摘要：Nitrogen(N) availability can be highly variable in tropical forests on regional and local scales. While environmental gradients influence Ncycling on a regional scale, topography is known to affect Navailability on a local scale. We compared natural abundance of15Nisotopes of soil profiles in tropical lowland forest, tropical montane forest, and subtropical Miombo woodland within the Congo Basin as a proxy to assess ecosystem-level differences in Ncycling. Soil15Nprofiles indicated that Ncycling in the montane forest is relatively more closed and dominated by organicNturnover, whereas the lowland forest and Miombo woodland experienced a more open Ncycle dominated by inorganicN. Furthermore, we examined the effect of slope gradient on soil15Nwithin forest types to quantify local differences induced by topography. Our results show that slope gradient only affects the soil15Nin the Miombo forest, which is prone to erosion due to a lower vegetation cover and intense rainfall at the onset of the wet season. Lowland forest, on the other hand, with a flat topography and protective vegetation cover, showed no influence of topography on soil15Nin our study site. Despite the steep topography, slope angles do not affect soil15Nin the montane forest, although stable isotope signatures exhibited higher variability within this ecosystem. A pan-tropical analysis of soil15Nvalues (i.e., from our study and literature) reveals that soil15Nin tropical forests is best explained by factors controlling erosion, namely mean annual precipitation, leaf area index, and slope gradient. Erosive forces vary immensely between different tropical forest ecosystems, and our results highlight the need for more spatial coverage of Ncycling studies in tropical forests, to further elucidate the local impact of topography on Ncycling in this biome.
