Ammonia (NH3) is pivotal to the fertilizer industry and one of the most commonly produced chemicals(1). The direct use of atmospheric nitrogen (N-2) had been challenging, owing to its large bond energy (945 kilojoules per mole)(2,3), until the development of the Haber-Bosch process. Subsequently, many strategies have been explored to reduce the activation barrier of the N equivalent to N bond and make the process more efficient. These include using alkali and alkaline earth metal oxides as promoters to boost the performance of traditional iron- and ruthenium-based catalysts(4-6)via electron transfer from the promoters to the antibonding bonds of N(2)through transition metals(7,8). An electride support further lowers the activation barrier because its low work function and high electron density enhance electron transfer to transition metals(9,10). This strategy has facilitated ammonia synthesis from N(2)dissociation(11)and enabled catalytic operation under mild conditions; however, it requires the use of ruthenium, which is expensive. Alternatively, it has been shown that nitrides containing surface nitrogen vacancies can activate N-2(refs.(12-15)). Here we report that nickel-loaded lanthanum nitride (LaN) enables stable and highly efficient ammonia synthesis, owing to a dual-site mechanism that avoids commonly encountered scaling relations. Kinetic and isotope-labelling experiments, as well as density functional theory calculations, confirm that nitrogen vacancies are generated on LaN with low formation energy, and efficiently bind and activate N-2. In addition, the nickel metal loaded onto the nitride dissociates H-2. The use of distinct sites for activating the two reactants, and the synergy between them, results in the nickel-loaded LaN catalyst exhibiting an activity that far exceeds that of more conventional cobalt- and nickel-based catalysts, and that is comparable to that of ruthenium-based catalysts. Our results illustrate the potential of using vacancy sites in reaction cycles, and introduce a design concept for catalysts for ammonia synthesis, using naturally abundant elements. Ammonia is synthesized using a dual-site approach, whereby nitrogen vacancies on LaN activate N-2, which then reacts with hydrogen atoms produced over the Ni metal to give ammonia.