Changes in ecosystem productivity and carbon sequestration in response to changes in climate and atmospheric CO2 concentration will be tightly constrained by the availability of nutrients like nitrogen and phosphorus. From the perspective of these constraints, it is informative to analyze changes in productivity and carbon sequestration based on ecosystem stoichiometry. In particular, changes in carbon storage must be accompanied by at least one of the following changes in the chemical properties of ecosystems:
(1) A change in the total nutrient capital of the ecosystem. Regions with large nutrient inputs (e.g., acid rain) would be expected to increase in productivity and carbon sequestration. Alternatively, areas where disturbance has resulted in major losses of nutrients through leaching would be expected to decrease productivity and carbon sequestration.
(2) A change in the carbon-to-nutrient ratio of ecosystem components. An often observed response to increased CO2 concentration is an increase in the carbon-to-nitrogen ratio of plant tissues.
(3) A redistribution of nutrients among ecosystem components that differ in their carbon-to-nutrient ratios. An increase in temperature should increase the turnover of soil organic matter. If the nutrients released as a result of this increased turnover are taken up and incorporated into plant tissues, then carbon sequestration by the whole ecosystem will increase because the carbon-to-nutrient ratios of plant tissues are generally much higher than those of soil organic matter.
The effects of changes in nutrient capital and of changes in carbon-to-nutrient ratios on productivity and carbon sequestration are well documented and can be demonstrated in relatively short-term (decade or less) experiments. However, the effects of redistributing nutrients between soils and vegetation is not yet fully appreciated. Because this redistribution involves the slow turnover of heartwood and soil organic matter, the effects on carbon storage can take many decades to become fully manifested. Thus, this important interaction between carbon storage and nutrient availability is impossible to test using short-term experiments. Nevertheless, our models indicate that the redistribution of nutrients can dominate the long-term responses of terrestrial ecosystems to changes in climate and CO2 concentration.
Key words: CO2 exchange, carbon-nutrient interactions, acclimation, carbon sequestration, climate change, CO2 response.
Correspondence: Edward B. Rastetter, The Ecosystem Center, Marine Biological Laboratory, Woods Hole, Massachusetts, U.S.A. 02543
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E-mail: erastett@lupine.mbl.edu