<p>Effectively managing and transforming organic waste into a valuable resource is critical for minimising environmental impact, conserving resources, and mitigating climate change. Anaerobic digestate of solid food waste has high ammonium (NH₄⁺) concentration and discharging it into the environment after nitrification may cause eutrophication. This study aimed to characterise (i) the adsorption of NH₄⁺ from anaerobic digestate of food waste by biochar derived from mixed eucalypt (<i>Eucalyptus</i> spp.) tree species before and after modification with HNO₃ and Na₂CO₃ and (ii) the use of NH₄⁺-loaded biochar as a potential N fertiliser. The isotherm and kinetics experiments using pure NH₄⁺ solution and anaerobic digestate solution revealed that modifications to eucalypt biochar enhanced NH₄⁺ adsorption capacity, predominantly through ion exchange and electrostatic interaction related to cation exchange capacity. A glasshouse pot study using maize plants was conducted with ammonium-loaded pristine and modified eucalypt biochar. The batch sorption data were fitted to Langmuir and Freundlich models for the isotherm and pseudo-first order, pseudo-second order, Elovich, and intraparticle diffusion models for the kinetics. The pseudo-secondorder and Elovich models closely matched the experimental breakthrough curves for NH₄⁺. Post-modification, the eucalypt biochar effectively extracted NH₄⁺ from pure NH₄⁺ solution and digestate, primarily through chemisorption via ion exchange, electrostatic interaction, cationic interaction. A glasshouse pot study using maize plants revealed that ammonium-loaded pristine and modified eucalypt biochar increased plant growth, suggesting that it can be used as an alternative to synthetic N fertiliser. Modified eucalypt biochar might be suitable for treating digestate liquid from various sources and reducing NH₄⁺ concentration and N losses from digestate liquid. Enriched modified biochar might be reused as nitrogen fertiliser in the soil.</p>