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Soil as a Sink for Nitrous Oxide?
Goldberg, S.D. and Gebauer, G. 2009. Drought turns a Central European Norway spruce forest soil from an N2O source to a transient N2O sink. Global Change Biology 15: 850-860.

According to the authors -- who cite the work of Rodhe (1990) -- nitrous oxide (N2O) has a "greenhouse effect per unit mass about 320 times stronger than CO2 on a 100-year time span." They also report that "the only sink for N2O considered in global models is the destruction of atmospheric N2O in the stratosphere through photolysis and photooxidation (IPCC, 2007)," and continuing to cite the most recent IPCC report, they say that "soils have been identified [therein] as the main sources [our italics] for atmospheric N2O."

So are these contentions of the IPCC really true? ... or do they fall just a little short of that lofty status?

What was done
Goldberg and Gebauer investigated the influence of drying and rewetting events on N2O emissions from the soil of a mature Norway spruce forest in Northeast Bavaria, Germany. This they did by inducing an artificial summer drought of 46 days duration (which was accompanied by a natural drought) via throughfall exclusion (TE) that was provided by special roof installations, which they followed with an experimental rewetting of 66 mm over two days, during which periods (and before and after them) they closely monitored N2O fluxes from the soils of the TE and unaltered control (C) plots that were exposed to the elements.

What was learned
"Before the drought," in the words of the two researchers, "both the C and TE plots showed slightly positive N2O fluxes from the soil to the atmosphere," in harmony with the sentiment of the IPCC. During the drought, on the other hand, they report that "the soil of both the throughfall exclusion and control plots served as an N2O sink," contrary to what might have been expected in light of IPPC statements; and they say that "the sink strength of the throughfall exclusion plots was doubled [our italics] compared with the control plots." Rewetting, however, "turned the soil into a source for atmospheric N2O again," but they found that "it took almost four months [our italics] to turn the cumulative soil N2O fluxes from negative (sink) to positive (source) values."

What it means
On the basis of their observations, Goldberg and Gebauer conclude that "long drought periods can lead to drastic decreases of N2O fluxes from soils to the atmosphere or may even turn forest soils temporarily to N2O sinks [our italics]," which situation may in some places persist for years at a time. It is also entirely possible, however, that over the entire terrestrial surface of the planet, the net result is indeed that "soils are the main sources for atmospheric N2O," as stated by the IPCC. Nevertheless, the two scientists conclude that the fact that there is, in fact, what they call an unbalanced global N2O budget "underlines the likelihood of a hitherto unconsidered sink function of soils."

IPCC. 2007. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Solomon, S., Qin, D., Manning, M., Marquis, M., Avery, K., Tignor, M.M.B., Le Roy Miller Jr., H. and Chen, Z. (Eds.), Cambridge University Press, Cambridge, UK, pp. 1-940.

Rodhe, H. 1990. A comparison of the contribution of various gases to the greenhouse effect. Science 248: 1217-1219.

Reviewed 15 July 2009