The unsustainable dependence of nitrogen fertilizers (NFs) production on energy-intensive processes and its association with nitrate-laden wastewater that fuels harmful algal blooms (HABs) necessitate innovative solutions. Here, we propose a paradigm shift: repurposing HABs biomass as carbon-based catalysts (Cu1Mo1/NC) for the ambient-condition electrosynthesis of NFs from NO3− and CO2. Remarkably, Cu1Mo1/NC delivers a high NFs yield rate of 2303μg h−1 mgcat−1 (772μg h−1mgcat−1 for urea and 1531μg h−1 mgcat−1 for ammonia) with a Faradaic efficiency (FE) of 68.4% (15.2% for urea and 53.2% for ammonia) at −1.05 V vs. RHE. Experimental and theoretical evidence reveal that Cu doping tunes the d-band center of Cu1Mo1/NC, bringing it closer to the Fermi level. This enhances the intermediate adsorption, thereby propelling the C-N coupling reaction. Carbon reduction potential analysis underscores the promising feasibility and sustainable value of the presented method.