Topography modulates effects of nitrogen deposition on soil nitrogen transformations by impacting soil properties in a subtropical forest

Soil nitrogen (N) transformations play key roles in ecosystem productivity and other functions of terrestrial ecosystems via regulating soil N availability. Although the effects of elevated atmospheric N deposition on soil N transformations have been intensively investigated, it remains unclear whether the effects are mediated by topography, which impacts multiple soil abiotic and biotic properties. Here, we conducted an N addition experiment consisting of three treatments: control (0 kg N ha(-1) yr(-1)), moderate N addition (50 kg N ha(-1) yr(-1)), and high N addition (100 kg N ha(-1) yr(-1)) in the valley and on the slope, respectively, of a subtropical karst forest. Under the control, protein depolymerization, amino acid uptake, nitrification and dissimilatory nitrate reduction to ammonium (DNRA) rates were significantly higher on the slope than in the valley attributed to the higher soil dissolved organic carbon, total dissolved N and pH on the slope. Nitrogen addition significantly increased the rates of protein depolymerization, amino acid uptake, mineralization, nitrification and DNRA through alleviating microbial carbon limitation in the valley, but decreased the rates of depolymerization, amino acid uptake, mineralization and DRNA by enhancing microbial carbon and phosphorus limitations on the slope. The increase in soil N transformations, microbial N use efficiency (NUE) and microbial biomass N but lowered microbial N turnover time resulted in a 94 % increase of total microbial necromass N in the valley under N addition. However, increase in NUE and microbial biomass N led to a 33 % decrease of necromass N due to enhanced microbial necromass destabilization on the slope under N addition. Our results suggest that the responses of soil N transformations to N addition may be mediated by topography, and hence highlight the importance of incorporating topography into Earth system models to better predict soil N dynamics in forest in the context of elevated atmospheric N deposition.