A full year of continuous net soil and ditch CO₂, CH₄, N₂O fluxes, soil hydrology and meteorology for a drained fen in Denmark

Abstract. We here present a detailed dataset of automated greenhouse gas (GHG) net soil and ditch fluxes of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) from a drained fen in Denmark covering a full year. The dataset resolves small scale spatial and hourly-daily-seasonal dynamics of GHG soil fluxes. The GHG flux dataset is accompanied by simultaneous time series of soil temperature and moisture, as well as groundwater table depth and covers spatiotemporal gradients in soil hydrological and climatic variability. The GHG fluxes of CO₂, CH₄ and N₂O were measured simultaneously by a high-precision cavity ring down laser spectrometer connected with a novel automated GHG system platform called SkyLine2D (Earthbound Scientific Ltd., UK) that allowed up to 27 individual chamber measurement points along a 24 meter transect. In total 47.483 chamber measurements were completed and after quality control 44.631 CO₂ fluxes, 44.099 N₂O and 42.515 CH₄ fluxes remained.

The average net soil CO₂ efflux observed at the site (2.55 μmol CO₂ m^-2 s^-1 or 35 tCO₂ ha^-1 y^-1) aligns with findings from similar drained fens in northern Europe. However, this transect average masks substantial spatial variability and highlights the role of episodic emission bursts related to hydrological variability. N₂O fluxes measured at this site were similarly variable in space, but displayed a more dynamic flux behaviour than CO₂, where increasing groundwater table depth in response to precipitation during warmer seasons lead to emission bursts of N₂O that dominated the annual budget and decreased to near-zero fluxes in drier periods. Soil CH₄ fluxes were near-zero and the site overall acted as a small net source, although net uptake was observed as well especially in drier conditions.

Diurnal and seasonal patterns of net soil CO₂ and N₂O fluxes align with expectations of soil temperature driven processes, but no clear patterns were observed for CH₄. Compared to soil GHG fluxes, ditch CO₂ and N₂O fluxes were 4-fold and 27-fold lower, respectively, while CH₄ fluxes were more than two orders of magnitude larger, confirming earlier findings that ditches can be CH₄ hotspots, where the CH₄ is emitted in bursts with little seasonal variability, including emissions as ebullitions.

The data set is well suited for testing and developing biogeochemical models, with emphasis on the soil thermal-hydrology interactions with the peat C and N cycles.