PARENT SESSION
Symposium 24: Progressive Nitrogen Limitation of Plant and Ecosystem Response to Rising CO₂
Organized by: Y Luo
Friday, August 8. 8:30 AM to 12:00 PM, SITCC Chatham Ballroom C.

Nitrogen limitation of ecosystem responses to rising atmospheric CO₂: Patterns and mechanisms.

Luo, Yiqi^*,1, NCEAS N working group, ², ¹ University of Oklahoma, Norman, OK² University of California, Santa Barbara, CA

ABSTRACT- One of the most controversial issues in global biogeochemistry is the regulation of terrestrial carbon (C) sequestration by nitrogen (N) availability. Such a controversy is translated to a great deal of uncertainties in predicted global terrestrial C sink in the next several decades. This talk will evaluate the C/N interactions under a new conceptual framework that is progressive N limitation (PNL). PNL takes place when available N in an ecosystem is allocated to long-lived biomass pools and/or immobilized to increased soil carbon stocks under elevated CO₂. Synthesis of experimental results from free-air CO₂ enrichment (FACE) and open-top chamber (OTC) studies suggests three types of ecosystem responses: no PNL, delayed PNL, and immediate N limitation. The immediate N limitation happens in ecosystems where elevated CO₂ could not stimulate plant biomass growth and generate no extra N demand. Delayed PNL occurs in ecosystems where CO₂ stimulation in biomass growth and soil C storage is initially strong and progressively declines. The delayed PNL is usually associated with short-term N stress alleviation mechanisms. Those mechanisms include N reallocation among different plant and soil pools, changes in plant and soil C/N ratios, and N mining via increased fine root growth. Those short-term mechanisms usually do not significantly increase total N stocks in an ecosystem but increase N use efficiency, supporting a positive ecosystem response to elevated CO₂ even if labile N availability in soil declines. However, these compensation mechanisms are not sustainable in the long term. The third type of ecosystems is that CO₂ stimulation of ecosystem productivity is sustained while no PNL is observed within the lifetime of a CO₂ experiment. The sustained CO₂ stimulation is likely supported by long-term N supply mechanisms. Such mechanisms balance extra N demand caused by additional long-lived biomass growth and increased soil C stocks under elevated CO₂.

Key words: biogeochemistry, global change, terrestrial ecosystems, modeling