Tropical forests are often heralded as the lungs of the Earth, serving as pivotal carbon sinks that help regulate the global climate. Yet, in an era of accelerating atmospheric CO₂ accumulation, the future of these ecosystems, particularly the Amazon rainforest, remains shrouded in uncertainty. A groundbreaking study originating from the Central Amazon, conducted by a collaborative team of researchers from the University of Vienna, the Technical University of Munich, and the National Institute for Amazon Research (INPA, Manaus), sheds new light on how Amazonian understory trees respond to elevated CO₂ levels. Their findings reveal a complex narrative: while these trees may initially enhance their carbon uptake and growth under increased CO₂, the prolonged availability of essential nutrients, especially phosphorus, could critically limit their carbon storage potential over time.
This study addresses a pivotal knowledge gap concerning the Amazon’s ability to continue functioning as a global carbon sink under shifting atmospheric conditions. The Amazon rainforest is a climatic linchpin within the Earth system, due to its enormous carbon storage capacity and its influence on hydrological and weather patterns. However, more than half of this forest floor is rooted in highly weathered, nutrient-poor soils that challenge plant growth and nutrient cycling. Specifically, phosphorus—a key mineral nutrient—exists only in scarce quantities in many parts of these old soils. Dr. Lucia Fuchslueger, co-lead author and researcher at the University of Vienna, explains that phosphorus scarcity could impede the forest’s ability to leverage elevated CO₂ for increased growth—a phenomenon known as CO₂ fertilization.
The Amazonian trees have, however, evolved remarkable adaptations to maximize nutrient acquisition despite these limitations. They engage in internal nutrient recycling by resorbing phosphorus and other nutrients from leaves before litter fall. The forest floor also hosts rapid microbial-driven organic matter decomposition, which can release nutrients for plant uptake. But whether these mechanisms can be further amplified in response to rising CO₂, and how sustainable such intensification might be, had remained unproven without rigorous experimental data from real forest settings.
To confront this challenge, the research team pioneered an innovative in situ experiment deploying open-top chambers (OTCs) within the Amazon rainforest understory. These chambers are constructed from transparent plexiglass, stand approximately three meters tall, and measure 2.5 meters in diameter. Their open design allows maintenance of natural temperature regimes and unaltered precipitation ingress—critical factors for preserving realistic understory conditions. Within these OTCs, atmospheric CO₂ concentrations were artificially elevated to simulate projected future conditions, enabling direct evaluation of tree physiological and ecological responses under elevated CO₂ in the complex tropical forest environment.
After one to two years of exposure, measurements confirmed that trees within these chambers exhibited increased carbon assimilation and growth rates, demonstrating the initial positive response to elevated CO₂. Their findings reveal that this growth stimulus is accompanied by a notable restructuring of root architecture, where trees extend root systems deeper and more extensively into the litter layer atop the soil in search of limiting nutrients, particularly phosphorus. This targeted root proliferation coincides with elevated production of enzymes designed to catalyze the breakdown of organic matter, liberating nutrients locked within the leaf litter before these elements diffuse into the less accessible mineral soil phases.
However, this nutrient acquisition strategy is not without ecological trade-offs. By intensifying nutrient scavenging from the litter, trees heighten competitive interactions with microbial decomposers, which play overlapping roles in nutrient mineralization. This dynamic could accelerate depletion of the finite organic phosphorus pool in the litter, potentially causing a gradual exhaustion of available nutrient resources critical to sustaining tree growth under elevated CO₂. Dr. Nathielly Martins, co-lead author from the Technical University of Munich and INPA, emphasizes that long-term nutrient depletion risks pose a significant constraint on the forest’s ability to continue storing carbon, despite the short-term growth enhancement observed.
The implications of these findings ripple beyond ecological theory into practical climate mitigation considerations. While tropical forests have been viewed as reliable carbon sinks capable of buffering anthropogenic emissions, this experiment reveals that nutrient limitation may undermine their resilience and carbon sequestration capacity in the long run. This caveat highlights the vulnerability of Amazonian forests under future climate scenarios characterized by elevated CO₂, nutrient constraints, and possibly exacerbated by other stressors such as drought and land-use changes.
Notably, this study functions as a pilot for the larger AmazonFACE project, an ambitious long-term, large-scale Free Air CO₂ Enrichment (FACE) experiment set to commence in the Amazon later this year. AmazonFACE aims to comprehensively examine tropical forest responses to elevated CO₂ within natural forest stands. It will provide unprecedented insights by deploying FACE technology in a biodiverse tropical forest context that has never before been replicated on such a scale. The project harnesses an interdisciplinary consortium of over 130 scientists, students, and professionals across 40 institutions worldwide, underscoring the international scope and critical importance of understanding tropical carbon dynamics.
FACE experiments in temperate and boreal forests have established foundational knowledge about ecosystem feedbacks to increased CO₂, but their transferability to tropical forests is uncertain due to stark differences in biodiversity, nutrient dynamics, and climatic regimes. AmazonFACE’s location—about 80 km north of Manaus within a typical terra firme, lowland rainforest—was chosen specifically to represent widespread Amazonian forest conditions, thereby ensuring relevance for continental-scale carbon cycle projections.
From a mechanistic perspective, the study conducted with the open-top chambers uncovers nuanced shifts in phosphorus acquisition strategies under higher CO₂ levels, revealing the forest’s capacity to modulate belowground nutrient foraging behavior. The enhanced root proliferation and enzymatic activity targeting the litter layer represent novel adaptive responses not previously documented in tropical forest ecosystems, emphasizing the complex interplay between plant physiology and soil biogeochemistry under climate change pressures.
Furthermore, this research contributes critical experimental data needed to parameterize and validate ecosystem and Earth system models tasked with predicting future carbon-climate feedbacks. Accurate representation of nutrient limitations in such models remains a key challenge, often oversimplified or omitted. Incorporating these new insights on phosphorus dynamics will refine predictions of tropical forest carbon storage trajectories, shaping more realistic climate mitigation scenarios.
The University of Vienna, with its remarkable heritage spanning over six centuries, continues to be a beacon of innovation and multidisciplinary research excellence. The Centre for Microbiology and Environmental Systems Science (CeMESS) exemplifies this spirit, uniting experts across microbiology, bioinformatics, ecology, and environmental geoscience to investigate planetary health through the lens of microorganism-environment interactions. This study embodies CeMESS’s commitment to addressing globally relevant environmental questions, combining molecular to ecosystem-level approaches.
This research is a tour de force showcasing how cutting-edge experimental design can unlock secrets of Earth’s most complex ecosystems. The interplay between rising atmospheric CO₂, nutrient cycling constraints, and forest productivity forms a delicate balance governing the future trajectory of the Amazon rainforest’s carbon sink function. As humanity grapples with escalating climate challenges, understanding and preserving these natural systems’ resilience emerges as a top priority. Ongoing and future research stemming from this work will be pivotal in guiding policy and conservation efforts, ensuring that tropical forests remain vital allies in climate mitigation for decades to come.
Subject of Research: Amazonian understory forests’ phosphorus acquisition strategies under elevated atmospheric CO₂ concentrations.
Article Title: Amazonian understory forests change phosphorus acquisition strategies under elevated CO₂.
News Publication Date: 28-Apr-2026.
Web References: http://dx.doi.org/10.1038/s41467-026-72098-0
Image Credits: Dado Galdieri.
Keywords: Amazon rainforest, tropical forests, elevated CO₂, phosphorus limitation, carbon sequestration, open-top chambers, nutrient cycling, AmazonFACE, climate change, tropical carbon sink, root adaptation, organic matter decomposition.
