Contributions of nitrification and denitrification to N2O emissions from soils at different water-filled pore space

A combination of stable isotope and acetylene (0.01% v/v) inhibition techniques were used for the first time to determine N₂O production during denitrification, autotrophic nitrification and heterotrophic nitrification in a fertilised (200 kg N ha^–1) silt loam soil at contrasting (20–70%) water-filled pore space (WFPS). ¹⁵N-N₂O emissions from ¹⁴NH₄¹⁵NO₃ replicates were attributed to denitrification and ¹⁵N-N₂O from ¹⁵NH₄¹⁵NO₃ minus that from ¹⁴NH₄¹⁵NO₃ replicates was attributed to nitrification and heterotrophic nitrification in the presence of acetylene, as there was no dissimilatory nitrate reduction to ammonium or immobilisation and remineralisation of ¹⁵N-NO₃^–. All of the N₂O emitted at 70% WFPS (31.6 mg N₂O-N m^–2 over 24 days; 1.12 g N₂O-N g dry soil^–1; 0.16% of N applied) was produced during denitrification, but at 35–60% WFPS nitrification was the main process producing N₂O, accounting for 81% of ¹⁵N-N₂O emitted at 60% WFPS, and 7.9 g ¹⁵N-N₂O m^–2 (0.28 ng ¹⁵N-N₂O g dry soil^–1) was estimated to be emitted over 7 days during heterotrophic nitrification in the 50% WFPS treatment and accounted for 20% of ¹⁵N-N₂O from this treatment. Denitrification was the predominant N₂O-producing process at 20% WFPS (2.6 g ¹⁵N-N₂O m^–2 over 7 days; 0.09 ng ¹⁵N-N₂O g dry soil^–1; 85% of ¹⁵N-N₂O from this treatment) and may have been due to the occurrence of aerobic denitrification at this WFPS. Our results demonstrate the usefulness of a combined stable isotope and acetylene approach to quantify N₂O emissions from different processes and to show that several processes may contribute to N₂O emission from agricultural soils depending on soil WFPS.