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

Area of Science:

• Ecology
• Biogeochemistry
• Climate Change Science

Background:

• Tropical forest ecosystems are sensitive to nutrient availability, particularly phosphorus, which is critical in weathered soils.
• Global land models often lack detailed microbial phosphorus cycling, limiting accurate simulation of tropical forest responses.
• Increasing atmospheric CO2 (iCO2) impacts forest carbon dynamics, but nutrient constraints, especially phosphorus, mediate this response.

Purpose of the Study:

• To investigate the role of microbial phosphorus cycling in Amazonian forests under varying soil phosphorus availability and increasing atmospheric CO2.
• To simulate coupled carbon-nutrient dynamics using a land surface model integrated with a microbial soil model.
• To identify phosphorus limitation thresholds and their impact on forest carbon allocation and nutrient stoichiometry.

Main Methods:

• Utilized the QUINCY land surface model coupled with the Jena Soil Model (JSM) for microbial-explicit simulations.
• Simulated coupled carbon-nutrient dynamics in Amazonian forests across a natural gradient of soil phosphorus (40-312 g P m-2).
• Analyzed model outputs for changes in wood production, carbon allocation, foliar N:P ratios, and phosphorus cycling under ambient and elevated CO2.

Main Results:

• Model accurately reproduced increased wood production with soil phosphorus, identifying a threshold (~80 g P m-2) for phosphorus limitation.
• Microbial mineralization of organic phosphorus was the primary source of soil solution phosphorus, especially in low-P sites.
• Increasing atmospheric CO2 (iCO2) enhanced plant and soil carbon stocks, but forests on low-P soils accumulated 40% less plant carbon under iCO2.
• Low-P forests showed greater fine-root growth and biochemical P mineralization under iCO2, increasing reliance on organic P cycling and depleting soil organic P pools.
• Phosphorus turnover increased across the gradient, with reallocation from soil to biomass, particularly in low-P sites, indicating progressive P limitation.

Conclusions:

• Microbial phosphorus cycling is crucial for tropical forest function and response to iCO2, especially in phosphorus-limited systems.
• Emerging progressive phosphorus limitation under iCO2 is characterized by shifts in carbon allocation and increased pressure on organic phosphorus pools.
• The capacity of low-phosphorus tropical forests to sequester carbon under future CO2 levels may be constrained by phosphorus availability and cycling dynamics.