Influence of elevated concentrations of atmospheric CO2 on CH4 and CO2 entrapped in rice-paddy soil

Controlled environment chambers were used to study the influence of elevated atmospheric CO2 concentration on CH4 and CO2 entrapped in soil bubbles and in solution in rice-paddy soil. Throughout the growing season, CO2 concentration was maintained at 383 ± 11 μmol mol−1 during the day and 446 ± 40 μmol mol−1 at night for ambient CO2 treatment, and at 706 ± 13 μmol mol−1 (day) and 780 ± 76 μmol mol−1 (night) for the elevated CO2 treatment. At the grain-filling stage of growth, rice plants in the chambers were supplied with 13C-enriched CO2 (δ13C = 413.9‰) for 3 days to study the allocation and transformation of photosynthetic carbon to root biomass, water-soluble organic carbon (WSOC) in soil solution, and CO2 and CH4 entrapped in the soil. Elevated atmospheric CO2 concentration not only directly increased the biomass above ground and in the roots by photosynthesis, but also indirectly increased the amounts of CH4 and CO2 entrapped in the soil. Most of the CO2 was dissolved in soil solution, but in contrast most of the CH4 existed in soil bubbles. When rice was fed with 13C-enriched CO2 at the grain-filling stage of growth, the increase in 13C of entrapped CO2 under ambient CO2 conditions accounted for 1.476% of the increase in 13C of the rice plants and for 1.845% of the increase in 13C of rice plants grown under elevated CO2 conditions. The increase in 13C of entrapped CH4 accounted for 0.178% and 0.234% of the increase in 13C of rice plants grown under ambient and elevated CO2 treatments, respectively. Under conditions of elevated CO2 the entrapped 13C–CO2 and 13C–CH4 increased by 57% and 65%, respectively. The increase in 13C after feeding with 13C-enriched CO2, as a proportion of the total C of plants before feeding, was higher for CH4 entrapped in rice-paddy soil than for CO2 entrapped in rice-paddy soil, WSOC in soil solution, aboveground biomass, and root biomass under both ambient and elevated CO2 treatments. This indicates that during the grain-filling stage of rice growth, photosynthesized carbon had the most impact on CH4 production and accelerated the CH4 turnover rate.