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Controls on Fluxes of Trace Gases from Brazilian Cerrado Soils

Virginia Institute of Marine Science, College of William and Mary, P.O. Box 1346, Gloucester Point, VA 23062;

Mark A. Poth

USDA Forest Service, Pacific Southwest Res. Stn., 4955 Canyon Crest Drive, Riverside, CA 92507.

^* Corresponding author (iris{at}vims.edu).

ABSTRACT

Tropical ecosystems play an important role in production or consumption of atmospheric trace gases including nitric oxide (NO), nitrous oxide (N₂O), carbon dioxide (CO₂), and methane (CH₄). Here we describe field and laboratory experiments, performed during 1994, to determine the influence of fire on processes responsible for fluxes of gases from cerrado sites burned 17 and 45 d earlier, and a control site, last burned in 1974. Burning stimulated gross N mineralization but depressed nitrification rates; however, rates were sufficient to support NO fluxes observed in a 1992 study at the same site. Extractable nutrients and fluxes of NO and N₂O from wetted and dry soils were measured prior to and for a 3-d period following burning. Over this period NO^–₂ declined to undetectable levels; NH⁺₄ increased, and NO fluxes remained relatively constant, suggesting that nitrifiers replaced the NO^–₂ reduced to NO. Soils at burned and unburned sites exhibited CH₄ uptake, which was inhibited by CH₃F, thereby converting soils from a strong sink to a weak source of CH₄. Carbon dioxide fluxes did not increase, and there were no detectable fluxes of N₂O following burning. In lab studies NO and N₂O emissions were inhibited by autoclaving, suggesting that nitrification was key to their production. However, addition of NO^–₂ to autoclaved soil resulted in large fluxes of NO but no detectable N₂O, suggesting that chemodenitrification may have been responsible for NO but not N₂O production. Further research is needed to determine whether NO is produced directly by nitrifier NO^–₂ reduction or indirectly by chemodenitrification of NO^–₂ produced by nitrifiers.

VIMS Contribution no. 2115.

Received for publication May 28, 1997.

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