Photosynthesis by terrestrial vegetations is the driving force for the C cycling between Earth and the atmosphere. To evaluate the impacts of photosynthates on the dynamics of soil organic matter (SOM) in a rice-soil system, we conducted an isotopic pulse-chase labeling experiment in a microcosm system. Pulse labeling of 13C was performed to rice plants at varying ages. Destructive samplings of plants and soils were conducted at the end of the growing season. The 13C distribution in the aboveground mass increased from 45--52% of the net assimilation during the tillering stage of rice to 78--90% of that during the maturing stage of rice, whereas the proportion that was retained in roots decreased from 28 to 2%. About 1--5% of the net assimilation was incorporated into soil and mean 19.5% was lost presumably by respirations of plants and soil microorganisms. The percentage distribution of assimilates to soil was significantly exponentially correlated to the rate of root growth. Over a growing season, the estimated net plant C input into soil was 200 kg C ha-1. Most of the incorporated organic C was present as humin-like substances at the end of the season. Rice plants also produced 865 kg C ha-1 of root residues which were retained in soil after harvest. It was, therefore, considered that totally about 1065 kg C ha-1 of the atmospheric CO2 was fixed in soil after a growing season of plants. The increase of soil organic C content, however, was less than the net plant C input, suggesting that the mineralization of native SOM occurred concomitantly.