The air-water interfacial nitrogen cycle produces irrigatable-level ammonium nitrate

We report a sustainable, air-based strategy for synthesizing ammonium nitrate (NH₄NO₃) by harnessing the redox-active properties of microscale air-water interfaces. The process proceeds through two sequential reactions: (1) the nitrogen oxidation reaction (NOR), generating nitrate (NO₃ ^-) from atmospheric N₂, and (2) the nitrate reduction reaction (NO₃RR), converting nitrate into ammonium ions (NH₄ ⁺). In the first step, ambient air is introduced into a recirculating microbubble system, where solar irradiation and a water-soluble photocatalyst drive the efficient oxidation of N₂ to nitrate, producing NO₃ ^- at a rate of 500 μmol L^-1 h^-1. In the second step, atomized water microdroplets are sprayed across a Fe₃O₄-Nafion-CuO mesh, generating an extensive air-water interfacial area that promotes the reduction of nitrate and nitric oxide intermediates to NH₄ ⁺. Over 12 hours, this tandem process yields a 50 mL aqueous solution containing 0.94 mM NH₄NO₃ and 4.42 mM HNO₃, derived entirely from air and water. This carbon-free and catalyst-assisted platform offers a decentralized and environmentally friendly approach to nitrogen fixation, with immediate applications in hydroponic systems, where controlled nutrient delivery is essential. The integration of solar photocatalysis with microdroplet interfacial chemistry establishes a viable foundation for next-generation green fertilizer technologies.