Most parts of Iran are located in the arid to semi arid region of the world. Therefore, lack of adequate water resulted in shortage of vegetation. This has resulted in low amounts of plant residues incorporated into the soil and hence low soil organic matter content. Nitrogen deficiency is predominantly observed in the arid to semi arid regions of the world because N is mostly concentrated in soil organic matter. Nitrogen deficiency on the other hand is one of the most important limiting factors in agroecosystems and therefore, legumes may play a key role in fixation and incorporation of N into soil and food chains. This is why legumes are recognized as an effective ecosystem component which may provide the soil food web with N and therefore can contribute to soil fertility improvement. Legume residues can potentially be considered as sources of soil N. The Nitrogen content of the plant residues are mostly in organic forms. Some of the organic forms of N can be dissolved in the soil water followed by incorporation in the soil. Soluble or insoluble forms of organic N are subjected to the N mineralizations which take place by the heterotrophic soil population. The objective of this study was to investigate the effects of legume residue application on the release of soluble inorganic and organic N in soil. For this purpose, two soil types were collected from 0-15 cm of Shervedan and Chelvan. The residues of sainfoin( Onobrychis viciifolia ), clover ( Trifolium repens ) and alfalfa ( Medicago sativa. L ) were dried paassed through 1-mm sieve and kept in the refrigerator. The plant residues were applied to the soils at the rate of 10 g kg-1 soil. control treatments (without application of the plant residues) were also considered for both soils. Stanford and Smith leaching columns were prepared for both residue-applied and control soils with three replications. The soils were kept in incubator for 20 weeks and the released inorganic and organic N was measured at different time intervals. Results indicated that application of the plant residues has resulted in soluble organic and inorganic N. Expectedly, the release of inorganic N was conformed to an exponential (first-order kinetic) model. In contrast, none of the treatments showed exponential increasing trend in the soluble organic N. In all of the time intervals, alfalfa residues brought about greater amounts of inorganic N. The lowest amount of inorganic N was observed in the clover-applied soils. The potentially mineralizable N was lowest in the control soils. The amount of N mineralized at the end of incubation was significantly correlated with the potentially mineralizable N (r = 0.6, P 0.001) and initial potential rates of N mineralization (r = 0.9, P 0.001). The soluble organic N was increasingly increased by time. In the Shervedan soil, the soluble organic N was equal among the treatments while, in the Chelvan, alfalfa-treated soils possessed the lowest amounts of soluble organic N. In both soils, the final concentrations of soluble organic and inorganic N were negatively associated (In the Shervedan soil r = 0.63, P 0.05 and in the Chelvan r = 0.6, P 0.05). Moreover, the soluble organic N was also negatively correlated with initial potential rates of N mineralization (In the Shervedan soil r = 0.58, P 0.05 and in the Chelvan r = 0.65, P 0.05).