The increasing on the greenhouse gases (GHG) emissions is today one of the main environmental problems, which can significantly affect human activities and land ecosystems. One of the main GHG is CO 2, which has been emitted indiscriminately due to the current lifestyle, as well as the intensification of agricultural activities. In this context, the objective of this investigation was to study the relationship between the spectral behavior of soybean during the crop cycle, using NDVI (Normalized Difference Vegetation Index), and the CO 2 fluxes, calculated by the eddy covariance method, generating information and methodology to investigate the carbon exchange in a soybean crop area in the State of Rio Grande do Sul, during the 2008/2009 soybean crop. For this, Landsat images 5 (TM), the phenological information and collected data from micrometeorological station throughout the development cycle of soybean were used. The results showed that the temporal pattern of CO 2 flux during the day was cyclical, showing negative values (capture) during daytime and positive values (liberation) at night. The global solar radiation determines the magnitude of the trapping of CO 2 by soybean, but the flow is modulated by the phenological stage of the crop. The photosynthetic activity of soybean plants is higher during the vegetative stage, coinciding to the higher incidence of solar radiation and the greater photosynthetic apparatus. The NDVI, obtained from Landsat images, is an indicator of the evolution of soybean biomass during the cycle. NDVI and negative CO 2 flow (capture) are correlated during the day. Therefore, remote sensing techniques show potentiality in generating of useful information on CO 2 exchange between the surface and atmosphere.
