CLIMATE AND NITROGEN CONTROLS ON THE GEOGRAPHY AND TIMESCALES OF TERRESTRIAL BIOGEOCHEMICAL CYCLING

We used the terrestrial ecosystem model ''Century'' to evaluate the re lative roles of water and nitrogen limitation of net primary productiv ity, spatially and in response to climate variability. Within ecology, there has been considerable confusion and controversy over the large scale significance of limitation of net primary production (NPP) by nu trients versus biophysical quantities (e.g., heat, water, and sunlight ) with considerable evidence supporting both views. The Century model, run to a quasi-steady state condition, predicts ''equilibration'' of water with nutrient limitation, because carbon fixation and nitrogen f luxes (inputs and losses) are controlled by water fluxes, and the capt ure of nitrogen into organic matter is governed by carbon fixation. Pa tterns in the coupled water, nitrogen, and carbon cycles are modified substantially by ecosystem type or species-specific controls over reso urce use efficiency (water and nitrogen used per unit NPP), detrital c hemistry, and soil water holding capacity. We also examined the coupli ng between water and nutrients during several temperature perturbation experiments. Model experiments forced by satellite-observed temperatu res suggest that climate anomalies can result in significant changes t o terrestrial carbon dynamics. The cooling associated with the Mount P inatubo eruption aerosol injection may have transiently increased terr estrial carbon storage. However, because processes in the water, carbo n, and nitrogen cycles have different response times, model behavior d uring the return to steady state following perturbation was complex an d extended for decades after 1- to 5-year perturbations. Thus conseque nces of climate anomalies are influenced by the climatic conditions of the preceding yeats, and climate-carbon correlations may not be simpl e to interpret.