Litterfall ¹⁵N abundance indicates declining soil nitrogen availability in a free-air CO₂ enrichment experiment

Forest productivity increases in response to carbon dioxide (CO ₂) enrichment of the atmosphere. However, in nitrogen-limited ecosystems, increased productivity may cause a decline in soil nitrogen (N) availability and induce a negative feedback on further enhancement of forest production. In a free-air CO₂ enrichment (FACE) experiment, the response of sweetgum (Liquidambar styraciflua L.) productivity to elevated CO₂ concentrations [CO₂] has declined over time, but documenting an associated change in soil N availability has been difficult. Here we assess the time history of soil N availability through analysis of natural ¹⁵N abundance in archived samples of freshly fallen leaf litterfall. Litterfall d¹⁵N declined from 1998 to 2005, and the rate of decline was significantly faster in elevated [CO₂]. Declining leaf litterfall d¹⁵N is indicative of a tighter ecosystem N cycle and more limited soil N availability. By integrating N availability over time and throughout the soil profile, temporal dynamics in leaf litterfall d¹⁵N provide a powerful tool for documenting changes in N availability and the critical feedbacks between C and N cycles that will control forest response to elevated atmospheric CO₂ concentrations.