Abstract. Nitrogen (N) availability can be highly variable in tropical forests on a regional and on a local scale. While environmental gradients influence N cycling on a regional scale, topography is known to affect N availability on a local scale. We compared stable isotope signatures (δ¹⁵N) 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 N cycling. Furthermore, we examined the effect of surface slope angles on δ¹⁵N in the same forests to quantify local differences induced by topography. Soil δ¹⁵N profiles indicated that the N cycling in in the montane forest is more closed and dominated by organic N turnover, whereas the lowland forest and Miombo woodland experienced a more open N cycle dominated by inorganic N. Furthermore, our results show that slope angles only affects the soil δ¹⁵N signature in the Miombo forest, which is prone to erosion due to the lower vegetation cover and intense rainfalls 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 soil N cycling. Values from the montane forest showed high variability in stable isotope signatures, but they were not constrained by topography. A pan-tropical analysis of soil δ¹⁵N values (i.e. from our study and the literature) reveals that soil δ¹⁵N is best explained by factors controlling erosion, namely mean annual precipitation, leaf area index, and slope angles. The erosive forces vary immensely between different tropical forest ecosystems and our results highlight the need of more spatial coverage of N-cycling studies in tropical forests, to further elucidate the local impact of topography on N cycling in this biome.

中文翻译：

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刚果盆地内环境-地貌梯度上土壤氮的稳定同位素特征

摘要。在区域和地方范围的热带森林中，氮（N）的利用率变化很大。虽然环境梯度会影响区域范围内的N循环，但已知地形会影响局部范围内的N可用性。我们比较稳定的同位素特征（δ ¹⁵在热带低地雨林，热带山地森林，亚热带林地米欧埔地区的刚果盆地内作为代理评估氮循环的生态系统水平的差异土壤剖面N）。此外，我们检测表面的倾斜角度的效果上δ ¹⁵ N的相同森林量化由地形引起的局部差异。土壤δ ¹⁵N剖面表明，山地森林中的N循环更封闭，主要受有机N转换的影响，而低地森林和Miombo林地则经历了更为开放的N循环，其中无机N占主导地位。此外，我们的结果表明，倾斜角度仅影响土壤δ ¹⁵在米欧埔地区的森林，这是容易受到侵蚀ň签名由于在雨季来临较低植被覆盖和强降雨。另一方面，低地森林地势平坦，植被被保护，对土壤氮循环没有影响。山地森林的值在稳定的同位素特征上显示出很高的变异性，但不受地形的限制。土壤的泛热带分析δ ¹⁵N个值（即我们的研究和文献）表明，土壤δ ¹⁵ N的最好通过控制侵蚀，即平均年降水量，叶面积指数，和边坡角因素来解释。不同热带森林生态系统之间的侵蚀力差异很大，我们的研究结果凸显了对热带森林中N循环研究进行更大空间覆盖的必要性，以进一步阐明地形对该生物群落中N循环的局部影响。