A new method for estimating carbon dioxide emissions from drained peatland forest soils for the greenhouse gas inventory of Finland

Abstract. Reporting the greenhouse gas (GHG) emissions from the LULUCF sector in the GHG inventory requires sound methods for estimating both the inputs and outputs of carbon (C) in managed ecosystems. Soil CO₂ balance of forests consists of the CO₂ released from decomposing soil organic matter (SOM) and the C entering the soil through aboveground and belowground plant litter input. Peatlands drained for forestry release soil C as CO₂ because the drainage deepens the oxic peat layer prone to SOM decomposition. IPCC Guidelines provide default CO₂ emission factors for different climatic zones and the defaults or locally adapted static emission factors are commonly in use in GHG inventory reporting for drained peatlands. In this paper, we describe a new dynamic method to estimate the CO₂ balance of drained peatland forest soils in Finland. Contrary to static emission factors, the annual CO₂ release from soil is in our method estimated using empirical regression models driven by time series of tree basal area (BA), derived from the national forest inventories in Finland, time series of air temperature and the drained peatland forest site type. Aboveground and belowground litter input is also estimated using empirical models with newly acquired turnover rates for tree fine roots and BA as a dynamic driver. All major components of litter input from ground vegetation and live, harvested and naturally died trees are included. Our method produces an increasing trend of emissions from 1.4 to 7.9 Mt CO₂ for drained peatland forest soils in Finland for the period 1990–2021, with a statistically significant difference between years 1990 and 2021. Across the period 1990–2021, annual emissions are on average 3.4 Mt and −0.3 Mt in southern and northern parts of Finland, respectively. When combined with data of the CO₂ sink created by trees, it appears that in 2021 drained peatland forest ecosystems were a source of 2.3 Mt CO₂ in southern Finland and a sink of 2.5 Mt CO₂ in northern Finland. We compare the emissions produced by the new method with those produced by the old GHGI method of Finland and discuss the strengths and vulnerabilities of our method in comparison to static emission factors.