Diffuse radiation impacts gross primary productivity of pasture ecosystems in Southern Amazonia

Biomass burning associated with Amazonia deforestation increases atmospheric aerosol loading, altering the partitioning of solar radiation and potentially affecting ecosystem carbon exchange. While diffuse radiation effects are documented, its influence on pasture remains uncertain. This study quantified aerosols and cloud effects on carbon uptake in a Brachiaria brizantha pasture in Southwestern Amazonia using eddy covariance and micrometeorological data (1999–2007) from the Fazenda Nossa Senhora site. Aerosol optical depth (AOD) increased from 0.10 (rainy) to 0.69 (dry season), showing a significant correlation with regional hotspots ( R 2 = 0.60, p < 0.05). Grassland ecosystems showed enhanced photosynthetic activity under diffuse radiation, as clouds and aerosols improved light use efficiency (LUE), promoting better energy utilization by the canopy. Variations in the diffuse fraction explained 34% of the LUE variability ( R 2 = 0.34, p < 0.05). Under cloudy conditions, Gross Primary Productivity (GPP) peaked at 0.6 gC m −2 s −1 , while the optimal photosynthetically active radiation (PAR) level for maximum GPP was approximately 2000 µmol m −2 s −1 . Increasing AOD shifted diffuse PAR from 50% to 90%, while total radiation decreased by 20% compared to clear-sky conditions. Atmospheric turbidity reduced canopy temperature by up to 10 °C and vapor pressure deficit from 2.5 kPa to 1.5 kPa, creating a cooler microclimate. Although GPP declined under extreme turbidity (AOD > 2.0), optimal carbon uptake occurred under intermediate conditions. These results demonstrate that moderate increases in diffuse radiation can enhance photosynthetic efficiency in Amazonian pastures.