Global distribution and climate sensitivity of the tropical montane forest nitrogen cycle

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与生物地球化学循环至关重要，但该生物群区中植物与微生物营养限制的地理分布格局仍未明确。长期以来存在一项公认概括：热带山地森林受氮（Nitrogen, N）限制，而低地森林通常氮素富集。然而，针对该假说的实证检验却得到了模棱两可的结果。本研究通过分析全球热带山地海拔梯度下表层土壤氮含量与稳定同位素（δ¹⁵N）的气候及地球物理调控机制，解析生态系统尺度下热带氮循环的地形特征。本研究观测到，随着海拔升高，土壤氮浓度显著升高，而δ¹⁵N值则持续降低，这与微生物氮周转过程减弱、气态氮损失减少的规律相符。温度可解释大部分氮素的变化特征，其表观温度敏感性（Q₁₀）约为1.9。尽管热带山地森林仅占全球热带林地总面积的11%，但我们估算其土壤氮库占全球热带森林土壤氮库的比例超过17%。本研究结果证实，热带山地森林生态系统中普遍存在微生物氮限制现象，且该生态系统对气候变暖具有较高敏感性。