The influence of free-air CO2 enrichment on microorganisms of a paddy soil in the rice-growing season

The atmospheric CO₂ concentration is dramatically rising, and this rise may affect soil methanogens, methanotrophs, nitrifiers, and denitrifiers, which are important microorganisms for the processes of carbon and nitrogen turnover. An experimental platform of free-air CO₂ enrichment (FACE) was established in mid-June of 2001 over a rice–wheat rotation ecosystem located in a suburb of Wuxi, China, and its CO₂ fumigation was continued until mid-February of 2004. Using the most probable number (MPN) method, we measured the numbers of methanogens, methanotrophs, nitrifiers, and denitrifiers by sampling fresh soils from the fields exposed to the elevated and ambient CO₂ during the rice-growing season in 2002. Our results show that the elevated CO₂ significantly increased methanogen populations of the cultivated soil layers during the entire rice-growing season. This positive effect of elevated CO₂ may be attributed to stimulated rice growth, which may provide more substrates for methanogens. The methanotroph population was decreased by elevated CO₂ in the upper soil layer (0–5 cm) but was increased in the lower one (5–10 cm) in most rice-growing stages, and the effect of CO₂ elevation was reversed at rice maturity. Elevated CO₂ increased nitrifier and denitrifier populations in most rice stages, but it occasionally decreased the number of nitrifiers late in the growing season and that of denitrifiers early. The methanogen population gradually increased until the filling stage of rice growth but then declined under either elevated or ambient CO₂. Meanwhile the numbers of methanotrophs and nitrifiers gradually decreased during the entire rice season. The number of denitrifiers in the wet/flooded soil during the growing season was also decreased as compared to the dry soil before rice season.