Hidden phosphorus recycling sustains Amazon growth amid rising CO₂ levels 

The Amazon rainforest may be able to sustain its important role as a natural CO₂ sink for longer than previously thought. Research by scientists from Vrije Universiteit Amsterdam, Max Planck Institute for Biogeochemistry and others shows that trees in large parts of the rainforest are becoming increasingly efficient in utilizing the scarce nutrient phosphorus.

This hidden belowground recycling helps the forest continue to grow, but also makes the ecosystem potentially more vulnerable to climate change.

The study, published in Global Change Biology, builds on recent experiments that demonstrated that Amazon trees adapt their root strategies when CO₂ concentrations in the atmosphere rise. “We wanted to know what happens when these adaptations continue for decades and occur on the scale of the entire Amazon rainforest,” says first author systems ecologist Katrin Fleischer.

Phosphorus a crucial factor

Using an advanced ecosystem model developed by the authors at Max Planck Institute for Biogeochemistry, the researchers simulated how phosphorus moves between soil, microorganisms, and plants. Unlike most biosphere models, this model explicitly takes soil microorganisms and the phosphorus cycle into account - two crucial factors for the growth of tropical forests. The results show that not all forest ecosystems in the Amazon Basin respond the same way. Forests on phosphorus-rich soils can largely utilize existing stocks in the soil. In phosphorus-poor areas, something else happens: there, trees invest more in phosphorus acquisition, and soil microorganisms work harder to release phosphorus from organic material. As a result, the same phosphorus is recycled over and over again.

“In the most nutrient-poor forests, an amount of phosphorus equal to the total phosphorus stock of the ecosystem is fully recycled on average once every three years. This rapid cycle makes it possible for the forests to continue growing despite a lack of new nutrient inputs,” says co-author Lin Yu of the University of Hamburg. However, this increased uptake comes at a cost, as trees have to invest more into fast-living root systems to acquire phosphorus. „This means that for the same amount of carbon uptake by trees, less becomes stored in aboveground biomass, thus limiting long-term carbon storage“, adds Prof. Sönke Zaehle, senior author of the study and director at the MaxPlanck Instutute for Biogeochemistry.

Climate disturbances are a problem

Because rising CO₂ in the atmosphere means that these forest ecosystems are becoming increasingly dependent on efficient phosphorus recycling, disturbances such as prolonged drought or other climate extremes can disrupt the cycle. If nutrients return to the trees more slowly or less efficiently, forests could become more vulnerable to stress, ultimately reducing their ability to take up CO₂. The study highlights that the future of the Amazon carbon sink depends not only on rising CO₂, but also on the forest's ability to keep nutrients moving through the ecosystem. Understanding these hidden belowground processes is essential for predicting how tropical forests will respond to climate change.

The next step is to test the model predictions against measurements from the AmazonFACE experiment, in which parts of the Amazon forest are exposed to elevated CO₂ concentrations over many years. The experiment will help reveal whether these belowground recycling mechanisms can continue to sustain forest growth under rising CO₂ and increasingly frequent climate extremes, such as drought.

[This press release was adopted based on permission by VU Amsterdam]