Abstract: Gaseous nitrogen (N) emission [nitric oxide (NO), nitrous oxide (N₂O), and nitrogen (N₂)] is an important pathway of soil N loss. Nitrification and denitrification are the main processes of gaseous N production in soil. However, the contribution of heterotrophic nitrification, co-denitrification, and anammox to gaseous N production remains uncertain. In a laboratory soil incubation experiment, we used the ¹⁵N labelling and pairing technique, combining the nitrification inhibitor dicyandiamide (DCD), to quantify the contribution of different microbial processes to soil NO, N₂O and N₂ production under anaerobic conditions. The results showed that after 24 h anaerobic incubation, the highest total ¹⁵N recovery of three gases occurred at 65% water filled pore space (WFPS), accounting for 20.0% of total added ¹⁵N. Denitrification contributed 49.9%-94.1%, 29.0%-84.7%, and 58.2%-85.8% to the production of NO, N₂O and N₂ respectively, suggesting that denitrification was the predominant process of those three N gases emission. Heterotrophic nitrification was an important pathway of NO and N₂O production, particularly at conditions with low soil water content (10% WFPS), with its contribution to those two N gases production being 50.1% and 42.8%, respectively. Co-denitrification contributed 10.6%-30.7% of N₂O production. For N₂ production, the total contribution of co-denitrification and anammox was 14.2%-41.8%. The role of co-denitrification can not be ignored for N₂O and N₂ production. Our results demonstrated that the ¹⁵N labelling and pairing technique is a promising tool to quantify the contribution of different microbial processes to gaseous N loss.

Key words: ¹⁵N labelling, ¹⁵N pairing technique, NO, N₂O, N₂, denitrification, heterotrophic nitrification, co-denitrification, soil moisture