Changes in nitrogen cycling during the past century in a northern hardwood forest.
Nitrogen (N) availability, defined here as the supply of N to terrestrial plants and soil microorganisms relative to their N demands, limits the productivity of many temperate zone forests and in part determines ecosystem carbon (C) content. Despite multidecadal monitoring of N in streams, the long-term record of N availability in forests of the northeastern United States is largely unknown. Therefore, although these forests have been receiving anthropogenic N deposition for the past few decades, it is still uncertain whether terrestrial N availability has changed during this time and, subsequently, whether forest ecosystems have responded to increased N deposition. Here, we used stable N isotopes in tree rings and lake sediments to demonstrate that N availability in a northeastern forest has declined over the past 75 years, likely because of ecosystem recovery from Euro-American land use. Forest N availability has only recently returned to levels forecast from presettlement trajectories, rendering the trajectory of future forest N cycling uncertain. Our results suggest that chronic disturbances caused by humans, especially logging and agriculture, are major drivers of terrestrial N cycling in forest ecosystems today, even a century after cessation.
McLauchlan
Kendra K
KK
Environmental Studies Program, Dartmouth College, 6182 Steele Hall, Hanover, NH 03755, USA. kendra.mclauchlan@dartmouth.edu
Craine
Joseph M
JM
Oswald
W Wyatt
WW
Leavitt
Peter R
PR
Likens
Gene E
GE
D017753:Ecosystem; D005618:Fresh Water; D019015:Geologic Sediments; D009518:New York; D009584:Nitrogen / Q000032:analysis / Q000378:metabolism*; D013997:Time Factors; D014197:Trees / Q000378:metabolism*; D014934:Wood / Q000378:metabolism*
D009584:Nitrogen