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

Ecosystem responses to increased CO2 are often constrained by nutrient limitation. We present a model of multiple-element limitation (MEL) and use it to analyze constraints imposed by N on the responses to an instantaneous doubling of CO2 concentration in a 350-yr-old eastern deciduous forest. We examine the effects of different exchange rates of inorganic N with sources and sinks external to the ecosystem (e.g., through deposition and leaching) and different initial ratios of net:gross N mineralization. Both of these factors influence the availability of N to vegetation and, therefore, have important effects on ecosystem responses to increased CO2. We conclude that reliable assessments of ecosystem responses to CO2 will require a better understanding of both these factors. The responses to increased CO2 appear on at least four characteristic time scales. (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 responds by increasing uptake effort for available N (e.g., through increased allocation of biomass, energy, and enzymes to fine roots). (3) On a time scale of decades, there is a net movement of N from soil organic matter to vegetation, which enables vegetation biomass to accumulate. (4) On the time scale of centuries, ecosystem responses are dominated by increases in total ecosystem N, which enable organic matter to accumulate in both vegetation and soils. In general, short-term responses are markedly different from long-term responses.