Controls on Fluxes of Trace Gases from Brazilian Cerrado Soils

Abstract

AbstractTropical 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 NO2− declined to undetectable levels; NH+4 increased, and NO fluxes remained relatively constant, suggesting that nitrifiers replaced the NO2− 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 NO2− 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 NO2− reduction or indirectly by chemodenitrification of NO2− produced by nitrifiers.