EQUILIBRATION OF THE TERRESTRIAL WATER, NITROGEN, AND CARBON CYCLES

Recent advances in biologically based ecosystem models of the coupled terrestrial, hydrological, carbon, and nutrient cycles have provided n ew perspectives on the terrestrial biosphere's behavior globally, over a range of time scales, We used the terrestrial ecosystem model Centu ry to examine relationships between carbon, nitrogen, and water dynami cs, The model, run to a quasi-steady-state, shows strong correlations between carbon, water, and nitrogen fluxes that lead to equilibration of water/energy and nitrogen limitation of net primary productivity. T his occurs because sis the water flux increases, the potentials for ca rbon uptake (photosynthesis), and inputs and losses of nitrogen, all i ncrease, As the flux of carbon increases, the amount of nitrogen that can be captured into organic matter and then recycled also increases, Because most plant-available nitrogen is derived from internal recycli ng, this latter process is critical to sustaining high productivity in environments where water and energy are plentiful. At steady-state, w ater/energy and nitrogen limitation ''equilibrate,'' but because the m ater, carbon, and nitrogen cycles have different response times, inclu sion of nitrogen cycling into ecosystem models adds behavior at longer time scales than in purely biophysical models, The tight correlations among nitrogen flu?ies with evapotranspiration implies that either cl imate change or changes to nitrogen inputs (from fertilization or air pollution) will have large and long-lived effects on both productivity and nitrogen losses through hydrological and tract gas pathways, Comp rehensive analyses of the role of ecosystems in the carbon cycle must consider mechanisms that arise from the interaction of the hydrologica l, carbon, and nutrient cycles in ecosystems.