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Evaluation of in situ capping with biochar (BC) to control ammonia nitrogen (NH₄⁺-N) release from the sediments of Baiyangdian Lake, the largest freshwater lake in northern China, was performed after 10 months of field and mesocosm experiments. The concentration of NH₄⁺-N in pore water and its resupply from native sediments and capped sediments were determined using the technique of diffusive equilibrium in thin films (DET) and the technique of diffusive gradients in thin films with CMI-7000 cation ion exchange membrane (CMI-DGT) binding layer. The results showed a low concentration of NH₄⁺-N in pore water and a low resupply from the sediment or capping materials after BC capping. The concentration of bioavailable NH₄⁺-N measured by DGT of the top 4 cm layer in BC capping layer was only 12.8%−29.2% of the native sediments. Flux _DET-NH₄⁺-N_, could be reduced from 12.74 to 3.44 mg·m⁻² …