Abstract
Currently, forests in the northeastern United States are net sinks of atmospheric carbon. Under future climate change
scenarios, the combined effects of climate change and nitrogen deposition on soil decomposition, aboveground processes,
and the forest carbon balance remain unclear. We applied carbon stock, flux, and isotope data from field studies
at the Harvard forest, Massachusetts, to the ForCent model, which integrates above- and belowground processes.
The model was able to represent decadal-scale measurements in soil C stocks, mean residence times, fluxes, and
responses to a warming and N addition experiment. The calibrated model then simulated the longer term impacts of
warming and N deposition on the distribution of forest carbon stocks. For simulation to 2030, soil warming resulted
in a loss of soil organic matter (SOM), decreased allocation to belowground biomass, and gain of aboveground carbon,
primarily in large wood, with an overall small gain in total system carbon. Simulated nitrogen addition resulted
in a small increase in belowground carbon pools, but a large increase in aboveground large wood pools, resulting in
a substantial increase in total system carbon. Combined warming and nitrogen addition simulations showed a net
gain in total system carbon, predominately in the aboveground carbon pools, but offset somewhat by losses in SOM.
Hence, the impact of continuation of anthropogenic N deposition on the hardwood forests of the northeastern United
States may exceed the impact of warming in terms of total ecosystem carbon stocks. However, it should be cautioned
that these simulations do not include some climate-related processes, different responses from changing tree species
composition. Despite uncertainties, this effort is among the first to use decadal-scale observations of soil carbon
dynamics and results of multifactor manipulations to calibrate a model that can project integrated aboveground and
belowground responses to nitrogen and climate changes for subsequent decades
