Responses of soil respiration to nitrogen addition are mediated by topography in a subtropical karst forest

Soil respiration is the largest terrestrial source of atmospheric carbon dioxide, and hence plays a vital role in the carbon cycle-climate feedback. Although the effects of elevated atmospheric nitrogen (N) deposition on soil respiration have been intensively investigated, it remains unclear whether the effects are mediated by topography, which impacts multiple soil abiotic and biotic properties. Here, a field N addition experiment with three rates, i.e., 0, 50 and 100 kg N ha(-1) yr(-1) was performed in the valley and on the slope of a subtropical karst forest in southwest China to explore whether and how topography mediates soil respiration. Nitrogen addition increased soil respiration by 55-63 % in the valley due to increased microbial activity. However, high N addition stimulated soil respiration by 56 % on the slope due to enhanced fine root biomass. The response of soil respiration to moderate N addition was significantly greater in the valley than on the slope, likely owing to the lower N availability in the valley. Our results demonstrate that the responses of soil respiration to N addition were mediated by topography, and hence highlight the importance of incorporating topography into Earth system models to better predict forest carbon dynamics in the context of elevated atmospheric N deposition.