Montados are evergreen oak woodlands dominated by Quercus species, which are considered to be key to biodiversity conservation and ecosystem services. This ecosystem is often used for cattle breeding in most regions of the Iberian Peninsula, which causes plants to receive extra nitrogen as ammonia (NH3) through the atmosphere. The effect of this atmospheric NH3 (NH3atm) on ecosystems is still under discussion. This study aimed to evaluate the effects of an NH3atm concentration gradient downwind of a cattle barn in a Montado area. Leaves from the selected Quercus suber L. trees along the gradient showed a clear influence of the NH3 on delta C-13, as a consequence of a strong limitation on the photosynthetic machinery by a reduction of both stomatal and mesophyll conductance. A detailed study of the impact of NH3atm on the photosynthetic performance of Q. suber trees is presented, and new mechanisms by which NH3 affects photosynthesis at the leaf level are suggested.