Towards a Representation of Flexible Canopy N Stiochiometry for Land-surface Models Based on Optimality Concepts

Abstract:

Foliar nitrogen (N) is know to acclimate to environmental conditions. One particular pertinent response is the general decline in foliar N following exposure to elevated levels of atmospheric CO₂ (eCO₂). Associated with reduced foliar N is an increased plant nitrogen-use efficiency, which contributes to the plants' sustained growth response to eCO₂ in the absence of any counteracting litter N feedbacks. Flexible leaf N thus has important consequences for the mid- to long-term response of terrestrial ecosystems to eCO₂.

The current generation of land-surface models including a prognostic N cycle generally employ heuristic, and simply mass-balancing parameterisations to estimate changes in stoichiometry given altered N and carbon (C) availability. This generation generally and substantially overestimates the decline of foliar N (and thus the increase in plant nitrogen use efficiency) observed in Free Air CO₂ Enrichment Experiments (FACE; Zaehle et al. 2014). In this presentation, I develop a simple, prognostic and dynamic representation of flexible foliar N for use in land-surface models by maximising the marginal gain of net assimilation with respect to the energy investment to generate foliar area and foliar N. I elucidate the underlying assumptions required to simulate the commonly observed decline in foliar N with eCO₂ under different scenarios of N availability (Feng et al. 2015).

References:

Zaehle, Sönke, Belinda E Medlyn, Martin G De Kauwe, Anthony P Walker, Michael C Dietze, Hickler Thomas, Yiqi Luo, et al. 2014. “Evaluation of 11 Terrestrial Carbon-Nitrogen Cycle Models Against Observations From Two Temperate Free-Air CO2 Enrichment Studies.” New Phytologist 202 (3): 803–22. doi:10.1111/nph.12697.

Feng, Zhaozhong, Tobias RUtting, Håkan Pleijel, GORAN WALLIN, Peter B Reich, Claudia I Kammann, Paul C D Newton, Kazuhiko Kobayashi, Yunjian Luo, and Johan Uddling. 2015. “Constraints to Nitrogen Acquisition of Terrestrial Plants Under Elevated CO 2.” Global Change Biology 21 (8): 3152–68. doi:10.1111/gcb.12938.