Modeling the carbon costs of plant phosphorus acquisition in Amazonian forests

Plants growing in low phosphorus (P) soils, such as in the predominant soils of Amazonia, are believed to devote more energy to acquiring P through absorptive root production, symbionts, and root exudates than plants in more fertile soils. Accounting for these energy costs in vegetation models is essential, as underestimating carbon (C) allocation to nutrient acquisition may lead to overestimating plant biomass growth. We developed a quantitative model to test a theoretical framework of C costs of P acquisition across soil P gradients. The model considers four strategies: P foraging via absorptive roots and arbuscular mycorrhizal fungi and P mining via root exudation of phosphatases and organic acids. We used field observations (i.e., soil data, plant biomass production, and stoichiometry of different organs) from ten sites across Amazonia to calibrate the model and explore different scenarios of (i) experimental soil P addition and (ii) elevated atmospheric CO₂ concentrations (eCO₂). Our model reproduced expected trends in P-acquisition strategies, with plants increasingly investing in foraging strategies as soil soluble inorganic P (Pi) increases and increasingly investing in mining strategies as total P and less available P forms decrease. Relative investment in P acquisition was within observed ranges. Plants, on average and across all sites, invested the equivalent of 20.5% of their estimated total net primary production (NPP) in P acquisition. On average, plants allocated 15.3% of their NPP to P acquisition in the three most fertile sites, compared to 29.0% in the least fertile sites. C allocation to arbuscular mycorrhizas, phosphatases, and organic acids, which are not commonly measured components of total NPP, was up to 25.8% (16.9% on average) of the total NPP. We highlight the need for quantitative data on plant C allocation to P acquisition from the soil to strengthen further model development and future model projections.