Higher N2O emissions from organic compared to synthetic N fertilisers on sandy soils in a cool temperate climate

The atmospheric increase in N2O is mainly derived from N fertilisation in agriculture, and improved emission estimates are needed for effective mitigation. This study presents first estimates of country-specific N2O emissions from synthetic and liquid organic fertilisers in Denmark. Representative crop rotations were established in four locations across Denmark to provide a realistic context for the estimation of N2O emissions, i.e., a dairy farm rotation in Western Denmark, dairy and pig farm rotations in Southwestern Denmark, and an arable rotation in Eastern Denmark. The four sites were light-textured and typical for Northern Europe, whereas rainfall varied considerably among sites and years. A randomised block design was used, and all crops were represented in triplicate each year with monitoring of N2O emissions between April 2020 and March 2022. Spring barley was part of all rotations, and here three synthetic fertilisers (NS, NPK and urea ammonium nitrate) and eight organic fertilisers (three cattle slurries, three pig slurries and two digestates) were applied in 1 m2 plots at either two or four sites in order to compare N2O emissions from the same N fertiliser materials under contrasting site conditions. Identical methodologies for management, fertiliser application, and N2O measurement and flux calculation, were used at all sites to ensure comparability. Manually operated chambers were used for N2O flux measurements. The continuous monitoring indicated a strong seasonal pattern across all four sites with the main part of N2O emissions during spring. The side-by-side comparison of several N sources at four sites in two years showed for synthetic fertilisers an average N2O emission factor for the spring period of 0.15% (95% C.I. −0.17 to 0.37%, n = 16), and for liquid organic fertilisers (pig and cattly slurries, and digestates) an average of 1.02% (95% C.I. 0.75 – 1.30%, n = 44). The higher N2O emissions from organic fertilisers, which was significant at each of the four sites, is in opposition to new N2O emission factors recently proposed in a refinement of the IPCC methodology for national inventories. The conflicting results are discussed with reference to region-specific site conditions and fertiliser types, and in particular the predominance of soils with a low clay content, and of liquid manure management, may explain the deviations from global estimates. A comparison of annual and spring N2O emissions indicated a difference in the order of 0.1 – 0.2% of the N input (n = 8), and the feasibility of estimating N2O emission factors based on emissions during the growing season only is discussed.