In this work, we developed electrochemical sensors capable of simultaneously monitoring ammonium and nitrate in various water sources. Our sensor design integrates electrodialysis and electrocatalysis (specifically electrochemical nitrate reduction to ammonium) to selectively measure these nitrogen species with minimal interference from other ions in solution. We utilized a capacitive sensing element that enables high sensitivity and incorporated it in the electrochemical reactor, creating a compact, all-in-one device. The capacitance changes can be directly related to ammonium concentration changes. After proof-of-concept demonstration, we optimized the electrodialysis and nitrate reduction steps independently to reduce the measurement time while still ensuring accuracy. Next, we integrated these electrochemical processes with capacitive sensing and systematically evaluated key performance metrics, including detection range, sensitivity, accuracy, detection rate, and energy demand. To confirm the robustness, we assessed the sensor’s selectivity and durability (baseline drift and signal to noise ratio) in diverse water compositions, elucidating the effects of competing ions on the electrochemical processes. The findings can be applied not only to nitrogen sensing but also more broadly to other electrodialysis and electrochemical nitrate reduction applications (e.g., wastewater treatment). Overall, our results highlight electrochemical ammonium-nitrate sensors as a promising, scalable solution for in-situ monitoring of nitrogen pollution.