Tropical Forests on a Phosphorus Loop: Internal Recycling Regulates Carbon Sink Capacity and Vulnerability under Global Change

Phosphorus (P) availability has regulated terrestrial productivity over geological time, leaving a persistent imprint on ecosystem structure, biodiversity, and function. In tropical forests, strong soil P gradients are overcome by fine-tuned P use and acquisition strategies, enabling forests to maintain high productivity despite large differences in soil P supply. Rising atmospheric CO₂ increases biological P demand, making P constraints a central factor for tropical forest carbon sink capacity and a major source of uncertainty in future model projections. Climate change and human disturbances further disrupt plant–soil–microbial interactions and reconfigure P losses and recycling, raising questions about forest functioning and vulnerability.

I synthesize current understanding of tropical forest functioning across soil P gradients, focusing on the co-evolution of soil P pools, vegetation P acquisition strategies, and consequences for the forest carbon (C) cycle. Evidence on P acquisition spans “foraging” strategies in relatively P-rich systems to “mining” of less accessible P forms in highly weathered soils.

Building on this framework, I present model results showing that internal recycling of organic P pools plays a critical role in shaping carbon sink capacity and vulnerability under rising CO₂. Simulations with a terrestrial biosphere model across the Amazon reveal that CO₂ fertilization effects depend not only on background soil P, but also on the capacity of forest ecosystems to enhance enzyme-driven acquisition of rapidly recycled organic P, intensifying internal P recycling. This strategy occupies an intermediate position between foraging and mining, relying on carbon investment to increase turnover of actively cycling P. Such recycling may support short-term forest functioning while increasing sensitivity to P disruption and loss under global change.

Finally, I highlight how an upcoming CO₂ enrichment experiment in the Amazon will provide a unique opportunity to directly test these mechanisms and provide empirical constraints on how internal phosphorus recycling shapes tropical forest carbon sink capacity and vulnerability under global change.