Input and distribution of photosynthesized carbon in a flooded rice soil

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 13 C was performed to rice plants at varying ages. Destructive samplings of plants and soils were conducted at the end of the growing season. The 13 C 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 CO 2 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.