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Effects of CO2 and Drought on Isoprene Emissions from Agriforest Cottonwood Plantations
Pegoraro, E., Abrell, L., van Haren, J., Barron-Gafford, G., Grieve, K.A., Malhi, Y., Murthy, R. and Lin, G.  2005.  The effect of elevated atmospheric CO2 and drought on sources and sinks of isoprene in a temperate and tropical rainforest mesocosm.  Global Change Biology 11: 1234-1246.

As described by the authors, "isoprene is the most abundant volatile hydrocarbon emitted by many tree species and has a major impact on tropospheric chemistry, leading to formation of pollutants and enhancing the lifetime of methane, a powerful greenhouse gas."  Hence, it is important to determine if it will become more or less abundant as the air's CO2 content continues to rise, and whether its consumption by soil will increase or decrease.

What was done
Working at the Biosphere 2 facility near Oracle, Arizona, USA, in enclosed ultraviolet light-depleted mesocosms (to minimize isoprene depletion by atmospheric oxidative reactions such as those involving OH), Pegoraro et al. studied the effects of atmospheric CO2 enrichment (1200 ppm compared to an ambient concentration of 430 ppm) and drought on the emission of isoprene from cottonwood (Populus deltoides Bartr.) foliage and its absorption by the underlying soil for both well-watered and drought conditions.

What was learned
In the words of the researchers, "under well-watered conditions in the agriforest stands, gross isoprene production (i.e., the total production flux minus the soil uptake) was inhibited by elevated CO2 and the highest emission fluxes of isoprene were attained in the lowest CO2 treatment."  In more quantitative terms, the elevated CO2 treatment resulted in a 46% reduction in gross isoprene production.  It was also determined that drought suppressed the isoprene sink capacity of the soil beneath the trees, but that "the full sink capacity of dry soil was recovered within a few hours upon rewetting."

What it means
Putting a slightly negative slant on their findings, Pegoraro et al. suggest that "in future, potentially hotter, drier environments, higher CO2 may not mitigate isoprene emission as much as previously suggested."  (See Isoprene in our Subject Index for reviews of earlier work on this subject.)  However, we note that climate models generally predict an intensification of the hydrologic cycle in response to rising atmospheric CO2 concentrations, and that the anti-transpirant effect of atmospheric CO2 enrichment typically leads to increases in the moisture contents of soils beneath vegetation (see Soil Water Status Field Studies and Growth Chamber Studies in our Subject Index).  Also, we note that over the latter decades of the 20th century, when climate alarmists claim the earth has warmed at a rate and to a level that is unprecedented over the past two millennia, soil moisture data from all around the world tend to display upward trendsRobock et al. (2000), for example, developed a massive collection of soil moisture data from over 600 stations spread across a variety of climatic regimes, including the former Soviet Union, China, Mongolia, India and the United States, determining that "in contrast to predictions of summer desiccation with increasing temperatures, for the stations with the longest records, summer soil moisture in the top 1 m has increased while temperatures have risen."  And in a subsequent study of "45 years of gravimetrically-observed plant available soil moisture for the top 1 m of soil, observed every 10 days for April-October for 141 stations from fields with either winter or spring cereals from the Ukraine for 1958-2002," Robock et al. (2005) say "the observations show a positive soil moisture trend for the entire period of observation," noting that "even though for the entire period there is a small upward trend in temperature and a downward trend in summer precipitation, the soil moisture still has an upward trend for both winter and summer cereals."  Consequently, in a CO2-enriched world of the future, we likely will have the best of both aspects of isoprene activity: less production by vegetation and more consumption by soils.

Robock, A., Mu, M., Vinnikov, K., Trofimova, I.V. and Adamenko, T.I.  2005.  Forty-five years of observed soil moisture in the Ukraine: No summer desiccation (yet).  Geophysical Research Letters 32: 10.1029/2004GL021914.

Robock, A., Vinnikov, K.Y., Srinivasan, G., Entin, J.K., Hollinger, S.E., Speranskaya, N.A., Liu, S. and Namkhai, A.  2000.  The global soil moisture data bank.  Bulletin of the American Meteorological Society 81: 1281-1299.

Reviewed 5 October 2005