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Effects of Drought on Trace Gas Emissions from the Soil of an Amazon Forest
Davidson, E.A., Ishida, F.Y. and Nepstad, D.C.  2004.  Effects of an experimental drought on soil emissions of carbon dioxide, methane, nitrous oxide, and nitric oxide in a moist tropical forest.  Global Change Biology 10: 718-730.

The authors say the climate of the Amazon Basin may become gradually drier due to the intensification of a number of different phenomena, including (1) less recirculation of water between the increasingly-deforested region and the atmosphere, (2) more rainfall inhibition by smoke caused by increased biomass burning, and (3) a warming-induced increase in the frequency and/or intensity of El Niņo events that have historically brought severe drought to the eastern Amazon Basin (Nepstad et al., 1999; but see Timmermann et al., 1999 as well).  Driven by concern about these potential problems, they devised an experiment to determine the consequences of the drying of the soil of an Amazonian moist tropical forest for the net surface-to-air fluxes of two important greenhouse gases: nitrous oxide (N2O) and methane (CH4).

What was done
In the Tapajos National Forest near Santarem, Brazil, Davidson et al. modified a one-hectare plot of land covered by mature evergreen trees so as to dramatically reduce the amount of rain that reached the forest floor - "throughfall" - while maintaining an otherwise similar one-hectare plot of land as a control for comparison.  Prior to making this modification, they measured the gas exchange characteristics of the two plots for a period of 18 months; then, after initiating the throughfall-exclusion treatment, they continued their measurements for an additional three years.

What was learned
The scientists report that the "drier soil conditions caused by throughfall exclusion inhibited N2O and CH4 production and promoted CH4 consumption."  In fact, they say "the exclusion manipulation lowered annual N2O emissions by >40% and increased rates of consumption of atmospheric CH4 by a factor of >4," which results they attributed to the "direct effect of soil aeration on denitrification, methanogenesis, and methanotrophy."

What it means
If global warming did indeed increase the frequency and/or intensity of El Niņo events - which real-world data clearly indicate it does not (see El Niņo - Relationship to Global Warming in our Subject Index) - the results of this study suggest that the anticipated drying of the Amazon Basin would initiate a strong negative feedback via (1) large drying-induced reductions in the evolution of both N2O and CH4 from its soils and (2) a huge drying-induced increase in the consumption of CH4 by its soils.  Although the authors envisage a more extreme second phase response, "in which drought-induced plant mortality is followed by increased mineralization of C and N substrates from dead fine roots and by increased foraging of termites on dead coarse roots" (an extreme response that would be expected to increase N2O and CH4 emissions), we note that the projected rise in the air's CO2 content would likely prohibit such extreme events from ever occurring, in light of the documented tendency for atmospheric CO2 enrichment to greatly increase the water use efficiency of essentially all plants (see Water Use Efficiency in our Subject Index, inclusive of the subsection Trees), which would enable the forest to continue to flourish under significantly drier conditions than those of the present.

Nepstad, D.C., Verissimo, A., Alencar, A., Nobre, C., Lima, E., Lefebvre, P., Schlesinger, P., Potter, C., Moutinho, P., Mendoza, E., Cochrane, M. and Brooks, V.  1999.  Large-scale impoverishment of Amazonian forests by logging and fire.  Nature 398: 505-508.

Timmermann, A., Oberhuber, J., Bacher, A., Esch, M., Latif, M. and Roeckner, E.  1999.  Increased El Niņo frequency in a climate model forced by future greenhouse warming.  Nature 398: 694-696.

Reviewed 4 August 2004