Influence of elevated concentrations of atmospheric CO 2 on CH 4 and CO 2 entrapped in rice-paddy soil

Abstract

Controlled environment chambers were used to study the influence of elevated atmospheric CO 2 concentration on CH 4 and CO 2 entrapped in soil bubbles and in solution in rice-paddy soil. Throughout the growing season, CO 2 concentration was maintained at 383 ± 11 μmol mol −1 during the day and 446 ± 40 μmol mol −1 at night for ambient CO 2 treatment, and at 706 ± 13 μmol mol −1 (day) and 780 ± 76 μmol mol −1 (night) for the elevated CO 2 treatment. At the grain-filling stage of growth, rice plants in the chambers were supplied with 13C-enriched CO 2 (δ 13C = 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 CO 2 and CH 4 entrapped in the soil. Elevated atmospheric CO 2 concentration not only directly increased the biomass above ground and in the roots by photosynthesis, but also indirectly increased the amounts of CH 4 and CO 2 entrapped in the soil. Most of the CO 2 was dissolved in soil solution, but in contrast most of the CH 4 existed in soil bubbles. When rice was fed with 13C-enriched CO 2 at the grain-filling stage of growth, the increase in 13C of entrapped CO 2 under ambient CO 2 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 CO 2 conditions. The increase in 13C of entrapped CH 4 accounted for 0.178% and 0.234% of the increase in 13C of rice plants grown under ambient and elevated CO 2 treatments, respectively. Under conditions of elevated CO 2 the entrapped 13C–CO 2 and 13C–CH 4 increased by 57% and 65%, respectively. The increase in 13C after feeding with 13C-enriched CO 2, as a proportion of the total C of plants before feeding, was higher for CH 4 entrapped in rice-paddy soil than for CO 2 entrapped in rice-paddy soil, WSOC in soil solution, aboveground biomass, and root biomass under both ambient and elevated CO 2 treatments. This indicates that during the grain-filling stage of rice growth, photosynthesized carbon had the most impact on CH 4 production and accelerated the CH 4 turnover rate.