Rising CO<sub>2</sub> and warming reduce global canopy demand for nitrogen
Ning Dong, Ian J. Wright, Jing M. Chen, Xiangzhong Luo, Han Wang, Trevor F. Keenan, Nicholas G. Smith, I. Colin Prentice
Abstract
Summary Nitrogen (N) limitation has been considered as a constraint on terrestrial carbon uptake in response to rising CO 2 and climate change. By extension, it has been suggested that declining carboxylation capacity ( V cmax ) and leaf N content in enhanced‐CO 2 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 CO 2 , 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.