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).d(-1) and Flux (DGT)-NH4+-N was further reduced from 13.38 to 1.59 mg.m(-2).d(-1). The calculated concentration gradient at the sediment-water interface following the capping treatment was lower than the control. These results implied that BC capping reduced the release of NH4+-N from the sediments. The low resupply from the sediments was further demonstrated by the smaller ratio of C-DGT to C-DET (R) in the BC capping layer, which their average values were 0.48, 0.59, 0.68 in the BC capping layer, BC/soil layer and native sediments, respectively. The physical, chemical and microbiological effects were attributed to inhibit the NH4+-N release from sediments by the BC capping.