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• Up to 93.45% of total nitrogen transformed into ammonium by hydrothermal process. • Activated hydrochar was synthesized to capture ammonium nitrogen. • 10.14% of total nitrogen was recovered after solubilization, capture, and available. • Nitrogen sorption by activated hydrochar was assessed by FTIR, SEM and BET. Nitrogen recovery from bio-based digestate has attracted considerable interests recently, but has been limited by the indirect capture of organic or insoluble nitrogen and the lower sorption ability of hydrochar. To mitigate these challenges, we used a hydrothermal pretreatment to promote the conversion of insoluble or organic nitrogen. Activated hydrochar was synthesized by using potassium hydroxide to enhance its sorption capacity, and the hydrochar was characterized by Fourier transform infrared spectroscopy, Brunauer-Emmett-Teller surface-area analysis, and scanning electron microscopy. An optimal efficiency was achieved at 210 °C, with 93.45% total nitrogen solubilization into ammonium. The sorption data of ammonium on the activated hydrochar fitted the pseudo second order kinetics and Langmuir model well. This reaction was dominated by chemisorption, which referring to the formation of bonds between (oxygen-containing) functional groups and ammonium. The maximum ammonium adsorption on the activated hydrochar at 260 °C (260AH) was 108.57 mg g−1. The ammonium sorption was endothermic, spontaneous and associated with a disorder increase on the solid-liquid interface. Once saturated, the ammonium was released continually from activated hydrochar and is anticipated to show an optimal performance in an alkaline environment. The amount of ammonium that was released from the saturated activated hydrochar at 260 °C after five extractions was 13.09 mg g−1. The nitrogen mass-balance calculations show that 10.14% total nitrogen was recovered after nitrogen solubilization, capture, and release. Therefore, potassium-hydroxide-activated hydrochar performed as a promising composite sorbent for ammonium. [ABSTRACT FROM AUTHOR]