Abstract: Nitrogen derived (Ndev) from water hyacinth (WH) residues and recovered (Nrec) in Komatsuna (Brassica rapa L.) was measured to evaluate the effect of different amounts of water hyacinth residues (0, 30, 45, 60 and 100%) added during komatsuna cultivation on the performance of the crop and amount of N released from WH residues into the system. There were no significant differences on dry matter production among treatments both in the 1st and 2nd sampling. N yield of Komatsuna decreased as the amounts of added WH residues increased in the 1st sampling but no significant differences were observed in N yield among treatments in the 2nd sampling. The Ndev from WH residues was significantly increased as amounts of added WH residues increased. In contrast, there were no significant differences found in Nrec from Komatsuna both in the 1st and 2nd sampling. In conclusion different quantities of added WH residues influenced the performance of Komatsuna, and the N dynamics in the soil-Komatsuna systems. The combination of 30% WH residues: 70% ammonium sulphate seems to be the most appropriate ratio that may be applied in the field.