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

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.