Topography modulates effects of nitrogen deposition on soil nitrous oxide sources in a subtropical forest

Atmospheric nitrogen (N) deposition triggers large N2O emissions from soils of subtropical/tropical forests via multiple, simultaneously occurring pathways. Nevertheless, it is still unclear whether topography modulates the magnitude and source partitioning of soil N2O emission under elevated N deposition. Here, an N addition experiment was conducted in the valley and on the slope of a subtropical secondary forest, respectively, with three treatments, i.e., 0, 50 and 100 kg N ha-1 year-1. A combination of the 15N–18O labelling technique, 15N pool mixing model and transcriptome analysis were used to identify contribution of seven pathways to soil N2O production. Autotrophic nitrification pathways (ammonia oxidization, nitrifier denitrification and nitrification coupled denitrification) accounted for > 70% of total N2O production, but denitrification pathways (heterotrophic denitrification and co–denitrification) were the minor sources of N2O at both topographic positions. In the valley, N addition stimulated ammonia oxidization–derived N2O mainly due to increase of ammonia oxidizing archaea (AOA) amoA gene expression, but inhibited nitrifier denitrification– and nitrification coupled denitrification–derived N2O primarily via suppressing ammonia oxidizing bacteria (AOB) amoA gene expression and nirS, nosZІІ gene expression, respectively. On the slope, N addition enhanced nitrifier denitrification–derived N2O through increasing AOB amoA and decreasing nirK gene expressions, but suppressed heterotrophic denitrification–derived N2O via increasing nosZІІ gene expression. Our results indicate that the effects of N deposition on N2O production pathways are likely topography–dependent.