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 CO₂and climate change. By extension, it has been suggested that declining carboxylation capacity (<em>V</em>_cmax) and leaf N content in enhanced-CO₂­ experiments and satellite records signify increasing N limitation of primary production. We predicted <em>V</em>_cmaxusing the coordination hypothesis, and estimated changes in leaf-level photosynthetic N for 1982–2016 assuming proportionality with leaf-level <em>V</em>_cmax at 25˚C. Whole-canopy photosynthetic N waas derived using satellite-based leaf area index (LAI) data and an empirical extinction coefficient for <em>V</em>_cmax, and converted to annual N demand using estimated leaf turnover times. The predicted spatial pattern of <em>V</em>_cmaxshares key features with an independent reconstruction from remotely-sensed leaf chlorophyll content. Predicted leaf photosynthetic N declined by 0.28 %/year, while observed leaf (total) N declined by 0.2–0.25 %/year. Predicted global canopy N (and N demand) declined from 1997 onwards, despite increasing LAI. Leaf-level responses to rising CO₂, and to a lesser extent temperature, may have reduced the canopy requirement for N by more than rising LAI has increased it. This finding provides an alternative explanation for declining leaf N that does not depend on increasing N limitation.