Rising CO2 and warming reduce global canopy demand for nitrogen

Published January 24, 2022 | Version v1

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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 (V_cmax) and leaf N content in enhanced-CO₂ experiments and satellite records signify increasing N limitation of primary production.
We predicted V_cmaxusing the coordination hypothesis, and estimated changes in leaf-level photosynthetic N for 1982–2016 assuming proportionality with leaf-level V_cmax at 25˚C. Whole-canopy photosynthetic N waas 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_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.

Notes

data input.

Name	Size	Download all
chl_vcmax25.nc md5:e3ce552fd07309984caf4794c0f985fe	112.0 MB	Download