Responses of N2O flux and functional genes to nitrogen addition and altered precipitation
N2O emissions showed a significant seasonal dynamic, with the maximum flux appearing in August (Figure 2A). Although the average flux was relatively small, the alpine steppe presented as the source of N2O (F > 0) during the growing season under different treatment conditions (Figure 2B). The addition of N resulted in a significant increase in N2O emissions (317%) (Figure 2B-C). However, N2O emissions were not significantly influenced by altered precipitation or the interaction between N addition and precipitation changes (Figure 2B). To a certain extent, the coupling of N and water alleviated the effect of N input on N2O emissions (178% and 100% vs 317%) (Figure 2B-C). Similarly, N2O flux during the emission peak was only affected by N addition (Figure 2D).
The amo A gene abundance of the nitrifier AOA was significantly affected by both N supply and precipitation changes (Figure 3A). However, the AOB amo A gene abundance was significantly elevated only by N supply (Figure 3B). Although the denitrifier nirS and nirK genes regulate the same step in denitrification (nitrite reduction: NO2→ NO), only the nirS gene abundance was significantly affected by the interaction of N addition and precipitation changes (Figure 3C). The abundance of the nirK gene did not significantly differ among the treatments (Figure 2D). The abundance of the nosZ gene was reduced under N addition, and altered precipitation did not significantly affect the nosZ gene abundance (Figure 3E).