Mangrove biochar residue was used for nitrogen adsorption and retention in sediment, which is beneficial for plant germination. The present study investigated the effect of contact time (5–360 min), biochar dosage (0.2–2 g L−1), pH (5–6), and initial concentration (2–10 mg L−1) on NH4+-N and NO3−-N adsorption. Three different adsorption isotherm models were used to fit the experimental data. Column leaching experiments were conducted to investigate the effect of biochar with sediment from a shrimp pond on nitrogen leaching at varying biochar dosages (0–8% w/w). The results showed that the maximum percentage of both NH4+-N and NO3−-N adsorption was achieved at an equilibrium contact time of 240 min, with an adsorbent dosage of 2 g L−1, and pH at 5.5 and 5, respectively. The adsorption of NH4+-N and NO3−-N were fitted to the Freundlich model and the adsorption process followed the physisorption and ion exchange. The addition of 8% biochar reduced both cumulative water volume and nitrogen leaching from the sediment. The biochar amendment increased the relative abundance of nitrifying and denitrifying bacteria in the sediment. This result suggested that biochar amended with sediment could be useful for nitrogen loss reduction.