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 decompo sition in rice soil. In three chambers, CO2 concentration was maintained at 353 +/- 15/396 +/- 23 mu mol mol(-1) (day/night; ambient CO2), while in an other three, CO2 was maintained at 667 +/- 36/700 +/- 41 mu mol mol(-1) (da y/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 dif ferent growth stages, soil samples were taken from surface (0-1 cm) and sub -surface (1-10 cm) 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. Separat e sets of soil samples were transferred to serum bottles and anaerobically incubated at 30 degreesC for 30 days to measure potential rates of N minera lization and C decomposition. Under an elevated atmospheric CO2 concentrati on, acetylene reduction activity significantly increased in the surface soi l layer during the early cultivation stages and in the sub-surface soil lay er during the latter part of cultivation. There was no difference in the am ount of NH4+ in fresh soils between elevated and ambient CO2 chambers, whil e the rate of N mineralization was increased by elevated CO2 during the lat ter part of cultivation. Soils from the elevated CO2 chambers had obviously higher rate of C decomposition than that from the ambient CO2 chambers. CH 4 production gradually increased with the growth of rice plants. These resu lts suggest that elevated CO2 affected biological N fixation, N mineralizat ion and C decomposition in submerged rice soil during the different growth stages of rice.