Effects of elevated carbon dioxide concentration on biological nitrogen fixation, nitrogen mineralization and carbon decomposition in submerged rice soil

Controlled-environment chambers were used to study the effects of elevated CO2 concentrations on biological N fixation, N mineralization and C decomposition in rice soil. In three chambers, CO2 concentration was maintained at 353±15/396±23 µmol mol–1 (day/night; ambient CO2), while in another three, CO2 was maintained at 667±36/700±41 µmol mol–1 (day/night; elevated CO2) throughout the growing season. Rice (var. Nipponbare) seedlings were grown under either ambient or elevated CO2 concentrations, and then transplanted into the soils in the corresponding chambers. At different growth stages, soil samples were taken from surface (0–1cm) and sub-surface (1–10cm) layers at the centre of four hills, then sieved (<1 mm) to remove root residues. Fresh soil was used to measure N fixation activity (using the acetylene reduction assay), NH4+ content and organic C. Separate sets of soil samples were transferred to serum bottles and anaerobically incubated at 30°C for 30 days to measure potential rates of N mineralization and C decomposition. Under an elevated atmospheric CO2 concentration, acetylene reduction activity significantly increased in the surface soil layer during the early cultivation stages and in the sub-surface soil layer during the latter part of cultivation. There was no difference in the amount of NH4+ in fresh soils between elevated and ambient CO2 chambers, while the rate of N mineralization was increased by elevated CO2 during the latter part of cultivation. Soils from the elevated CO2 chambers had obviously higher rate of C decomposition than that from the ambient CO2 chambers. CH4 production gradually increased with the growth of rice plants. These results suggest that elevated CO2 affected biological N fixation, N mineralization and C decomposition in submerged rice soil during the different growth stages of rice.