Nutrient constraints on the Amazon carbon sink: from field measurements to model projections

The Amazon rainforest faces immense pressures from human-induced deforestation and climate change and its future existence is largely indeterminate. Accurately projecting the forest’s response to future conditions, and thus preparing for the best possible outcome, requires a sound process-based understanding of its ecological and biogeochemical functioning. The intact forest acts as a sink of atmospheric carbon dioxide (CO₂), however, this invaluable function is slowing down for unclear reasons, according to long-term plot measurements of tree growth. Earth system models, on the other hand, assume a continuous sink of carbon into the 21^st century, predominantly driven by CO₂ fertilization, concurrently buffering against adverse effects by climate change. Advancing empirical and experimental evidence points to strong nutrient constraints on the Amazon carbon sink, foremostly by phosphorus and other cations, so that the projected strength of the future carbon sink is certainly unrealistic. It is highly uncertain, however, to which degree nutrients are and will diminish elevated CO₂-induced productivity, and to which extent plant-based mechanisms may upregulate phosphorus supply or optimize phosphorus use to facilitate the increasing demand by elevated CO₂. Site-scale ecosystem model ensemble analysis underscores the diverging hypotheses on phosphorus feedbacks we are currently facing. In addition, heterogeneous soil phosphorus availability across the Amazon basin, in combination with a hyperdiverse plant community, challenges current efforts to project phosphorus constraints on the future of the Amazon carbon sink. We here give an outlook of current progress and future research needs of model-experiment integration to tackle this pressing question.