CARBON-MONOXIDE FLUXES FROM NATURAL, MANAGED, OR CULTIVATED SAVANNA GRASSLANDS

As part of a comprehensive study on tropical land use change and its e ffect on atmospheric trace gas fluxes, we report the CO fluxes recorde d at a natural grassland site and the changes produced when this ecosy stem was managed or cultivated. The field site is located in the centr al part of the savannah climatic region of Venezuela. Fluxes were meas ured in the dark using the enclosed chamber technique. CO was analyzed with a reduction-gas detector in combination with a molecular sieve 5 A columm for CO separation, At all sites, CO fluxes exhibited a strong diurnal variation, with net emission during daytime and consumption o r no fluxes during nighttime. In unplowed soils no differences were ob served between dry and rainy season. A large disparity was observed be tween unplowed and plowed grassland soils. Plowed soil shows a much sm aller emission during daytime and a larger consumption at night. The 2 4-hour integrated fluxes indicate that the nonperturbed grassland swit ches from being a net source of CO (3.4 x 10(10) molecules cm-2 s-2) t o being a net sink (-1.6 x 10(10) molecules cm-2 s-2) after plowing. I t is likely that burial of surface litter reduces the production of CO in the top soil and that the diffusion of CO to deeper layers (where CO is consumed by microbiological processes) is promoted in decompacte d soils. As the rainy season progressed the plowed soil gradually comp acted and CO fluxes changed back, and after 3 months the fluxes from p lowed soils and the original unplowed soils were equal. Even though th e various cultivated fields (com, sorghum, and pasture) received diffe ring inorganic fertilization treatments, no significant difference in the CO fluxes resulted. Measurements during the dry season suggest tha t ''degrading dry (dead) vegetation'' produces CO under dark condition s.