The role of phosphorus in limiting Amazon rainforest productivity under global environmental change

Despite being suggested for more than 20 years the existence, magnitude and duration of a supposed CO2-fertilization effect in tropical forests, a process of utter importance for the resilience of tropical forests and maintenance of the global carbon cycle, still remains largely undetermined. The vegetation response to elevated atmospheric CO2 is poorly constrained for tropical ecosystems due to a lack of direct experimental evidence. It is assumed that a potential growth limitation by phosphorus (P) plays an important role. The low nutrient availability, in particular P, is might even be a driver of an observed decline in the Amazon carbon sink. Only few global land-surface models include P cycle dynamics so far as development has been hampered by lack of observations and limited process understanding. The goal of the proposed project is therefore to evaluate the role of P cycling in tropical rainforest productivity under global environmental change. We propose to develop a detailed P cycle sub-model for the well-established dynamic vegetation model LPJ-GUESS. The advantage of this model is that it accounts for above- and belowground biogeochemical cycles (C, N) which makes it suitable for a straightforward implementation of the P cycle. The model also considers detailed processes of vegetation dynamics and succession, which are assumed to be important drivers for Amazonian biomass change over time and variability across space. For model development, existing model representations will be combined with novel modelling approaches. New observation data from the AmazonFACE (Amazon Free Air CO2 Enrichment) und AFEX (Amazon Fertilization Experiment) experiments as well as existing data from measurements throughout the Amazon basin will be available from Brazilian partners and aid in the parametrization and evaluation of the model. Different process formulations and plant growth strategies, in particular for the acquisition of P by plants will be tested and employed to derive hypotheses of ecosystem functioning under a changing climate and elevated atmospheric CO2 concentration. Model-based hypotheses will form the basis for guidance of field measurements of the AmazonFACE experiment and will lead to new insights into governing drivers of forest ecosystem functioning under global change in Amazonia.