Contradicting default nitrous oxide emission factors: Average nitrous oxide emissions from mixed organic fertilizer application are higher than those from synthetic nitrogen fertilizers on Danish agricultural soils

Nitrous oxide emissions from Danish agriculture account for about 25 % of its greenhouse gases. This study aimed to establish country-specific emission factors for synthetic and mixed organic nitrogen fertilizers under typical Danish farming conditions to support improved monitoring and mitigation. Over three years (2022 – 2024), 28 individual field trials were conducted in spring barley and winter wheat, representing 86.5 % of Danish soil types. Nitrous oxide emissions were measured growing seasons following fertilization with organic fertilizers (cattle slurry, pig slurry, digestate) supplemented with synthetic starter fertilizer, or synthetic ammonium nitrate fertilizer. Cumulative nitrous oxide emissions of synthetic fertilizers ranged from 0.12 to 1.05 kg N 2 O-N ha −1 (spring barley), and from 0.08 to 1.17 kg N 2 O-N ha −1 (winter wheat). Cumulative emissions of mixed organic fertilizers ranged from 0.95 to 1.41 kg N 2 O-N ha −1 (spring barley), and from 0.19 to 1.30 kg N 2 O-N ha −1 (winter wheat). Emissions were generally low, but influenced by soil moisture and precipitation, with higher fluxes in wetter conditions. Emission factors during the growing season for synthetic fertilizers averaged 0.11 ± 0.03 % (spring barley) and 0.16 ± 0.06 % (winter wheat), and were higher for mixed organic fertilizers: 0.38 ± 0.05 % (spring barley, cattle slurry), 0.38 ± 0.09 % (winter wheat, pig slurry), and 0.37 ± 0.10 % (winter wheat, digestate). These results contradict the IPCC refinement for wet climates, assuming lower emissions from organic fertilizers. The findings highlight the need for region-specific emission factors and a Tier 2 approach in Denmark’s national greenhouse gas inventory. • Nitrous oxide emissions from organic fertilizers exceeded those from synthetic sources. • Emission factors were ∼0.38 % for organics vs. ∼0.13 % for synthetic N. • Study provides Denmark-specific data to improve national GHG inventories.