Plasticity of nutrient mechanism acquisition in the Amazonian understory under elevated atmospheric carbon dioxide

Approximately 60% of the Amazon rainforest grows on highly weathered soils with low levels of rock-derived nutrients. In such nutrient-limited ecosystems, plant communities' functional capacity and adaptability to facilitate efficient nutrient acquisition is decisive for a potential CO2 fertilization effect. To address this uncertainty, we designed an in situ open-top chamber (OTC) experiment, elevating CO2 (eCO2) by 250 ppm in the understory of a highly phosphorus-limited mature Amazonian tropical forest in Manaus, Brazil. Our results showed that under eCO2, plants intensified root foraging in the litter layer, adopting a “do-it-yourself” strategy, increasing both root length and area by 328% and 217%, respectively. In contrast, roots in the soil followed an “outsourcing” strategy by doubling arbuscular mycorrhizal colonization. Furthermore, eCO2 enhanced direct biochemical phosphorus (P) mineralization in the litter layer by increasing P release by 11% without changing litter decomposition. This suggests that under the eCO2 understory, plants maximize nutrient acquisition and maintain stand-level biomass growth by tackling different P sources within the litter-soil continuum, switching from mineral soil to litter-derived organic P sources. This mechanism directly influences Amazonian rainforest C and nutrient cycling and its resilience to climate change.