Tropical forests are pivotal to global climate and biogeochemical cycles, yet the geographic distribution of nutrient limitation to plants and microbes across the biome is unresolved. One long-standing generalization is that tropical montane forests are nitrogen (N)-limited whereas lowland forests tend to be N-rich. However, empirical tests of this hypothesis have yielded equivocal results. Here we evaluate the topographic signature of the ecosystem-level tropical N cycle by examining climatic and geophysical controls of surface soil N content and stable isotopes (δ¹⁵N) from elevational gradients distributed across tropical mountains globally. We document steep increases in soil N concentration and declining δ¹⁵N with increasing elevation, consistent with decreased microbial N processing and lower gaseous N losses. Temperature explained much of the change in N, with an apparent temperature sensitivity (Q₁₀) of ~1.9. Although montane forests make up 11% of forested tropical land area, we estimate they account for >17% of the global tropical forest soil N pool. Our findings support the existence of widespread microbial N limitation across tropical montane forest ecosystems and high sensitivity to climate warming.

热带森林对全球气候和生物地球化学循环至关重要，但整个生物群落中植物和微生物营养限制的地理分布尚未解决。一个长期存在的概括是热带山地森林的氮 (N) 有限，​​而低地森林往往富含氮。然而，对该假设的实证检验得出了模棱两可的结果。在这里，我们通过检查全球热带山脉分布的海拔梯度对地表土壤氮含量和稳定同位素 (δ ¹⁵ N)的气候和地球物理控制，评估生态系统级热带氮循环的地形特征。我们记录了土壤 N 浓度的急剧增加和 δ ^15的下降N 随着海拔的升高，与微生物 N 加工减少和气态 N 损失减少一致。温度解释了 N 的大部分变化，表观温度敏感性 ( Q ₁₀ ) 约为 1.9。尽管山地森林占热带森林覆盖面积的 11%，但我们估计它们占全球热带森林土壤氮库的 17% 以上。我们的研究结果支持热带山地森林生态系统中广泛存在的微生物 N 限制和对气候变暖的高度敏感性。