Reconstructing terrestrial nitrogen cycling on century to millennial timescales in temperate forested ecosystems

Background/Question/Methods The long-term (century to millennial scale) record of nitrogen availability in forests of the northern United States is largely unknown. A multitude of anthropogenic changes are currently affecting temperate forests in North America, including nitrogen deposition, fire suppression, and climate change. Although these drivers are likely to affect terrestrial nitrogen cycling, it is still uncertain whether terrestrial nitrogen availability has changed during the past few decades and more generally how forest ecosystems have responded to environmental change in the past. The objective of this study was to reconstruct terrestrial nitrogen cycling from lacustrine and dendrochronological records in two different temperate forests: a mixed hardwood and a coniferous forest. Results/Conclusions Here, we reconstructed changes in vegetation and other ecosystem characteristics using a millennial-scale lacustrine sediment record and high-intensity sampling of wood from living trees in the watershed of Mirror Lake, New Hampshire, USA. Our results indicate that nitrogen availability in that watershed has declined over the past 75 years, likely due to ecosystem recovery from Euro-American land use. We compare these results with a complete Holocene record of stable nitrogen isotopes in sediments from Deming Lake, Minnesota, USA. Significant changes in terrestrial nitrogen cycling occurred at both sites prior to the onset of Euro-American land use. We interpret presettlement trajectories of ecosystem change in the watershed of the two lakes within the context of ecosystem development theory and modern anthropogenic change.