Effects of soil moisture on microbial processes of soil nitrogen gases production under anaerobic conditions

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.