SOIL FLUXES AND ATMOSPHERIC CONCENTRATION OF CO AND CH4 IN THE NORTHERN PART OF THE GUAYANA SHIELD, VENEZUELA

Soil fluxes and atmospheric concentrations of CH4 and CO were measured in a scrub grass savanna in Venezuela, during October 1988. Soil flux measurements were also made in a nearby semideciduous forest. Emission of CH4 was observed from the "dry" savanna soils; emissions do not show any significant trend with the time of day and have an average of 3.5 +/- 5.0 X 10(10) molecules cm-2 s-1. CO fluxes show a strong diurnal variation, with net emission during daytime and consumption during nighttime. Twenty-four-hour integrated soil fluxes give a small net emission of CO of 5.6 X 10(9) molecules cm-2 s-1; this result contrasts with previous net sink estimates given in the literature. Forest soils consume both CH4 and CO at rates of -5.0 X 10(10) and -2.4 X 10(11) molecules cm-2 s-1, respectively. Therefore, long-range forest degradation should produce an increase in the emission of CO and CH4 from these tropical soils. At the savanna site, higher atmospheric concentrations of CH4 and CO were observed during nighttime. Minimum concentrations, representative of boundary layer conditions, were observed during hours of high solar irradiation with values of 1.78 +/- 0.04 ppm and 224 +/- 61 ppb for CH4 and CO, respectively. Using the methane concentration increase inside the nocturnal mixing height, a total methane source of 4.9 X 10(11) molecules cm-2 s-1 from the surface was calculated, showing that a significant dispersed source, likely from termites and/or small tracts of flooded soils, is present in the tropical savanna region. Extrapolating this value to the world savanna regions, an emission of 61 Tg CH4 is estimated, this is about 10% of the global source of methane. From chamber measurements a distinct nighttime soil consumption of CO was observed; however, the concentration remained fairly constant during the night, indicating a surface source of CO of the order of 5 X 10(10) molecules cm-2 s-1. Even though anthropogenic sources cannot be ruled out completely, termites and vegetation emission are, most likely, the dispersed sources of CO in the savanna region.