Litterfall 15 N abundance indicates declining soil nitrogen availability in a free‐air CO 2 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 CO 2 enrichment (FACE) experiment, the response of sweetgum ( Liquidambar styraciflua L.) productivity to elevated CO 2 concentrations [CO 2 ] 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 15 N abundance in archived samples of freshly fallen leaf litterfall. Litterfall δ 15 N declined from 1998 to 2005, and the rate of decline was significantly faster in elevated [CO 2 ]. Declining leaf litterfall δ 15 N 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 δ 15 N 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 CO 2 concentrations.