Topography-driven soil properties modulate effects of nitrogen deposition on soil nitrous oxide sources in a subtropical forest.

An ex-situ ¹⁵ N–¹⁸O tracing experiment with soils collected from the valley and slope, respectively, of a subtropical secondary karst forest with three N addition levels, i.e., 0, 50, and 100 kg N ha⁻¹ year⁻¹ for each topographic position to investigate N₂O production pathways. Autotrophic nitrification pathways (ammonia oxidation, nitrifier denitrification, and nitrification-coupled denitrification) accounted for > 70% of total N₂O production, but denitrification pathways (heterotrophic denitrification and co-denitrification) were the minor source of N₂O at both topographic positions. In the valley, chronic N addition stimulated ammonia oxidation-derived N₂O, which was paralleled by increased ammonia-oxidizing archaea (AOA) amoA gene transcript abundance, but inhibited nitrifier denitrification- and nitrification-coupled denitrification–derived N₂O along with suppressed ammonia-oxidizing bacteria (AOB) amoA gene transcript abundance and stimulated nosZII gene transcript abundance, respectively. On the slope, chronic N addition stimulated ammonia oxidation-derived N₂O along with increased AOB amoA gene transcript abundance, and enhanced nitrifier denitrification-derived N₂O congruent with increased AOB amoA and decreased nirK gene transcript abundances. In addition, chronic N addition reduced the relative contribution of heterotrophic denitrification to N₂O production but had no significant influence on heterotrophic denitrification-derived N₂O on the slope. Overall, our results provide a comprehensive view in terms of how topography-driven soil properties regulate N₂O production and its pathways in a subtropical forest. [ABSTRACT FROM AUTHOR]