Upsetting the balance of nature?

Study finds soil N key to plant response and rising CO₂

In CDR LTER’s BioCON experiment we grew 296 field plots (each 2m x 2m) containing different numbers and combinations of perennial grassland species under ambient and elevated atmospheric CO₂ and with either ambient or enriched soil N supply. In three elevated-CO₂ rings, a free-air CO₂ enrichment system was used during each growing season to maintain the CO₂ concentration at an average of 560 µmol mol-1, a concentration likely to be reached this century. Our results are consistent with the idea that although some models indicate a considerable capacity of land ecosystems to sequester large amounts of C in the coming century, this C accumulation is likely to be constrained over time by N availability. Photo: Peter Reich

Lack of nitrogen may make the earth’s terrestrial ecosystems a smaller carbon sink than previously thought. That is because the global nitrogen limitation in terrestrial ecosystems likely results in the earth’s vegetation not absorbing as great a share of the rising atmospheric carbon dioxide as some models have predicted, according to a study led by Peter B. Reich at the Cedar Creek LTER site. In April 2006 Reich, a professor in the Department of Forest Resources at the University of Minnesota, and researchers from six universities published a paper, "Nitrogen limitation constrains sustainability of ecosystem response to CO₂," in the journal Nature showing how plant communities facing nitrogen (N) supply limitation were also limited in their ability to absorb extra carbon dioxide (CO₂) in an elevated CO₂ environment. Given that a large proportion of the world’s soils are nitrogen-limited, the study suggested that the rate of increase in atmospheric CO₂ levels could speed up if nitrogen-limited plants lose their ability to take advantage of the extra CO₂.

The findings derive from an experiment during which researchers planted 16 different perennial grassland species, one, four, nine, or 16 species at a time, in 296 permanent field plots. They exposed the plots to ambient or elevated atmospheric carbon dioxide levels using a free-air CO₂ enrichment technique, and ambient or N-enriched soils. They sampled biomass (both aboveground and roots) in every plot twice a year, and after four to six years, noticed that plots receiving added nitrogen acquired three times as much extra carbon under higher carbon-dioxide conditions than did plots without any additional nitrogen supply. The presence of legumes, capable of converting atmospheric nitrogen gas into biologically usable forms of nitrogen, did not alter the responses of the plant communities. Plants in plots with and without legumes were statistically indistinguishable in their ability to increase their growth in response to elevated CO₂ levels. Nor did it matter whether the plots contained one, four, nine or 16 species of plants. The researchers concluded that soil nitrogen supply (in limiting response to CO₂) was paramount as it occurred across many different kinds of plant communities.

The 6-year study, which is supported by the National Science Foundation’s Biocomplexity Program, the U.S. Department of Energy, and LTER, is one of the few long-term research projects looking at how soil nitrogen affects the abilities of long-lived plants to absorb extra CO₂ in realistic "natural" open-air ecosystems. Only two other long-term experiments in the world have been asking similar questions for more than four years. The study is currently in its 9th field season and the investigators hope to be able to run the study for at least 12-15 years to assess long-term plant-soil feedbacks and the ways in which changing community composition may alter responses to CO₂. A very recent review of all studies of CO₂ x N interaction, published in the Annual Review of Ecology, Evolution and Systematics as "Carbon-Nitrogen Interactions in Terrestrial Ecosystems in Response to Rising Atmospheric CO₂" by Reich and colleagues Bruce Hungate and Yiqi Luo, found similar results, including in forests and agricultural crop systems, as in the perennial grasslands at Cedar Creek. Thus reports, including the Intergovernmental Panel on Climate Change’s "Climate Change 2001: The Scientific Basis," that predict that terrestrial plants will be a considerable "sink" for excess CO₂ are probably overly optimistic in this regard. Since rising atmospheric CO₂ levels are the largest cause of global "greenhouse" warming, this nitrogen interaction raises the possibility of accelerated global climate change.

References

Reich, PB , SE Hobbie, T Lee, DS Ellsworth, JB West, D Tilman, J Knops, S Naeem, J Trost. 2006. Nitrogen limitation constrains sustainability of ecosystem response to CO₂. Nature 440: 922-925.

Reich, PB, BA Hungate, Y Luo. 2006. Carbon-Nitrogen Interactions in Terrestrial Ecosystems in Response to Rising Atmospheric CO₂. Annual Review of Ecology, Evolution, and Systematics 37: 611-636.