Declining Soil Nitrogen in a Free Air CO2-Enrichment Experiment (FACE)
The sustainability of higher ecosystem production under elevated atmospheric carbon dioxide (CO,2) is unknown. Nitrogen (N) is often limited in ecosystems as a result of N incorporation into long-lived biomass and soil organic matter. As a result, N limitation could eventually limit or nullify increasing forest productivity under elevated CO2 (i.e., “Progressive N Limitation”). In the first six years of the Oak Ridge National Laboratory (ORNL) FACE experiment, there was no apparent evidence that N limitation was exacerbated by elevated CO2 or that N limitation reduced sweetgum tree growth. However, the CO2 stimulation of sweetgum tree growth has more recently declined and was tentatively attributed to N limitation. Using stable N isotopes, temporal trends in sweetgum leaf litterfall 15N abundance provided strong evidence that N availability in the ORNL FACE plots has in fact declined over time, and declined faster in plots exposed to elevated CO2, providing evidence for progressive N limitation. Although these results cannot be generalized for other FACE sites, examination of leaf litterfall d15N may provide an accurate indicator of soil N availability and progressive N limitation.
Garten Jr., C. T., C. M. Iversen, and R. J. Norby. 2011. “Litterfall 15N Abundance Indicates Declining Soil Nitrogen Availability in a Free Air CO2 Enrichment Experiment,” Ecology 92, 133–39 [doi:10.1890/10-0293.1].