Site-specific effects of nitrification inhibitors on N2O emissions from manure and urea ammonium nitrate

Nitrification inhibitors (NIs) can be an effective measure to mitigate nitrous oxide (N₂O) emissions from agricultural soils. Meta-analyses indicate that the efficacy of NIs is influenced by soil properties and climate, but such effects have rarely been investigated by direct comparisons across field sites. Using four experimental field sites across Denmark, this study investigated the interacting effects of fertiliser type, soil type and rainfall on N₂O emissions and effects of two different NIs after spring fertilisation of spring barley in 2022 and 2023. The fertiliser materials included cattle slurry (CS), pig slurry (PS) and urea ammonium nitrate (UAN, 2022 only), applied at recommended rates with or without 3,4-dimethylpyrazole phosphate (DMPP) or 2-chloro-6-(trichloromethyl) pyridine (nitrapyrin, NP), along with an unfertilised control. Rainfall was close to the long-term average at Askov and Vejen in 2022 but below average in the six other monitoring periods. The increase in net cumulative N₂O emissions from manure application without NIs varied between 0.16 and 1.5 kg N ha⁻¹ while with UAN it was between 0.05 and 0.46 kg N ha⁻¹. The amendment of NIs significantly reduced N₂O emissions but only the coarse sandy soil at Vejen showed consistent and large reductions with NI amendment leading to average reductions of 83 %, 67 %, and 77 % for cattle slurry, pig slurry, and UAN, respectively. Specific site effects of NI were attributed to differences in gas and solute diffusivity, and NI availability, across sites and years. In this study, NI amendment to synthetic N fertiliser and, especially, manure applied to sandy soil had the greatest N₂O mitigation potential, suggesting targeted use of NIs on sandy soil as a strategic N₂O mitigation scenario.