Rising CO2 and warming reduce global canopy demand for nitrogen

Nitrogen (N) limitation has been considered as a constraint on terrestrial carbon uptake in response to rising CO₂ and climate change. By extension, it has been suggested that declining carboxylation capacity (V_cmax) and leaf N content in enhanced-CO₂­ experiments and satellite records signify increasing N limitation of primary production. We estimated changes in V_cmax andbased on optimality principles over decades, and the changes in leaf-level photosynthetic N assuming proportionality with leaf-level V_cmax at 25˚C. using satellite-based and predicted , and converted to annual N demand using estimated leaf turnover times.The predicted spatial pattern of V_cmax shares key features with an independent reconstruction based on remotely-sensed leaf chlorophyll content. Leaf-level responses to rising CO₂, and to a lesser extent temperature, may have reduced the canopy requirement for N by more than greening has increased it. Our finding provides an alternative explanation for declining N that does not depend on increasing N limitation, also could use as evidence for recently increasing N limitation on primary production.