Horizontal and vertical CO2 fluxes and gradients were made in an Amazon tropical rain forest, the Tapajós National Forest Reserve (FLONA-Tapajós: 54°58′W, 2°51′S). Two observational campaigns in 2003 and 2004 were conducted to describe subcanopy flows, clarify their relationship to winds above the forest, and estimate how they may transport CO2 horizontally. It is now recognized that subcanopy transport of respired CO2 is missed by budgets that rely only on single point eddy covariance measurements, with the error being most important under nocturnal calm conditions. We tested the hypothesis that horizontal mean transport, not previously measured in tropical forests, may account for the missing CO2 in such conditions. A subcanopy network of wind and CO2 sensors was installed. Significant horizontal transport of CO2 was observed in the lowest 10 m of the canopy. Results indicate that CO2 advection accounted for 73% and 71%, respectively, of the carbon budget for all calm nights evaluated during dry and wet periods. We found that horizontal advection is likely important to the canopy CO2 budget even for conditions with the above-canopy friction velocity higher than commonly used thresholds.