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

Abstract

Forests are increasingly recognized as a significant source of atmospheric nitrous oxide (N2O) under elevated atmospheric nitrogen (N) deposition. Nevertheless, it is still unclear whether topography modulates the responses of soil N2O emission to elevated N deposition. Here, N was applied as NH4NO3 in three levels, i.e., 0 (Control), 50 (moderate N) and 100 (high N) kg N ha–1 yr–1 in the valley and on the slope of a subtropical forest in southwest China. Soil N2O emission along with microbial functional gene abundances (ammonia–oxidizing bacteria (AOB) and archaea (AOA) amoA, nirK, nirS, fungal nirK, nosZI and nosZII) and soil properties were measured in responses to three years of N addition. The cumulative N2O fluxes were 1.15 ± 0.25 kg N ha–1 in the valley and 1.25 ± 0.35 kg N ha–1 on the slope under the control. Nitrogen addition increased cumulative soil N2O emission by 87–176% in the valley and 27–143% on the slope. In the valley, the stimulation of N2O emission by N addition was mainly due to increase of ammonium availability and AOB amoA abundances but decrease of AOA:AOB amoA ratio. On the slope, the stimulation was mainly explained by increase of nitrate availability, AOB amoA and nirK abundances. Furthermore, the stimulation of N2O emission by moderate N addition was significantly greater in the valley than on the slope largely owing to the lower N status in the valley. This work highlights the importance of topography in regulating the responses of soil N2O emission to elevated N deposition.