Increased nitrogen (N) deposition has been found controversial affecting soil CO 2 emission in terrestrial ecosystems, which leads to serious debate on the efficiency of estimated C sequestration induced by N enrichment. The forms of input N might be responsible for this controversy. This study aims to explore the effects of NH 4 + (reduced N) and NO 3 − (oxidized N) on soil CO 2 flux and the underlying microbial mechanisms. An N addition experiment, two N fertilizers (NH 4 Cl and NaNO 3 ) and two rates (40 and 120 kg N ha −1 year −1 ), was carried out in a slash pine plantation of southern China. Soil-atmospheric CO 2 exchange, soil microbial biomass, and community composition were measured using static chamber-gas chromatography and phospholipid fatty acid (PLFA) analyses in the active growing and nonactive growing seasons, respectively. Low level of NaNO 3 addition significantly increased soil CO 2 flux in the active growing season, whereas other N treatments did not change soil CO 2 flux. High level of NH 4 Cl addition significantly reduced soil fungal biomass (fungal PLFA) and changed microbial community composition (ratio of fungal to bacterial (F/B) PLFAs). The positive relationships between the change in soil CO 2 flux and the change in fungal biomass, as well as between the change in soil CO 2 flux and the change in community composition, were observed in the nonactive growing season. The N forms as NO 3 − or NH 4 + are important factors affecting C cycles in the subtropical coniferous plantation. These results suggested that the variations of soil CO 2 emission and microbial biomass and community composition in the subtropical plantation depended on the seasons and the levels and forms of N addition.