The productivity of rainforests growing on highly weathered tropical soils is expected to be limited by phosphorus availability¹. Yet, controlled fertilization experiments have been unable to demonstrate a dominant role for phosphorus in controlling tropical forest net primary productivity. Recent syntheses have demonstrated that responses to nitrogen addition are as large as to phosphorus², and adaptations to low phosphorus availability appear to enable net primary productivity to be maintained across major soil phosphorus gradients³. Thus, the extent to which phosphorus availability limits tropical forest productivity is highly uncertain. The majority of the Amazonia, however, is characterized by soils that are more depleted in phosphorus than those in which most tropical fertilization experiments have taken place². Thus, we established a phosphorus, nitrogen and base cation addition experiment in an old growth Amazon rainforest, with a low soil phosphorus content that is representative of approximately 60% of the Amazon basin. Here we show that net primary productivity increased exclusively with phosphorus addition. After 2 years, strong responses were observed in fine root (+29%) and canopy productivity (+19%), but not stem growth. The direct evidence of phosphorus limitation of net primary productivity suggests that phosphorus availability may restrict Amazon forest responses to CO₂ fertilization⁴, with major implications for future carbon sequestration and forest resilience to climate change.

生长在高度风化的热带土壤上的热带雨林的生产力预计会受到磷可用性的限制¹。然而，受控施肥实验无法证明磷在控制热带森林净初级生产力中的主导作用。最近的合成表明，对氮添加的响应与对磷的响应一样大²，对低磷可用性的适应似乎使净初级生产力能够在主要的土壤磷梯度中保持³. 因此，磷的可用性限制热带森林生产力的程度是高度不确定的。然而，亚马逊流域的大部分地区的特点是土壤中的磷含量比进行大多数热带施肥试验的土壤更贫瘠². 因此，我们在一个古老的亚马逊热带雨林中建立了磷、氮和碱阳离子添加实验，土壤磷含量低，代表亚马逊流域约 60%。在这里，我们表明净初级生产力仅随着磷的添加而增加。2 年后，在细根 (+29%) 和冠层生产力 (+19%) 中观察到强烈反应，但在茎生长中没有观察到。磷限制净初级生产力的直接证据表明，磷的可利用性可能会限制亚马逊森林对 CO ₂施肥的反应⁴，这对未来的碳封存和森林对气候变化的适应力具有重大影响。