Rising CO2 and warming reduce global canopy demand for nitrogen

• Published in: The New phytologist Vol. 235; no. 5; pp. 1692 - 1700
• Main Authors: Dong, Ning, Wright, Ian J., Chen, Jing M., Luo, Xiangzhong, Wang, Han, Keenan, Trevor F., Smith, Nicholas G., Prentice, Iain Colin
• Format: Journal Article
• Language: English
• Published: Lancaster Wiley Subscription Services, Inc 01.09.2022; Wiley
• Subjects: acclimation; Canopies; Canopy; Carbon dioxide; Carboxylation; Chlorophyll; Chlorophylls; Climate change; CO2 fertilization; coordination hypothesis; Demand; ENVIRONMENTAL SCIENCES; Extinction coefficient; Leaf area; Leaf area index; leaf chlorophyll; Nitrogen; nitrogen cycle; nitrogen demand; Photosynthesis; photosynthetic capacity; Plant cover; Plant Sciences; Primary production; Remote sensing; Satellites; Temperature requirements; Uptake
• ISSN: 0028-646X; 1469-8137; 1469-8137
• DOI: 10.1111/nph.18076

Summary 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 (Vcmax) and leaf N content in enhanced‐CO2 experiments and satellite records signify increasing N limitation of primary production. We predicted Vcmax using the coordination hypothesis and estimated changes in leaf‐level photosynthetic N for 1982–2016 assuming proportionality with leaf‐level Vcmax at 25°C. The whole‐canopy photosynthetic N was derived using satellite‐based leaf area index (LAI) data and an empirical extinction coefficient for Vcmax, and converted to annual N demand using estimated leaf turnover times. The predicted spatial pattern of Vcmax 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. See also the Commentary on this article by Smith, 235: 1683–1685.