Amazon Forest Response to CO2 Fertilization Dependent on Plant Phosphorus Acquisition
Global ecosystem models currently predict that the Amazon rainforest will continue to act as a carbon sink in the future due to rising atmospheric carbon dioxide (CO2) concentrations, effectively enhancing ecosystem resilience and reducing the pace of climate change. However, the uncertainty of future carbon uptake is still large.
It is widely accepted that soil phosphorus impoverishment in most of the Amazon basin limits biomass growth. However, the role of phosphorus availability in limiting future Amazon forest carbon uptake has not been considered in global ecosystem model ensembles, e.g., during the Coupled Model Intercomparison Project for the 5th Assessment Report of the United Nations Intergovernmental Panel on Climate Change (IPCC). Therefore, previous model projections agree on a sustained fertilization effect of CO2 on the Amazon C sink but have not considered feedbacks from low soil P availability. Our study demonstrates that, based on the current generation of CNP models, the omission of P feedbacks is highly likely to cause an overestimation of the Amazon rainforest’s capacity to sequester atmospheric CO2. By overestimating the CO2 fertilization effect on biomass C, models may inadvertently underestimate the vulnerability of the forest to higher temperatures and changing rainfall patterns.
We simulate the planned Free-Air CO2 Enrichment (FACE) experiment in the Amazon with an ecosystem model ensemble (n=14) including models accounting for nitrogen and phosphorus feedbacks. We show that incorporating a phosphorus limitation on productivity reduces CO2-induced biomass carbon sequestration to only 0.08 kg C m-2 yr-1 over 15 years, a reduction of 43-50% compared to carbon and carbon-nitrogen models. The model predictions vary due to contrasting assumptions regarding flexibility in plant phosphorus use and acquisition strategies that allow a CO2-induced biomass gain despite low phosphorus availability to plants.