EFs for specific applications, it is preferable to pool data from regions with similar climatic conditions rather than restricting analyses to unnecessarily small datasets defined by national borders or aggregating data across overly broad and climatically diverse zones.

We reassessed forest-soil GHG emissions in the Baltic states by supplementing regional data with additional sites sharing the same Dfb climate zone under the Köppen–Geiger climate classification. This approach expanded the dataset from 56 to 98 sites, improving EF accuracy and enhancing comparability between neighbouring GHG emission estimates, regardless of whether countries rely solely on domestic data. While such analyses typically focus on drained organic soils, we also included undrained soils to support the establishment of emission baselines for assessing forest management impacts.

Average annual organic-soil emissions in the Dfb climate zone  (mean ± SE, per hectare of forest) were estimated at 0.22 ± 0.18 t CO₂-C, 1.17 ± 1.58 kg CH₄, and 2.82 ± 0.59 kg N₂O for drained soils, and −0.60 ± 0.37 t CO₂-C, 92.88 ± 78.05 kg CH₄, and 2.81 ± 1.07 kg N₂O for undrained soils. Expressed as CO₂ equivalents using AR5 GWPs, total emissions were 1.59 ± 0.46 t CO₂ eq. for drained and 1.15 ± 6.70 t CO₂ eq. for undrained soils. Owing to high natural variability between site-level fluxes, the effect of drainage on GHG emissions remains uncertain. Although the mean difference between drained and undrained soils (0.44 t CO₂ eq.) may indicate a long-term drainage effect, this estimate is highly doubtful. The dataset indicated that simple averaging across all sites is not well suited to deriving EFs, as CO₂ emissions from drained organic soils showed dependence on nutrient status and linkage to dominant tree species and stand age. Drained soils in young stands tended to act as emission sources, whereas older stands increasingly functioned as carbon sinks, with a transition at approximately 25 years of stand age. However, additional observations are required to accurately quantify this dynamic across the forest growth cycle. To illustrate the implications for national upscaling, we derived a Latvia-specific drained organic-soil CO₂ EF that accounts for the distribution of dominant tree species and stand types characterising soil nutrient availability, yielding a weighted EF of 0.14 t CO₂-C ha⁻¹ yr⁻¹.

This work was supported by PeatTransform with co-funding from the European Union and the State Budget of Latvia (6.1.1.2/1/25/A/001).