The greenhouse gas balance of a drained fen peatland is mainly controlled by land-use rather than soil organic carbon content

Abstract. Drained organic soils are considered to be hotspots for greenhouse gas (GHG) emissions. Arable lands and intensively used grasslands, in particular, have been regarded as the main producers of carbon dioxide (CO₂) and nitrous oxide (N₂O). However, GHG balances of former peatlands and associated organic soils not considered to be peatland according to the definition of the Intergovernmental Panel on Climate Change (IPCC) have not been investigated so far. Therefore, our study addressed the question to what extent the soil organic carbon (SOC) content affects the GHG release of drained organic soils under two different land-use types (arable land and intensively used grassland). Both land-use types were established on a Mollic Gleysol (labeled C_medium) as well as on a Sapric Histosol (labeled C_high). The two soil types differed significantly in their SOC contents in the topsoil (C_medium: 9.4–10.9 % SOC; C_high: 16.1–17.2 % SOC). We determined GHG fluxes over a period of 1 or 2 years in case of N₂O or methane (CH₄) and CO₂, respectively. The daily and annual net ecosystem exchange (NEE) of CO₂ was determined by measuring NEE and the ecosystem respiration (R_ECO) with the closed dynamic chamber technique and by modeling the R_ECO and the gross primary production (GPP). N₂O and CH₄ were measured with the static closed chamber technique. Estimated NEE of CO₂ differed significantly between the two land-use types, with lower NEE values (−6 to 1707 g CO₂-C m⁻² yr⁻¹) at the arable sites and higher values (1354 to 1823 g CO₂-C m⁻² yr⁻¹) at the grassland sites. No effect on NEE was found regarding the SOC content. Significantly higher annual N₂O exchange rates were observed at the arable sites (0.23–0.86 g N m⁻² yr⁻¹) than at the grassland sites (0.12–0.31 g N m⁻² yr⁻¹). Furthermore, N₂O fluxes from the C_high sites significantly exceeded those of the C_medium sites. CH₄ fluxes were found to be close to zero at all plots. Estimated global warming potential, calculated for a time horizon of 100 years (GWP₁₀₀) revealed a very high release of GHGs from all plots ranging from 1837 to 7095 g CO_{2 eq.} m⁻² yr⁻¹. Calculated global warming potential (GWP) values did not differ between soil types and partly exceeded the IPCC default emission factors of the Tier 1 approach by far. However, despite being subject to high uncertainties, the results clearly highlight the importance of adjusting the IPCC guidelines for organic soils not falling under the definition in order to avoid a significant underestimation of GHG emissions in the corresponding sectors of the national climate reporting. Furthermore, the present results revealed that mainly the type of land-use, including the management type, and not the SOC content is responsible for the height of GHG exchange from intensive farming on drained organic soils.