Contrasting effects of ammonium and nitrate inputs on soil CO2 emission in a subtropical coniferous plantation of southern China

Increased nitrogen (N) deposition has been found controversial affecting soil CO2 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 NH4 (+) (reduced N) and NO3 (-) (oxidized N) on soil CO2 flux and the underlying microbial mechanisms. An N addition experiment, two N fertilizers (NH4Cl and NaNO3) 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 CO2 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 NaNO3 addition significantly increased soil CO2 flux in the active growing season, whereas other N treatments did not change soil CO2 flux. High level of NH4Cl 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 CO2 flux and the change in fungal biomass, as well as between the change in soil CO2 flux and the change in community composition, were observed in the nonactive growing season. The N forms as NO3 (-) or NH4 (+) are important factors affecting C cycles in the subtropical coniferous plantation. These results suggested that the variations of soil CO2 emission and microbial biomass and community composition in the subtropical plantation depended on the seasons and the levels and forms of N addition.