Stable Isotope Constraints on Net Ecosystem Production Under Elevated CO₂

Net ecosystem production (NEP) is the total carbon accumulated in an ecosystem over a time interval, which is usually a year. The C is stored above- and below ground in plants, animals, and soils, and has residence times ranging from less than one to several hundred years or more. NEP varies in response to disturbance and during plant succession, and human-induced perturbations to climate and atmospheric constituents also affect NEP at local to global scales. Because NEP determines how much carbon dioxide is taken up from the atmosphere by terrestrial ecosystems, it is critical to evaluate what forcing factors might be regulating variations in NEP at various temporal and spatial scales. In particular, the effects of elevated atmospheric CO₂ may be altering NEP in many ecosystems, and techniques such as stable isotope analysis are improving researchers' ability to evaluate the magnitude of these effects.