Plant phosphorus acquisition mechanisms in the Amazon rainforest: From model projections to field measurements.

Katrin Fleischer, Lin Yu and Sönke Zaehle, Department Biogeochemical Signals, Max Planck Institute for Biogeochemistry, Jena, Germany, Katrin Fleischer, Tatiana Reichert and Anja Rammig, Land Surface-Atmosphere Interactions, Technical University of Munich, Freising, Germany, Lucia Fuchslueger, Department of Microbiology & Ecosystem Science, University of Vienna, Austria, Laynara F. Lugli and Carlos Alberto Quesada, Coordination of Environmental Dynamics, National Institute of Amazonian Research (INPA), Manaus, Brazil, Lin Yu, Centre for Environmental and Climate Research, Lund University, Lund, Sweden

Background/Question/Methods
The intact Amazon rainforest acts as a sink of atmospheric carbon dioxide (CO2), according to long-term plot measurements of tree growth. Earth system models generally project a continuation of the carbon sink into the 21st century, predominantly driven by CO2 fertilization, counterbalancing carbon losses induced by higher temperatures. Advancing empirical evidence points to nutrient constraints on the Amazon carbon sink, foremost by phosphorus and essential cations. Accounting for nutrients in process-based ecosystem models strongly reduces the projected strength of the carbon sink.
It is uncertain to what extent nutrients limit plant productivity and will diminish elevated CO2-induced carbon sink in the future. Plant-based mechanisms may upregulate phosphorus supply under eCO2, partly meeting increased demand for nutrients. To which degree, however, remains speculative, since no experimental evidence on the response of mature tropical trees to eCO2 exists, to date. To move forward, we employ state-of-the-art ecosystem models to simulate phosphorus feedbacks on the eCO2 response in a mature tropical forest and conduct a synthesis of the current literature on the dominant phosphorus acquisition mechanisms in the Amazon basin.
Results/Conclusions
Simulations from process-based model ensemble, carried out for the AmazonFACE project, underscores the importance of increased plant carbon investment in fine roots and belowground carbon export, as well changes in plant tissue stoichiometry to overcome limitation induced by eCO21. The nutrient-enabled models embed different assumptions on phosphorus feedbacks to plant growth, projecting a lower carbon biomass growth to eCO2 than carbon-only models, but diverge significantly among themselves. The ensemble thus derives directly testable hypotheses on these diverging carbon-phosphorus feedbacks.
The scientific literature on field observations and manipulation experiments reveals our current understanding of dominant plant-based phosphorus acquisition mechanisms in the Amazon rainforest2 including (1) leaf P resorption, (2) fine-root P foraging, (3) arbuscular mycorrhizal symbiosis, (4) root phosphatase production and (5) low-molecular-weight organic-acids exudation. Empirical evidence on how plant phosphorus acquisition strategies vary with soil phosphorus availability and changing nutrient regimes in manipulation experiments allows constraining their importance along soil fertility gradient in the Amazon, as well as their potential plasticity. Based on this synthesis, a framework of phosphorus acquisition mechanisms in the Amazon basin provides a roadmap for future model development and field observations. • Fleischer, K., et al. Amazon forest response to CO2 fertilization depend on plant phosphorus acquisition. Nat. Geosci. (2019).
• Reichert, T., et al. Plant phosphorus use and acquisition strategies in the Amazon rainforest. in prep (2021).