One possible management option for farmers to improve the soil nitrogen (N) supply for rice production is the cultivation of a prior legume. The objective of this study was to investigate the value of such an option in the lowland of the north-east of Thailand. Two experiments were established in 2 typical locations in a split-plot design with 4 replicates. The main plots included 3 nitrogen levels (0, 30, and 60 kg N/ha) and the subplots, 4 pre-rice managements: (i) fallow with weeds removed (FW–); (ii) with weeds incorporated before the rice crop (FW+); (iii) mungbean incorporated at flowering as green manure (MGM); or (iv) incorporated after grains harvest (MR+). In both experiments the difference in rice yield between MGM and MR+ was not significant. In Expt 1, in contrast to Expt 2, the rice yield increase due to MR+ was significant and significantly higher than that due to application of 60 kg N/ha. Moreover, significantly higher apparent recovery of N (ANR_m, kg N uptake increase/kg N supplied by residues), probably due to the continuous flooding of the soil surface, was achieved in this experiment. The low values of internal efficiency of N (IEN, kg total grains/kg total N uptake), ANR_f (Δ kg N uptake/kg N supplied by fertiliser), and of ANUE_f (Δ kg grains/kg applied N fertiliser) recorded in the MR+ treatment of Expt 1, suggest that no application of N fertiliser is needed where the soil water conditions allow high recovery of the N supplied by a preceding mungbean crop.