Abstract: The organic acids were detected in soybean root exudates in order to understand the effect of the two N formations and N concentration on root exudates under hydroponics culture.Two cultivars,HF25 and SN10,were chosen in this study.HF25 is a susceptible genotype for root rot,while SN10 is resistant one.Two nitrogen sources were CO(NH2)2 and Ca(NO3)2,and N levels were 2.010-3 and 5.010-3 mol/L.Rhizobium was inoculated in each N treatment with a comparison to the un-inoculation one.In this research,the organic acids in soybean root exudates were detected by HPLC at seedling stage and flowering stage.The results of seedling stage showed that rhizobium had a neglect effect on the species and concentration of organic acids in HF25 root exudates under CO(NH2)2 solution compared to noninoculation one.While the species of organic acids in Ca(NO3)2 solution with rhizobium became less.As for SN10,rhizobium decreased the species of organic acids in CO(NH2)2 solution,but no effect in Ca(NO3)2 solution.In flowering stage,rhizobium increased the organic acids in HF25 root exudates both in CO(NH2)2 and Ca(NO3)2 solutions.There were no significantly different in SN10 under the same conditions.The concentration of organic acids in both soybean genotypes root exudates at flowering stage was higher than that in seedling stage.Data also showed that the citric acid in root exudates disappeared as the concentration of Ca(NO3)2 in solution increased from 2.010-3 to 5.010-3 mol/L,which indicated that the citric acid exudation from root was influenced by nitrogen concentration.It can be concluded that organic acids exudation from soybean root was affected by N source,N concentration,rhizobium,and soybean genotype.