Photosynthesis of forest around the world is constrained by phosphorus

A new study by an international research team lead by Prof. David Ellsworth from Western Sydney University in published in Nature Communication elucidates phosphorus constraints to photosynthesis in tropical forests and shows how accounting for them improves terrestrial biosphere models.

Tropical forests are hotspots for biodiversity and range among the most productive ecosystems on the planet. Tropical forests absorb more carbon from the atmosphere than any other biome, and are thus key modulators of global climate due their important role in the carbon cycle. The process by which plants absorb carbon from the atmosphere utilising light energy is called photosynthesis. Only about half of the carbon adsorbed is converted into biomass, while the remaining is released back to the atmosphere within a year by plant respiration. The understanding of photosynthesis and its environmental controls is pivotal for our capacity to predict the response of the biosphere to increasing atmospheric carbon dioxide concentrations and climate change, and therefore for predicting future climate change.

Based on a compilation of nearly 18,000 measurements of photosynthesis and additional data, and a true international effort involving 12 different countries, the research team performed the most comprehensive analysis to date of photosynthetic biochemistry across plant families. The analysis shows for the first time that phosphorus is a major constraint on the photosynthetic capacity of leaves worldwide: the higher the concentration of phosphorus in leaves, the higher their capacity to adsorb carbon dioxide. A finding which has far reaching implications.

Phosphorus is a major plant nutrient, and its availability in soils is often sub-optimal as illustrated by the positive response of plant growth to addition of phosphorus in the form of fertilisers. The team derived a mathematical formulation describing the link between photosynthesis and leaf phosphorus, and deployed it within one of the few global biosphere models which resolves phosphorus cycling. Using the model, they demonstrate how this factor can be accounted for in predictions of global photosynthesis, and show that tropical forests could adsorb much more carbon from the atmosphere if the ecosystems were richer in phosphorus than they actually are. "These new insights not only affect our understanding about the response of tropical forests to global change, but also open up possibilities to artificially enhance the productivity of tropical forests." says Daniel Goll who led the modelling work for this study.

Ellsworth, D.S., Crous, K.Y., De Kauwe, M.G. et al. Convergence in phosphorus constraints to photosynthesis in forests around the world. Nat Commun 13, 5005 (2022). https://doi.org/10.1038/s41467-022-32545-0