Simulated CO₂ Fertilization Drives Progressive Phosphorus Limitation via Accelerated Organic Cycling in Amazonian...

Tropical forest responses to increasing atmospheric CO₂ (iCO₂) depend on nutrient constraints, with phosphorus emerging as a central limiting factor. However, current global land models rarely include explicit representations of microbial-driven phosphorus cycling, which is critical in highly weathered tropical soils. Here, we use the QUINCY land surface model coupled with the microbial-explicit Jena Soil Model (JSM) to simulate coupled carbon-nutrient dynamics in Amazon forests along a natural gradient in soil phosphorus availability (40-312 g P m^-2). The model reproduced observed patterns of increasing wood production with soil phosphorus and identified a threshold of ~80 g P m^-2 in the topsoil below which plants experienced phosphorus limitation, shifting carbon allocation belowground and reducing foliar nitrogen : phosphorus ratios. Across all sites, microbial mineralization of organic phosphorus supplied the greatest share of soil solution phosphorus, with stronger reliance on organic phosphorus cycling in low-phosphorus sites. The observed rise in atmospheric CO₂ between 1901 and 2019 increased simulated plant and soil carbon stocks by 23.3% and 1.5%, respectively. However, forests on low-phosphorus soils accumulated 40% less additional plant carbon under iCO₂ than forests on high-phosphorus sites. CO₂ stimulated greater fine-root carbon growth (+29.5%) and higher biochemical phosphorus mineralization (+24.5%) in low-phosphorus sites. These responses increased reliance on organic phosphorus cycling and depleted mineral-associated organic phosphorus pools. In contrast, phosphorus demand in high-phosphorus forests was largely met by inorganic soil phosphorus pools. Across the gradient, phosphorus turnover increased and phosphorus was reallocated from slow-turnover soil pools into living biomass, with the strongest shifts in low-phosphorus sites. Together, these responses indicate emerging progressive phosphorus limitation under iCO₂, characterized by increased belowground carbon allocation, greater reliance on organic phosphorus cycling, and increasing pressure on organic phosphorus pools through enhanced turnover that may not be sustained over time.