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 key 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, an N addition experiment with three N addition levels, i.e., 0, 50 and 100 kg N ha-1 yr-1, was carried out in the valley and on the slope of a subtropical karst forest. Nitrogen addition increased soil respiration by 55-63% in the valley attributed to increased microbial respiration. In contrast, high N addition stimulated soil respiration by 56% on the slope due to enhanced fine root biomass. Stimulation of soil respiration by moderate N addition was significantly greater in the valley than on the slope largely owing to the lower N status in the valley. Our results demonstrate that the mechanisms underlying the impacts of N addition on soil respiration may be mediated by topography, and hence highlight the importance of incorporating topography into Earth system models to better predict the responses of soil respiration to atmospheric N deposition.