Field experiments on dynamics of soil soluble organic N (SON) and correlations with soil microbial biomass N (MBN) and N uptake of rice in rice-duck, rice-fish ecosystems in subtropical region of China were conducted from May to September in 2007. The experimental field was divided into nine plots for three different treatments, and the three experimental treatments were (1) conventional rice field (CK), (2) rice-duck ecosystem (RD) and (3) rice-fish ecosystem (RF). The results showed that soil SON constituted a predominant reservoir of soluble N in paddy fields, and the sizes of soil SON pools in CK, RD and RF were 121.16, 109.30 and 113.71 kg N hm-2,respectively, higher than the sizes of soil soluble inorganic N (SIN) pools in the corresponding treatments. During rice growth, SON was significantly positively related to SIN; moreover, the content of SON decreased gradually along with rice growth. Due to the presence of ducks and fish, the amounts of SON were significantly lower in RD and RF than in CK. The results also indicated SON was significantly negatively related to N uptake of rice, which showed that rice growth strongly affected SON. In the forepart periods of rice growth, contents of different formations of N all were the highest. Dissolved organic N (DON) was the main formation of N in percolation water in paddy fields; moreover, statistical analysis indicated that compared to CK, RD significantly decreased the leakage of SON and RF slightly decreased the leakage of SON. During rice growth, soil MBN changed constantly, and the presence of ducks and fish significantly increased soil MBN; moreover, due to N uptake of rice and N leaching, the soil MBN was not closely related to SON. In conclusion, soil SON was regulated by the interactions of N uptake of rice plants, microbial uptake and decomposition, and leaching in paddy fields during rice growth.