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 present a detailed dataset (10.60612/DATADK/BZQ8JE, Skov Nielsen et al., 2025) 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 m 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 (±SE) net soil CO2 efflux observed at the site (2.6 ± 0.02 μmol CO2 m-2 s-1 or 35 ± 0.3 tCO2 ha-1 yr-1) aligns with findings from similar drained fens in northern Europe covering substantial spatial variability. The organic soil at the site was a larger net source of N2O (8.9 ± 0.1 nmol N2O m-2 s-1 or 123 ± 1.4 kg N2O ha-1 yr-1) to the atmosphere compared to other temperate drained organic grassland soils in northern Europe with similar spatial variability as soil CO2 effluxes. However, the temporal variability of N2O fluxes were closely linked to fluctuations of the groundwater table depth with emission bursts of soil N2O emissions during low water table depth. N2O fluxes decreased to near-zero fluxes when the water table depth increased. Net soil CH4 fluxes were near-zero and the site overall acted as a smaller net source (0.18 ± 0.06 nmol CH4 m-2 s-1 or 0.91 ± 0.3 kg CH4 ha-1 yr-1) compared to other drained organic grassland soils, although net uptake of atmospheric CH4 was observed as well especially in drier conditions. Compared to the peat soil, the ditch was a smaller net source of CO2 (0.94 ± 0.05 μmol CO2 m-2 s-1 or 1.3 ± 0.7 tCO2 ha-1 yr-1) and N2O (0.35 ± 0.03 nmol N2O m-2 s-1 or 4.9 ± 0.4 kg N2O ha-1 yr-1). The ditch emission of CH4 (161 ± 13 nmol CH4 m-2 s-1 or 812 ± 66 kg CH4 ha-1 yr-1) average of diffusive and ebullition fluxes) to the atmosphere was more than two orders of magnitude larger than the net soil CH4 emissions. The very large number of fluxes of CO2, N2O and CH4 for peat soils and a ditch linked to both groundwater table data, soil moisture/temperature as well as groundwater and soil physicochemical parameters are unique to northern temperate peatlands and holds a potential for exploring and testing basic hypothesis on the simultaneous regulation of these gas fluxes by both soil hydrology and temperature, including soil and groundwater chemistry. The high temporal detail also allows for time series analyses as well as investigations into diurnal and seasonal patterns of fluxes in response to physical drivers. Similarly, the high frequency of measured variables and the large number of spatial replicates are furthermore well suited for testing biogeochemical models as it is possible to have both calibration and validation dataset covering the same period. Furthermore, the surprisingly large spatial variability of flux data is ideal to include in model sensitivity tests which can aid in constraining model outputs and develop model routines.