Dynamics of immobilization of the labeled substrate N by microorganism and soil clay and its residue fractions in soils from the plough layers of two subtropical paddy soils (Reddish clayey soil, Purple alluvial soil) in the Dongting Lake floodplain were studied. A laboratory-flooded incubation experiment was composed of three treatments: control (CK), labeled ammonium sulphate group (15NA) and the combined of labeled ammonium sulphate and rice straw group (S + 15NA). During the incubation, microbial biomass N (BN) increased firstly and then those decreased and tended to stable subsequently, while the content of fixed ammonium changed little. Native BN acted as the major N pool because the percentage-of labeled substrate BN in reddish clayey soil and purple alluvial soil were 0.30%-6.67% and 1.00%-3.47%, respectively. The combined application of rice straw and chemical fertilizer (S + 15NA) improved the immobilization of inorganic N by microorganism, because the immobilization ratio of substrate N in reddish soil and purple alluvial soil were 6.78% and 10.78%, respectively, for S + 15NA treatment, higher than those for 15NA treatment. The immobilization ratio of substrate N by soil clay in reddish clayey soil and purple alluvial soil were 2.48%-10.57% and 12.55%-30.04%, respectively. However, the immobilization ratio of substrate N by soil clay in the two soils were 7.14% and 21.53%, respectively, for S + 15NA treatment, lower than those for NA treatment. The incorporation of chemical fertilizer and rice straw increased the N remain percent. The main residue formation of the labeled substrate N was total hydrolysable N (> 72%) in Reddish clayey soil, while it was total hydrolysable N (44.0%-53.2%) and fixed ammonium (35.2%-37.5%) in Purple alluvial soil. The residue of mineral nitrogen ranged 10%-20% in the two soils. In conclusion, fertilization method and the type of soil clay had important effects on the immobilization and mineralization of substrate N. The combined application of chemical fertilizer and straws increased the immobilization of inorganic nitrogen by microorganism and decreased immobilization of inorganic nitrogen by soil clay. The combined application of chemical fertilizer and straws decreased the loss of chemical fertilizer N, increased residue of nonhydrolysable N, and decreased residue of mineral nitrogen.