Dynamics of soil organic carbon (SOC) under cultivation shifts has been in debate in soil C cycling studies. In this study, 2 adjacent fields of Wunitu paddy soil (one with rice-rape rotation and another with double corn for 3 years after rice and rape) were chosen to study the SOC dynamics. Both topsoil and whole profile was sampled. C pools of total organic carbon (TOC), dissolved organic carbon (DOC) and soil microbial biomass carbon (SMBC) was determined for bulk soils from the two fields respectively. Soil particle size factions (PSFs) were separated using low energy ultrasonic dispersion and the TOC of them determined using CNS analyzer. Selected samples of bulk soil and PSFs from both rice and corn fields were used for ¹³C natural abundance measurement with Finnigan MAT251 mass spectrometer. TOC of topsoil decreased drastically after 3 years of continuous corn cultivation although marked increase of DOC and SMBC was observed in the corn field. This was in coincident with the decrease of SOC in the sand PSF despite no remarkable changes in the other PSFs from the corn filed. Significantly heavier carbon could be detected either in bulk samples or in a single PSF from the corn field than from rice field. Calculation using the data of δ¹³C ‰(PDB) indicated that 80% of young carbon inputted by corn residues was allocated in the topsoil of 0～20cm and mainly found in the coarse PSF as well as in the pools of DOC and SMBC. The calculated short mean residence time (MRT) found with bulk sample and coarse fraction from the corn field in contrast to from the rice field suggested a coupling of rapid TOC decrease and fast aggregate turnover in the cornfield. Therefore, the TOC decline in the corn field was attributed to the disruption of aggregates that, in turn, released labile carbon from physical protection. Drastic decline of indigenous SOC at C 4 t hm^-2 a^-1 in the paddy soil after corn cultivation was in disagreement with the findings reported for American and European dry croplands and implicated a possible instability under alternation of cropping systems of SOC accumulated in paddies due to destruction of physical protection in coarse PSFs.