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 predicted V_cmax using the coordination hypothesis and estimated changes in leaf-level photosynthetic N for 1982–2016 assuming proportionality with leaf-level V_cmax at 25°C. The whole-canopy photosynthetic N was derived using satellite-based leaf area index (LAI) data and an empirical extinction coefficient for V_cmax, 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 from remotely sensed leaf chlorophyll content. Predicted leaf photosynthetic N declined by 0.27% yr⁻¹, while observed leaf (total) N declined by 0.2–0.25% yr⁻¹. Predicted global canopy N (and N demand) declined from 1996 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.