Forest productivity increases in response to carbon dioxide (CO2) 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 δ15N declined from 1998 to 2005, and the rate of decline was significantly faster in elevated [CO2]. Declining leaf litterfall δ15N 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 δ15N 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.