The impact of mineral soil cover fill on N2O emissions in peatland drained for agriculture

Drainage for agriculture has converted peatlands from a carbon sink to one of the world’s major greenhouse gas (GHG) sources. In order to improve the sustainability of peatland management in agriculture, and to counteract soil subsidence, mineral soil coverage is becoming an increasingly used practice in Switzerland. Cover fills may change the GHG balance from the corresponding organic soil. To explore the effect of cover fill on soil N₂O emissions, we carry out a field experiment in the Swiss Rhine Valley and measure the soil – borne N₂O exchange from two adjacent sites: drained organic soil without mineral soil cover (DN), and drained organic soil with mineral soil cover (DC). Mineral soil material was applied 12 years ago and varies in thickness between 20 – 80 cm. Both sites have the identical farming practice (intensive permanent meadow). In our experiment, an automatic chamber system is used for collecting the N₂O at an interval of 3 h. Soil moisture, expressd as volumetric water content (VWC), is recorded every 10 min. After ten month (303 days) of continous measurement, the data reveal that: (1) The average N₂O emission from DN is higher than DC by a factor of 11 (11.24 ± 3.46 vs 0.97 ± 0.22 mg N₂O-N m^-2 day^-1). Hence, mineral soil cover of organic soil seems to induce a strong reduction in N₂O emissions. (2) Exogenous N inputs (mineral N fertilizer and cow slurry) are the main drivers of N₂O emissions. N₂O peaks occured shortly after the N application and lasted for 2 to 3 weeks before returning to background N₂O emission. At the DC site post N- input N₂O emissions accounted for 68 % of the total N₂O emission over the whole measurement period. An equivalent of around 1 % of the exogenous N- input was emitted as N₂O. At the DN site, emission peaks after fertilization accounted for 79 % of the total N₂O emission, equivalent to around 13 % of the exogenous N- input. Background emissions between peak events shows no significant difference between DC (0.51± 0.15 mg N₂O-N m^-2 day^-1) and DN (2.73± 2.44 mg N₂O-N m^-2 day^-1). The comparison of peak and background fluxes tentatively indicates that higher average emission rates from the DN site are related directly to fertilization. Finally, surface soil characteristics (soil pH, bulk density, and soil N) changed after mineral soil cover, and soil moisture content differed between sites. During the experimental period, the mean daily soil moisture from DN site (24.1 % VWC – 60.18 % VWC) is higher than DC site (20.17 % VWC – 51.26 % VWC). In summary, our data from this first experimental period suggest that mineral soil cover fill could strongly reduce the N₂O emission from drained organic soil, and may therefore be an interesting GHG mitigation option in agriculture.