Amazonian understory forests change phosphorus acquisition strategies under elevated CO2

The potential for the Amazon forest to continue functioning as a carbon (C) sink strongly depends on soil nutrient availability, particularly phosphorus (P), and on plants’ ability to adjust nutrient acquisition strategies. However, limited experimental evidence constrains the mechanistic representation of nutrient–carbon interactions in climate change models. We conducted an experiment in a P-depleted Amazonian understory forest, increasing atmospheric CO₂ in-situ by approximately 300 ppm using open-top chambers. Eight polypropylene open-top chambers (OTCs), each with a diameter of 2.4 m and a height of 3 m, were randomly installed in the forest understory. To prevent root proliferation and other effects from plants outside the OTCs, each chamber was surrounded by a circular soil trench measuring 30 cm in width and 50 cm in depth. The chambers were arranged in pairs, with four control chambers maintained at ambient CO2 (aCO2) and four elevated CO2 (eCO2) treatments, in which CO2 concentration was increased on average by ~ 300 ppm relative to the respective ambient OTC. Within the OTCs, we conducted various experiments to understand the effects of eCO2 on root parameters in the litter layer and in the soil, soil nutrient concentrations, microbial activity, and leaf litter decomposition. Our findings suggest that eCO₂ induced contrasting responses by roots along the litter–soil continuum that could facilitate nutrient uptake. In addition, intensifies the competition for P between plant roots and soil microorganisms, leading to changes in litter and soil P pools, exacerbating already strong nutrient constraints.