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. Nitrogen (N) availability can be highly variable in tropical forests on regional and local scales. While environmental gradients influence N cyclingon a regional scale, topography is known to affect N availability on a local scale. We compared natural abundance of 15N isotopes of soilprofiles in tropical lowland forest, tropical montane forest, and subtropical Miombo woodland within the Congo Basin as a proxy to assessecosystem-level differences in N cycling. Soil δ15N profiles indicated that N cycling in the montane forest is relatively moreclosed and dominated by organic N turnover, whereas the lowland forest and Miombo woodland experienced a more open N cycle dominated byinorganic N. Furthermore, we examined the effect of slope gradient on soil δ15N within forest types to quantify local differencesinduced by topography. Our results show that slope gradient only affects the soil δ15N in the Miombo forest, which is prone toerosion due to a lower vegetation cover and intense rainfall at the onset of the wet season. Lowland forest, on the other hand, with a flattopography and protective vegetation cover, showed no influence of topography on soil δ15N in our study site. Despite the steeptopography, slope angles do not affect soil δ15N in the montane forest, although stable isotope signatures exhibited highervariability within this ecosystem. A pan-tropical analysis of soil δ15N values (i.e., from our study and literature) reveals thatsoil δ15N in 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 spatialcoverage of N cycling studies in tropical forests, to further elucidate the local impact of topography on N cycling in this biome.

Stable isotope signatures of soil nitrogen on an environmental–geomorphic gradient within the Congo Basin