RESPONSES OF N-LIMITED ECOSYSTEMS TO INCREASED CO2 - A BALANCED-NUTRITION, COUPLED-ELEMENT-CYCLES MODEL

Ecosystem responses to increased CO, are often constrained by nutrient limitation. We present a model of multiple-element limitation (MEL) a nd use it to analyze constraints imposed by N on the responses to an i nstantaneous doubling of CO2 concentration in a 350-yr-old eastern dec iduous forest. We examine the effects of different exchange rates of i norganic N with sources and sinks external to the ecosystem (e,g., thr ough deposition and leaching) and different initial ratios of net:gros s N mineralization. Both of these factors influence the availability o f N to vegetation and, therefore, have important effects on ecosystem responses to increased CO2. We conclude that reliable assessments of e cosystem responses to CO2 will require a better understanding of both these factors. The responses to increased CO2 appear on at least four characteristic time scares. (1) There is an instantaneous increase in net primary production, which results in an increase in the vegetation C:N ratio. (2) On a time scale of a few years, the vegetation respond s by increasing uptake effort for available N (e.g., through increased allocation of biomass, energy, and enzymes to fine roots). (3) On a t ime scare of decades, there is a net movement of N from soil organic m atter to vegetation, which enables vegetation biomass to accumulate. ( 4) On the time scale of centuries, ecosystem responses are dominated b y increases in total ecosystem N, which enable organic matter to accum ulate in both vegetation and soils. In general, short-term responses a re markedly different from long-term responses.