Abstract: 【Objective】It is important to study the soluble organic nitrogen extracting in water-logged incubation for evaluating soil N-supplying capacity. 【Method】Initial soluble organic nitrogen(SONi) and mineral nitrogen(Nmin), cumulative soluble organic nitrogen(SONC) and NH4+-N(Nmin(C) ) in leaching solution during water-logged incubation, decomposable nitrogen mineralization potentials and resistant nitrogen mineralization potentials (NDPM and NRPM ), and their correlations with crop N-uptake of 10 kinds of farmland soils with different physical and chemical properties on the Loess Plateau were investigated. Effect of soluble organic nitrogen (SONi and SONC) on evaluating soil N-supplying capacity was studied. 【Result】The results showed that mean content of SONi (23.9mg/kg) of 10 soils was 28.8% of initial total soluble nitrogen (TSNi) and 2.4% of total nitrogen (TN). The percent of SONC in leaching total soluble nitrogen(TSNC) during incubation was higher and as high as Nmin(C) . After 217d water-logged incubation, the mean SONC content was 118.1mg/kg, was 46.4% of TSNC. NDPM had close correlation with TN, the correlation coefficient were 0.92(p<0.01, without SONC) and 0.88(p<0.01, with SONC). Different types of soils had different NDPM and the mineralization velocity of decomposable plant materials (KD). NDPM of Los-Orthic Entisols and Ust-Sandiic Entisols were lower than that of Eum-Orthrosols. KD decreased and the mineralization velocity of resistant plant materials (KR) increased when considering SONC. 【Conclusion】Nmin(C) was a good evaluating index of soil N-supplying not only for the first season crop, but also for two successive season crops. NDPM was a more correct index in revealing the potential mineralizable nitrogen when considering SONC, although SONC alone was not a good index for the potential of mineralizable nitrogen. And TSNC could also be taken as an index for the potential mineralizable nitrogen to some extent. Nmin(C) , TSNC and NDPM were good indexes for the potential mineralizable nitrogen, especially in estimating the potential mineralizable nitrogen of the soil after two successive season crops. And TSNC and NDPM considering SONC in evaluating the permanence of soil N-supply had better effect.

Key words: water-logged incubation, nitrogen mineralization, soil organic nitrogen, Soil N-supplying capacities