© 2019 American Chemical Society. Boron nitride (BN), with outstanding stability and robustness in diverse polymorphs, possesses many advantageous properties for industrial applications. Activation of BN materials for nonmetal catalysts is among the most revolutionary and challenging tasks. Taking advantage of quantum size effect and synergistic effect, here we exploit boron nitride nanotubes (BNNTs) encapsulating early transition metal nanowires, which is experimentally feasible, for nitrogen fixation and ammonia synthesis. Using first-principles calculations and microkinetic modeling, we show that the coexisting occupied and unoccupied p states of B atoms in filled BNNTs can effectively mimic the d states of transition metal. They act as electron reservoirs with tunable orbital energies and occupancy, which are beneficial for associative N2 adsorption and hydrogenation. Due to the competition between thermodynamics of gas adsorption and kinetics of hydrogenation reaction, the activity can be optimized by controlling the type of metal filler and size of BN nanotube, achieving a turnover frequency competitive to that of benchmark Fe catalyst. These results manifest a universal strategy for activating BN nanomaterials as a promising family of robust and efficient catalysts and provide vital insights into the activity-band structure relationship for p-block nonmetal catalysts.