Observations of NO_x exchange between the atmosphere and vegetation have been widely reported. However, the magnitude, direction, and mechanism of this atmosphere–biosphere exchange remain uncertain across different ecosystems. We use laboratory measurements to study the rates of NO_x deposition to the leaves of a California oak tree species. We detect no evidence of NO_x emission and find that NO_x loss to oak leaves is substantial even at low NO_x concentrations relevant to forested environments.