<p>Nitrogen fixation heavily relies on the energy-intensive Haber-Bosch process, necessitating renewable alternatives. Here, we introduce a non-equilibrium spatially distributed electric field (SD-EF) strategy for nitrogen fixation in ambient air plasma. The optimized SD-EF strategy gives a NO_x⁻ yield of 9.8 mmol/h, tripling that of a uniform electric field and the N₂ conversion is three times higher than most discharge configurations at similar or lower energy consumptions. This high NO_x⁻ yield is achieved through simultaneously activating two beneficial kinetic networks via SD-EF by having both high and low electric fields present: O₃ and vibrational excitation of N₂ (N₂(v)) sub-mechanisms, which are revealed by developing a photonic crystal fiber diagnostic for in-situ quantification of molecules and ions (NO, NO₂, N₂O, O₃, NO₃⁻, and NO₂⁻) in gas-liquid plasma. The establishment of the SD-EF strategy, coupled with in-situ gas-liquid diagnostics, is broadly applicable to plasma-assisted nitrogen fixation and holds promise for other plasma-assisted chemical conversion processes.</p>