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

Abstract

Controlled-environment chambers were used to study the effects of elevated CO 2 concentrations on bi- ological N fixation, N mineralization and C decomposi- tion in rice soil. In three chambers, CO 2 concentration was maintained at 353±15/396±23 µmol mol -1 (day/ night; ambient CO 2 ), while in another three, CO 2 was maintained at 667±36/700±41 µmol mol -1 (day/night; el- evated CO 2 ) throughout the growing season. Rice (var. Nipponbare) seedlings were grown under either ambient or elevated CO2 concentrations, and then transplanted in- to 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), NH 4 + content and organic C. Separate sets of soil samples were transferred to se- rum bottles and anaerobically incubated at 30°C for 30 days to measure potential rates of N mineralization and C decomposition. Under an elevated atmospheric CO 2 concentration, acetylene reduction activity signifi- cantly 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 CO 2 chambers, while the rate of N mineraliza- tion was increased by elevated CO 2 during the latter part of cultivation. Soils from the elevated CO 2 chambers had obviously higher rate of C decomposition than that from the ambient CO 2 chambers. CH 4 production gradually increased with the growth of rice plants. These results suggest that elevated CO 2 affected biological N fixation, N mineralization and C decomposition in submerged rice soil during the different growth stages of rice.