High rates of net ecosystem carbon assimilation by Brachiara pasture in the Brazilian Cerrado.

To investigate the consequences of land use on carbon and energy exchanges between the ecosystem and atmosphere, we measured CO₂ and water vapour fluxes over an introduced Brachiara brizantha pasture located in the Cerrado region of Central Brazil. Measurements using eddy covariance technique were carried out in field campaigns during the wet and dry seasons. Midday CO₂ net ecosystem exchange rates during the wet season were -40μmol m^-1s^-1, which is more than twice the rate found in the dry season -15μmol m^-1s^-1. This was observed despite similar magnitudes of irradiance, air and soil temperatures. During the wet season, inferred rates of canopy photosynthesis did not show any tendency to saturate at high solar radiation levels, with rates of around 50μmol m^-2s^-1 being observed at the maximum incoming photon flux densities of 2200 μmol m^-2s^-1. This contrasted strongly to the dry period when light saturation occurred with 1500 μmol m^-1s^-1 and with maximum canopy photosynthetic rates of only 20μmol m^-2s^-1. Both canopy photosynthetic rates and night-time ecosystem CO₂ efflux rates were much greater than has been observed for cerrado native vegetation in both the wet and dry seasons. Indeed, observed CO₂ exchange rates were also much greater than has previously been reported for C₄ pastures in the tropics. The high rates in the wet season may have been attributable, at least in part, to the pasture not being grazed. Higher than expected net rates of carbon acquisition during the dry season may also have been attributable to some early rain events. Nevertheless, the present study demonstrates that well-managed, productive tropical pastures can attain ecosystem gas exchange rates equivalent to fertilized C₄ crops growing in the temperate zone. [ABSTRACT FROM AUTHOR]