cades through the decomposition of wood or soil organic matter. Here we identify “equilibration” of the nitrogen budget with water/energy and carbon fluxes as an additional process causing decadal and longer time scale behavior.

Just as perturbations to the climate system can cause the nitrogen and carbon systems to respond ( 4 , 24 ), perturbations of the N inputs from air pollution or fertilization will also cause long-lived ecosystem changes ( 34 , 36 , 44 ). Analyses of the past interannual variability of the carbon cycle, and of its potential future behavior, must consider mechanisms that act through the coupled water/energy, carbon, and nitrogen cycles.

We acknowledge the assistance of Rebecca McKeown, Melannie Hartmann, and Hank Fisher with conducting and analyzing global Century runs, with special thanks to Becky for her exceptional effort in completing the global calculations for this paper. Dennis Ojima, Beth Holland, Alan Townsend, and Jason Neff all helped conceive of or design the model experiments. This research was supported by the National Aeronautics and Space Administration Earth Observing System Interdisciplinary Science Program, and by the National Center for Atmospheric Research. The National Center for Atmospheric Research is sponsored by the National Science Foundation.

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