Tracing Changes in Ecosystem Function under Elevated Carbon Dioxide Conditions

Responses of ecosystems to elevated levels of atmospheric carbon dioxide (CO₂) remain a critical uncertainty in global change research. Two key unknown factors are the fate of carbon newly incorporated by photosynthesis into various pools within the ecosystem and the extent to which elevated CO₂ is transferred to and sequestered in pools with long turnover times. The CO₂ used for enrichment in many experiments incorporates a dual isotopic tracer, in the sense that ratios of both the stable carbon-13 (¹³C) and the radioactive carbon-14 (¹⁴C) isotopes with respect to carbon-12 are different from the corresponding ratios in atmospheric CO₂. Here we review techniques for using ¹³C and ¹⁴C abundances to follow the fate of newly fixed carbon and to further our understanding of the turnover times of ecosystem carbon pools. We also discuss the application of nitrogen, oxygen, and hydrogen isotope analyses for tracing changes in the linkages between carbon, nitrogen, and water cycles under conditions of elevated CO₂.