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<p>Nitrogen (N) limitation constrains the magnitude of terrestrial carbon uptake in response to CO<sub>2&#160;</sub>fertilization and climate change. However, the trajectory of N demand, and how it is influenced by continuing changes in CO<sub>2&#160;</sub>and climate, is incompletely understood. We estimate recent changes in global canopy N demand based on a well-tested optimality hypothesis for the control of photosynthetic capacity (<em>V</em><sub>cmax</sub>). The predicted global pattern of optimal leaf-level <em>V</em><sub>cmax&#160;</sub>is similar to the pattern derived from remotely sensed chlorophyll retrievals. Over the period from 1982 to 2015, rising CO<sub>2&#173; </sub>and warming both contributed to decreasing leaf-level N demand. Widespread increases in green vegetation cover over the same period (especially in high latitudes) imply increasing total canopy N demand. The net global trend is, nonetheless, a decrease in total canopy N demand. This work provides a new perspective on the past, present and future of the global terrestrial N cycle.</p>
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Leaf-level photosynthetic capacity dynamics in relation to soil and foliar nutrients along forest–savanna boundaries in Ghana and Brazil
