Abstract

Drained agricultural boreal peatlands comprise a large source of nitrous oxide (N₂O) and carbon dioxide (CO₂) but a small sink or source of methane (CH₄). N₂O fluxes have high spatial and temporal variability and are often measured with the chamber technique. Therefore, continuous measurements of N₂O fluxes are needed to better understand how N₂O emissions are triggered and to reduce the uncertainty of annual N₂O budget estimations. Here we present a two-year-long time series of continuous measurements of CO₂ and N₂O fluxes of a shallow-peated drained agricultural boreal peatland cultivated for grass silage. The fluxes were measured with the area-averaging eddy covariance technique. Several NO peak events were observed throughout all seasons. The peaks were associated with meteorological or management events, such as soil thawing or freezing, precipitation, fertilization and glyphosate application. The annual N₂O budget was 4.74 ±0.47 and 6.08 ±0.49 kg NO-N ha⁻¹ y⁻¹ in 2020 and 2021, respectively. The annual CO₂ budget, comprising the sum of net ecosystem exchange and biomass export, was 3.70 ±0.22 and 5.54 ±0.33 t CO₂-C ha⁻¹ y⁻¹ in 2020 and 2021, respectively. The N₂O budget during the first, warmer winter was 106% higher than during the second, meteorologically more typical winter, due to the higher frequency of soil freezing–thawing cycles. The average annual NO budget was 36%–50% lower than the IPCC Emission Factor (EF) while the CO₂ budget was in accordance with the IPCC EF. CO₂ emissions dominated the total CO₂-eq emissions of our site but N₂O also had a significant contribution of 12%. Our results also suggest that glyphosate application enhanced N₂O emissions in the last quarter of 2021. However, the full rotation should be measured to confirm whether there is a need to re-evaluate the N₂O IPCC EF for ‘grassland drained boreal’ land-use class.