Responses of soil nitrous oxide emission to nitrogen addition at two topographic positions of 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. High N addition significantly increased N2O emission in the valley from 2017–2019, while significantly increased N2O emission on the slope only in 2017. The cumulative N2O fluxes across three 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. High N addition stimulated 113.3% increase in soil N2O emission in the valley due to increased ammonium, nitrate and dissolved organic carbon availabilities and AOB amoA abundances. High N addition stimulated 84.3% increase in soil N2O emission on the slope due to increased nitrate and carbon availabilities, 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 N status in regulating the responses of soil N2O emission to elevated N deposition.