Flow of photosynthesized carbon from rice plants into the paddy soil ecosystem at different stages of rice growth

To elucidate the flow of C assimilated by rice plants into soil C, soil biomass C, and emitted CH₄ at different rice growth stages, ¹³C pulse-labeling was conducted at the maximum-tiller-number (first experiment), booting (second experiment), and milky stages (third experiment) for potted rice grown outdoors under flooded conditions. The distribution of the assimilated C into each fraction was traced during a 14-day period. The atom-¹³C% excess of shoots was the highest just after the 6 h feeding of ¹³CO₂ and decreased until days 14, 7, and 3 in the first, second, and third experiments, respectively. Translocation of the assimilated C into roots was largest in the first experiment, 13%, while that into ears was more than 50% in the third experiment. The proportion of the rice-assimilated C recovered in soil organic matter increased with time after labeling and reached 3.4%, 3.0%, and 1.7% on day 14 in the first, second, and third experiments, respectively. Incorporation of the rice-assimilated C was faster into soil microbial biomass than into gross soil organic matter or the 0.5 M K₂SO₄-extractable fraction. Although the percent of labeled soil C that is in the microbial biomass on day 0 was much larger at the maximum-tiller-number stage (42%) than at the milky stage (5%), its variation among growth stages was small on day 14 (10 to 15%). The percent of the rice-assimilated C emitted as CH₄ during the 14-day period at the maximum-tiller-number, booting, and milky stages was 0.003%, 0.26%, and 0.30%, respectively.