Responses of Soil Nitrous Oxide Emission to Nitrogen Addition at Two Topographic Positions of a Subtropical Forest.

Topography can influence nitrous oxide (N2O) emission via its influences on soil nutrient availability, moisture, and microbial communities. Nevertheless, it is still unclear whether topography modulates the responses of soil N2O emissions to elevated N deposition. Here the N addition experiment was conducted in the valley and on the slope of a subtropical karst forest in southwest China. Nitrogen was applied as NH4NO3 in two levels, that is, 50 (moderate N) and 100 (high N) kg N ha−1 yr−1 with no N addition plots as the control. Nitrogen addition consistently increased N2O emission in the valley, but only high N addition significantly increased N2O emission on the slope in 2017. The cumulative N2O fluxes across the 3 years were 1.16 ± 0.24 kg N ha−1 in the valley and 1.50 ± 0.06 kg N ha−1 on the slope under the control. Nitrogen addition stimulated N2O emission by 88.7%–113.3% in the valley due to increased ammonium, nitrate and dissolved organic carbon availabilities and ammonia‐oxidizing bacteria (AOB) amoA abundance. High N addition stimulated N2O emission by 84.3% on the slope owing to increased nitrate and carbon availabilities, AOB amoA, and nirK abundances. The stimulation of N2O emission by moderate N addition was more pronounced in the valley than on the slope largely owing to the lower N status in the valley. This work highlights the importance of N status in regulating the responses of soil N2O emissions to elevated N deposition. Plain Language Summary: Atmospheric N deposition is identified as one of the strongest driving factors responsible for the increase of forest soil N2O emission, and topography can influence N2O emission via its influences on soil nutrient availability, moisture, and microbial communities. However, there is uncertainty about whether topography modulates the responses of soil N2O emissions to elevated N deposition. We investigated soil N2O emission and related microbial functional gene abundances at two topographic positions under three N addition levels in a subtropical region of China. We find that the N addition consistently increased N2O emission in the valley, but only stimulated N2O emission at the early stage on the slope. Stimulation of soil N2O emission by moderate N addition was more pronounced in the valley than on the slope. Our findings highlight the importance of N status in regulating the response of soil N2O emissions to elevated N deposition. Key Points: N addition increased soil N2O emission in the valley across the 3 yearsHigh N addition stimulated N2O emission on the slope at the early stageModerate N stimulated greater N2O emission in the valley than on the slope [ABSTRACT FROM AUTHOR]