The purpose of the Chronic Nitrogen Amendment is to increase our understanding of ecosystem nitrogen dynamics in response to elevated nitrogen inputs. Plots were established in 1988 as part of the National Science Foundation Long-Term Ecological Research program, at Harvard Forest. The plots are maintained by The Forest Ecosystems Studies Group at the University of New Hampshire’s Earth Systems Research Center.  See their site for details of the study. In 1991 the Nadelhoffer lab applied 15N tracers to the Chronic N Amendment and has subsequently followed the fate of the tracer.